Prathum Saraf - December 11, 2023
Dissertation Title: Charge order and structural transition in topological semimetal family Aal4
Date and Time: Monday, December 11, 1:00 pm
Location: Toll 2219
Dissertation Committee Chair: Johnpierre Paglione
Committee:
Nicholas Butch
Jeffrey Lynn
Richard Greene
John Cumings
Abstract:
The BaAl4-type structure hosts a variety of interesting and exotic properties, with descendant crystal structure resulting numerous interesting ground states of matter including magnetic, super-conducting and strongly correlated electron phenomena. BaAl4 itself has recently been shown to host a non-trivial topological band structure but is otherwise a paramagnetic metal. However, the other members of the 1-4 family, such as SrAl4 and EuAl4, exhibit symmetry-breaking ground states including charge density wave (CDW) and magnetic order, respectively. SrAl4 hosts a second transition at 94K that is hysteretic in temperature and is a structural transition to a monoclinic structure. Here I report on the charge density wave in SrAl4 and the effect of the structural transition on the physical and electronic properties of the material. The structural transition is extremely subtle with deviation of around 0.5 degrees from the tetragonal structure but shows significant changes resistivity, hall and magnetic susceptibility measurements. This transition is extremely sensitive to disorder and can be suppressed completely by substituting 1% Ba nominally or using less pure Sr during crystal growth. Furthermore, magnetoresistance in this material is extremely large, and can be up to 140 times at 2K. A combination of magnetoresistance and hall measurements are used to fit the data to a two band model to extract carrier density and mobility of the charge carriers at 2K. Finally, work was done on the evolution of the charge-ordered state in high quality single crystals of the solid solution series Ba1−xSrxAl4, using transport, thermodynamic and scattering experiments to track the 243 K CDW order in SrAl4 as it is suppressed with Ba substitution until its demise at x =0.5. Neutron and x-ray diffraction measurements reveal a nearly commensurate CDW state in SrAl4 with ordering vector (0,0,0.097) that evolves with Ba substitution to (0,0,0.18) and (0,0,0.21) for x=0.8 and x=0.55, respectively. DFT calculations show a softening of phonons in SrAl4 hinting at EPC strength being the source of the charge order in this material. Similar calculations are done on the Ba substitutions to investigate the nature of the charge density waves. With very little change in the lattice parameters in this series, this evolution raises important questions about the nature of the electronic structure that directs a dramatic change in charge ordering.
Alexander Wikner - November 6, 2023
Dissertation Title: Combining Physics-based Modeling, Machine Learning, and Data Assimilation for Forecasting Large, Complex, Spatiotemporally Chaotic Systems
Date and Time: Monday, November 6, 12:00 pm
Location: AV Williams 2460
Dissertation Committee Chair: Prof. Edward Ott
Committee:
Prof. Michelle Girvan
Prof. Thomas Antonsen
Prof. Rajarshi Roy
Prof. Brian Hunt (Dean’s Representative)
Abstract:
We consider the challenging problem of forecasting high-dimensional, spatiotemporal chaotic systems. We are primarily interested in the problem of forecasting the dynamics of the earth's atmosphere and oceans, where one seeks forecasts that (a) accurately reproduce the true system trajectory in the short-term, as desired in weather forecasting, and that (b) correctly capture the long-term ergodic properties of the true system, as desired in climate modeling. We aim to leverage two types of information in making our forecasts: incomplete scientific knowledge in the form of an imperfect forecast model, and past observations of the true system state that may be sparse and/or noisy. In this thesis, we ask if machine learning (ML) and data assimilation (DA) can be used to combine observational information with a physical knowledge-based forecast model to produce accurate short-term forecasts and consistent long-term climate dynamics.
We first describe and demonstrate a technique called Combined Hybrid-Parallel Prediction (CHyPP) that combines a global knowledge-based model with a parallel ML architecture consisting of many reservoir computers and trained using complete observations of the system's past evolution. Using the Kuramoto-Sivashinsky equation as our test model, we demonstrate that this technique produces more accurate short-term forecasts than either the knowledge-based or the ML component model acting alone and is scalable to large spatial domains. We further demonstrate using the multiscale Lorenz Model 3 that CHyPP can incorporate the effect of unresolved short-scale dynamics (subgrid-scale closure).
We next demonstrate how DA, in the form of the Ensemble Transform Kalman Filter (ETKF), can be used to extend the Hybrid ML approach to the case where our system observations are sparse and noisy. Using a novel iterative scheme, we show that DA can be used to obtain training data for successive generations of hybrid ML models, improving the forecast accuracy and the estimate of the full system state over that obtained using the imperfect knowledge-based model.
Finally, we explore the commonly used technique of adding observational noise to the ML model input during training to improve long-term stability and climate replication. We develop a novel training technique, Linearized Multi-Noise Training (LMNT), that approximates the effect of this noise addition. We demonstrate that reservoir computers trained with noise or LMNT regularization are stable and replicate the true system climate, while LMNT allows for greater ease of regularization parameter tuning when using reservoir computers.
Yi-Mu Chen - November 3, 2023
Dissertation Title: Search for emerging-jet signatures in pp collisions at 13TeV with the CMS detector using a fully data-based method for background extraction
Date and Time: Friday, November 3, 11:00 am
Location: Zoom
Dissertation Committee Chair: Alberto Belloni
Committee:
Sarah Eno
Christopher Palmer
Zackaria Chacko
Manuel Franco Sevilla
Massimo Ricotti
Abstract:
A search for emerging jets produced in proton-proton collisions at a center-of-mass energy of 13 TeV is presented in this thesis. This search examines a hypothetical dark QCD sector that couples to the standard model (SM) through a scalar mediator. The scalar mediator decays into a standard model quark and a dark sector quark. As the dark sector quark showers and hadronizes, it produces long-lived dark mesons that subsequently decay into SM particles, resulting in a jet with multiple displaced vertices, known as an emerging jet. This search extends the existing efforts using data collected with the CMS detector by including the possibility of a flavored coupling between the standard model sector and dark sector, which results in emerging jets containing both long-lived and prompt decays. This search looks for pair production of the scalar mediators at the large hardon collider, yielding events with two SM jets and two emerging jets.
As the detector signature left by such dark sector showers significantly deviates from the assumptions made when designing the CMS detector, a fully data-based method is used for evaluating the number of standard model events that can be mistaken as signal events. The data-based method works by comparing the probability of standard model jets being misidentified as emerging jets in a signal-free data set and then using this probability to calculate a per-event weighting factor applied to compare events in the signal-depleted region. The weighted sum can be used to estimate the standard model number of events that enter a signal-enriched region.
The search is carried out on data collected by the CMS experiment corresponding to an integrated luminosity of 138/fb . No significant excess over the expected background is observed. The results are interpreted using two dark sector models and exclude mediator masses up to 1750 GeV for an unflavored dark QCD model and up to 2000 GeV for a flavor-aligned dark QCD model.
Hadi Vafaii - November 2, 2023
Dissertation Title: Unveiling secrets of brain function with generative modeling: Motion perception in primates & Cortical network organization in mice
Date and Time: Thursday, November 2, 3:30 pm
Location: BRB-1103
Dissertation Committee Chair: Luiz Pessoa
Committee:
Daniel Butts (Co-Chair)
Michelle Girvan
Evelyn MR Lake
Tom Goldstein (Dean’s Representative)
Abstract:
Generative modeling seeks to understand and replicate the underlying structure of a dataset, which enables the interpretation of observed data, such as images, based on some latent (or unobserved) variables. Here, I will demonstrate how generative modeling can be used to effectively analyze neuronal data and answer questions within diverse domains of systems neuroscience.
In the first part, I draw inspiration from Helmholtz's “Perception as Unconscious Inference”, a concept paralleled in modern machine learning by generative models like variational autoencoders (VAE) and their hierarchical variants. Here, I evaluate the role of hierarchical inference and its alignment with brain function in the domain of motion perception. To this aim, I introduce a novel synthetic data framework that enables control over motion statistics and their causes. I introduce a new hierarchical VAE architecture and test it against alternative models on two downstream tasks: predicting ground truth variables (e.g., self-motion) and predicting the responses of neurons in the motion processing pathway of primates. The findings reveal that hierarchical latent structure improves the linear decodability of ground truth variables and does so in a sparse and disentangled manner. Furthermore, hierarchical VAEs surpass previous state-of-the-art models in predicting neuronal responses by over 2x and result in sparse latent-to-neuron relationships. Collectively, these results support the notion that hierarchical Bayesian inference underlies the brain's understanding of the world, and hierarchical VAEs offer an effective means of capturing this understanding.
In the second part, I shift focus to the mouse cortical network structure using a simultaneous dataset combining fMRI with wide-field calcium imaging (Ca2+). The goal is to understand the intrinsic functional architecture of the mouse cortex in the absence of controlled external stimulation and test whether different imaging modalities reveal the same underlying organization. I achieve this by employing a generative algorithm to decompose the correlation structure of spontaneous brain fluctuations into overlapping networks of brain regions. Comparative analyses across imaging modalities reveal that many features of fMRI network organization are reproduced by Ca2+ data, particularly in a lower temporal frequency band. However, notable differences also emerge in the overall network structure and measures of regional functional connectivity strength and diversity, suggesting that network topologies are ultimately different across imaging modalities. In conclusion, wide-field Ca2+ imaging uncovered overlapping functional cortical organization reflecting several but not all properties observed with fMRI-BOLD signals.
Kate S. Collins - November 2, 2023
Dissertation Title: Analog—Digital Quantum Simulations with Trapped Ions
Date and Time: Thursday, November 2, 12:00 pm
Location: PSC 1136
Dissertation Committee Chair: Chris Monroe
Committee:
Zohreh Davoudi
Alexey Gorshkov
Chris Jarzynski
Qudsia Quraishi
Abstract:
Since its inception in the early 1920s, the theory of quantum mechanics has provided a framework to describe the physics of nature; or at least our interpretations about systems in nature. However, even though quantum theory works, the unsettling question of ``why?' still remains. The field of quantum information science and technology (QIST) has brought together a collection of disciplines forming a united multidisciplinary collaborative effort towards realizing a large-scale quantum processor as an attempt to understand quantum mechanics better. It has been established in the field that the most efficient architectural design of this quantum processor would be composed of numerous individual quantum computers, quantum simulators, quantum networks, quantum memories, and quantum sensors that are ``wired'' together creating just the hardware layer in the full stack of the machine. Realizing a module-based quantum processor on such a macroscopic scale is an ongoing and challenging endeavor in itself.
However, existing noisy intermediate-scale quantum (NISQ) devices across all the quantum applications above are still worth building, running, and studying. NISQ quantum computers can still provide quantum advantages over classical computation for given algorithms, and quantum simulators can still probe complex many-body dynamics that remain improbable to consider even on the best supercomputer. One such system is the trapped-ion quantum simulator at the center of this dissertation. Using 171Yb+ ions, we expand our “analog” quantum simulation toolbox by incorporating “digital” quantum computing techniques in each of the three experiments presented in this work.
In the first experiment, we perform a quantum approximate optimization algorithm (QAOA) to estimate the ground-state energy of a transverse-field antiferromagnetic Ising Hamiltonian with long-range interactions. For the second project, we develop and demonstrate dynamically decoupled (DD) quantum simulation sequences in which the coherence in observed dynamics evolving under the unitary operator of the target Hamiltonian is extended while the known noise is suppressed. Finally, in the third project, we implement an experimental protocol to measure the spectral form factor (SFF) and its generalization, the partial spectral form factor (PSFF), in both an ergodic many-body quantum system and in a many-body localized (MBL) model. Consequently, a quantum simulator can be utilized to test universal random matrix theory (RMT) predictions, and simultaneously, probe subsystem eigenstate thermalization hypothesis (ETH) predictions of a quantum many-body system of interest.
Kristi Engel - October 26, 2023
Dissertation Title: All-Sky Search for Very-High-Energy Emission from Primordial Black Holes and Gamma-Ray Bursts with the HAWC Observatory
Date and Time: Thursday, October 26, 11:00 am
Location: PSC 2136
Dissertation Committee Chair: Jordan Goodman
Committee:
J. Patrick Harding
Kara Hoffman
Gregory Sullivan
M. Coleman Miller
Abstract:
Transient sources of very-high-energy gamma rays are short-lived astrophysical phenomena often associated with catastrophic events that change their brightness over relatively short timescales. The search for and study of such objects, especially in the TeV energy regime, has the possibility to shed light not only on the physics at play within the enigmatic, chaotic environments that produce such emission, but also to answer several remaining questions in fundamental physics. In this dissertation, we leverage the sensitivity and characteristics of the HAWC Observatory in pursuit of gaining insight into these areas.
The High-Altitude Water Cherenkov (HAWC) Observatory, located on the side of the side of the Sierra Negra volcano in Puebla, Mexico at an altitude of 4,100 m above sea level, is an extensive-air-shower array sensitive to gamma rays from a ~0.1 to >100 TeV that has been in operation since March of 2015. It has a wide field of view of ~2 sr and a large operational duty cycle (>95%). Herein, we present an improved characterization for the larger of the two Photomultiplier Tubes used by HAWC, as well as the custom testing apparatus designed and constructed for this purpose.
With HAWC's wide field of view, near-continuous uptime, and large archival dataset, it serves as an ideal observatory with which to search for transient sources of all kinds. We apply these advantages to perform searches for two types of transient sources— Primordial Black Holes (PBHs) and Gamma-Ray Bursts (GRBs). The first of these, a search for emission signatures of evaporating PBHs, is performed on 959 days of HAWC data for remaining lifetimes of 0.2, 1, 10, and 100 s, assuming radiation development according to the Standard Emission Model. Finding no significant emission for any duration, we place upper limits at the 99% confidence level on the local burst rate density.
For the second of these source types, we apply the low-energy improvements recently made to the HAWC data reconstruction procedure to search for very-high-energy emission within the first 0.1, 1, 10, and 100 s of emission for 93 GRBs within HAWC's field of view at their reported T0 over the first 7 years of HAWC operations. Finding no significant emission for any duration, we place upper limits at the 95% confidence level on the intrinsic flux for all GRBs. For those GRBs with external fluence models available from other gamma-ray detectors, we compare the HAWC limits to those models in order to constrain the possible emission in the TeV regime with respect to that at lower energy values.
Finally, we speculate about the future of searches for PBHs and GRBs with the next-generation wide-field-of-view instrument, the Southern Wide-field Gamma-ray Observatory (SWGO), presenting projected performance for these two types of transient sources.
Arthur Lin - October 26, 2023
Dissertation Title: Tight-binding simulations of random alloy and strong spin-orbit effects in InAs/GaBiAs quantum dot molecules
Date and Time: Thursday, October 26, 1:00 pm
Location: Toll 2219
Dissertation Committee Chair: Jay Deep Sau
Committee:
Garnett W. Bryant, Advisor/Co-Chair
Andrew M. Childs
Matthew F. Doty
Theodore L. Einstein
Michael Gullans
Abstract:
Self-assembled InAs quantum dots (QDs), which have long hole-spin coherence times and are amenable to optical control schemes, have long been explored as building blocks for qubit architectures. One such design consists of vertically stacking two QDs to create a quantum dot molecule (QDM). The two dots can be resonantly tuned to form "molecule-like" coupled hole states with the hybridization of hole states otherwise localized in each respective dot. Furthermore, spin-mixing of the hybridized states in dots offset along their stacking direction enables qubit rotation to be driven optically, allowing for an all-optical qubit control scheme. Increasing the magnitude of this spin-mixing is important for optical quantum control protocols. We introduce the incorporation of dilute GaBixAs1–x alloys in the barrier region between the two dots, as GaBiAs is expected to provide an increase in spin-mixing of the molecular states over GaAs.
Using an atomistic tight-binding model, we compute the properties of GaBixAs1–x and the modification of hole states that arise when the alloy is used in the barrier of an InAs QDM. We show that an atomistic treatment is necessary to correctly capture non-traditional alloy effects of GaBiAs. Additionally, an atomistic model allows for the study of configurational variances and clustering effects of the alloy. We find that in InAs QDMs with a GaBiAs inter-dot barrier, hole states are well confined to the dots up to an alloy concentration of 7%. By independently studying the alloy-induced strain and electronic scattering off Bi and As orbitals, we conclude that an initial increase in QDM hole state energy at low Bi concentration is caused by the alloy-induced strain. Additionally, a comparison between the fully alloyed barrier and a partially alloyed barrier shows that fully alloying the barrier applies an asymmetric strain between the top and bottom dot.
By lowering the energetic barrier between the two dots, GaBiAs is able to promote the tunnel coupling of hole states in QDMs. We obtain a three-fold increase of hole tunnel coupling strength in the presence of a 7% alloy. Additionally, we show how an asymmetric strain between the two dots caused by the alloy results in a shift in the field strength needed to bring the dots to resonance. We explore different geometries of QDMs to optimize the tunnel coupling enhancement the alloy can provide, as well as present evidence that the change in tunnel coupling may affect the heavy-hole and light-hole components of the ground state in a QDM.
The strong spin-orbit coupling strength of GaBiAs allows for the enhancement of spin-mixing in QDMs. A strong magnetic field can be applied directly in the TB Hamiltonian. In order the fit the TB results to a simple phenomenological Hamiltonian, we found it necessary to include second order magnetic field terms in the phenomenological Hamiltonian as a diamagnetic correction to the hole state energies. Fitting to the corrected phenomenological model, we obtain a three-fold enhancement for the spin-mixing strength of offset dots at 7% Bi. Additionally, at higher alloy concentrations, a combination of enhanced spin-mixing and increase resonance change in g-factor results in intra-dot spin-mixing between Zeeman split states of the lower energy dot. A perturbative analysis of the magnetic field shows that both the spin-mixing and resonance g-factor change are effects of the Peierls contribution, or the component of the magnetic field applied to the effective spatial angular momentum of the wavefunction. When spin-orbit coupling is removed from the system, there is no longer a preferred alignment between the spin of the system and the Peierls effective angular momentum, thus removing any magnetic field effects of the Peierls contribution.
The analysis of spin-orbit effects can be extended to single dots with in-plane magnetic and electric fields. This thesis concludes with some preliminary results utilizing electric fields, in conjunction with spin-locking effects provided by spin-orbit coupling, to manipulate the spin polarization in single dots. TB calculations with a magnetic field is preformed to show the preferred alignment of the effective angular momentum, given by the geometry of the dot, also spatially locks the spin polarization of hole states. An electric field can then be applied to bias the charge density to either side of the dot, using the spatial texture of the spin to obtain a spin polarized in z while both the magnetic and electric field is in the xy-plane. The same perturbative analysis with the QDMs can be applied to show sufficient spin-orbit coupling is needed to generate such an effect. We propose the utilization of spin texture and electric fields as a novel method for rotating the spin in QDs.
John Silk - July 27, 2023
Dissertation Title: IDENTIFICATION OF TRACE KRYPTON IN THE LUX-ZEPLIN DARK MATTER SEARCH
Date and Time: Thursday, July 27, 2:00 pm
Location: PSC 3150
Dissertation Committee Chair: Professor Carter Hall
Committee:
Professor Xiangdong Ji
Professor Zackaria Chacko
Professor Jason Kahn
Dr. Jon Balajthy
Abstract:
Searches for Weakly Interacting Massive Particles (WIMPs) carried out by liquid xenon time projection chambers (TPCs) require a careful accounting of all background sources. Because WIMPs are a leading dark matter candidate, their possible existence is of great interest to particle physicists, astrophysicists, and cosmologists. The LUX-ZEPLIN (LZ) detector has completed an initial science run finding no evidence for WIMP scattering events. The data excludes scattering cross sections above 6.5x10−48 cm2 for a WIMP mass of 30 GeV/c2. Background contributions from the beta decay of dispersed 85Kr were reduced prior to the initial science run using charcoal chromatography to remove trace krypton. Over 10 tonnes of xenon were processed, and a custom mass spectrometry system measured a final mass averaged krypton concentration of 123 ± 22 parts-per-quadrillion (ppq) gram/gram natKr/natXe . A delayed coincidence β - γ search was also conducted to identify rare decays from 85Kr in the LZ WIMP search data. The 11.0 ± 4.0 identified events are equivalent to a concentration of 183 ± 67 ppq. The total background contribution from 85Kr to the WIMP search region of interest is 30 ± 11 electron recoil events.
Scott Hancock - July 27, 2023
Dissertation Title: Spatiotemporal optical vortices
Date and Time: Thursday, July 27, 9:00 AM
Location: ERF 1207
Dissertation Committee Chair: Howard Milchberg
Committee:
Julius Goldhar
Phillip Sprangle
Eric Rosenthal
Timothy Koeth
Abstract:
Light beams carrying orbital angular momentum (OAM) have become a mainstay of optical science and technology. In these beams, well-known examples of which are the Laguerre-Gaussian and Bessel-Gaussian beams, the OAM vector points parallel or anti-parallel to propagation, and is associated with a phase winding in the plane transverse to the propagation direction, where integer is the winding order or the “topological charge”. Such beams can be monochromatic.
Recently, our group discovered a new type of OAM structure that naturally emerges from nonlinear self-focusing, which we dubbed the spatio-temporal optical vortex (STOV). Here, the phase winding exists in a spatiotemporal plane, with the OAM pointing transverse to propagation. In this dissertation, we extend the generation of STOV-carrying pulses to the linear regime, demonstrating their generation using a pulse shaper and measuring their free-space propagation using a new ultrafast single-shot space- and time-resolving diagnostic, TG-SSSI (transient-grating single-shot supercontinuum spectral interferometry). We then demonstrate that transverse OAM is a property of photons by experimentally confirming the conservation of transverse OAM in second harmonic generation. Because the field of STOVs is so new, a first principles theory for their transverse OAM was lacking. We developed such a theory for transverse OAM that predicts half integer values of OAM and the existence of a STOV polariton in dispersive media. The surprise of half-integer OAM values launched a debate in the OAM community, which has been resolved in favor of our theory by our most recent experiments. These explore how phase and amplitude perturbations can impart spatiotemporal torques to light. We find that transverse OAM can be imparted to light pulses only for (1) sufficiently fast transient phase perturbations or (2) energy removal from a pulse already possessing transverse OAM.
Jaron Shrock - July 26, 2023
Dissertation Title: Multi-GeV Laser Wakefield Acceleration in Optically Generated Plasma Waveguides
Date and Time: Wednesday, July 26, 1:00 pm
Location: IREAP Large Conference Room
Dissertation Committee Chair: Howard Milchberg
Committee:
Phillip Sprangle
Wendell Hill III
Drew Baden
Ki-Yong Kim
Abstract:
Plasma based electron accelerators offer a promising path to overcoming the significant technological and economic challenges facing the evolution to higher energies by radiofrequency (RF) accelerator technology. In particular, laser-driven wakefield acceleration in (LWFA) in plasmas can produce accelerating gradients 1000 times larger than linear RF accelerators, enabling the production of GeV-scale electron bunches in just a few centimeters of acceleration. Efficient LWFA of electrons to this energy scale requires the use of optical guiding to maintain drive laser intensity over much longer distances than the characteristic diffraction length of the pulse.
In this dissertation, I will present the first successful implementations of optically generated plasma waveguides in multi-GeV laser wakefield acceleration. I will focus on three primary topics: (1) experimental considerations for generating and diagnosing meter-scale plasma waveguides and the wakefield acceleration process, (2) the experimental demonstration of electron bunches accelerated up to 5 GeV in an all-optical LWFA, and (3) development of a model of drive pulse evolution and electron injection in agreement with a broad range of our experimental results, including the demonstration of localized electron injection through modification of the waveguide properties.
Pradeep Niroula - July 25, 2023
Dissertation Title: Phase Transitions in Random Quantum Circuits
Date and Time: Tuesday, July 25, 10:00 am
Location: Atlantic 3100A and Zoom
Dissertation Committee Chair: Maissam Barkeshli
Committee:
Michael Gullans (co-chair, co-advisor)
Alexey Gorshkov (co-chair, co-advisor)
Christopher Monroe
Christopher Jarzynski (Dean’s Representative)
Abstract:
Random Circuits has emerged as an invaluable tool in the quantum computing toolkit. On one hand, the task of sampling outputs from a random circuit has established itself as a leading contender to experimentally demonstrate the intrinsic superiority of quantum computers using near-term, noisy platforms. On the other hand, random circuits have also been used to deduce far-reaching conclusions about the theoretical foundations of quantum information and communication.
One intriguing aspect of random circuits is exemplified by the measurement-induced entanglement phase transition that occurs in monitored quantum circuits, where unitary gates compete with projective measurements to determine the entanglement structure of the resulting quantum state. When the measurements are sparse, the circuit is unaffected and entanglement grows ballistically; when the measurements are too frequent, the unitary dynamics is arrested or frozen. The two phases are separated by a sharp-phase transition. In this work, we discuss an experiment probing such phases using a trapped-ion quantum computer.
While entanglement is an important resource in quantum communication, it does not capture the non-classicality needed to achieve universal quantum computation. A new family of measures, termed magic, is used to quantify the extent to which a quantum state is non-classical or can enable universal quantum computation. In this dissertation, we also discuss a newly uncovered phase transition in magic, using quantum circuits that implement a random stabilizer code. This `magic' phase transition is intimately related to the error-correction threshold. In this work, we present numerical and analytic characterizations of the magic transition.
Finally, we use a statistical mechanical map from random circuits acting on qubits to Ising models to suggest thresholds in error mitigation whenever the underlying noise of a quantum device is imperfectly characterized. We show the existence of an error-mitigation threshold for random circuits in dimensions larger than one.
Yizhou Huang - July 25, 2023
Dissertation Title: Improvements and Studies of Planar Transmon Qubits
Date and Time: Tuesday, July 25, 10:30am
Location: PSC 2136
Dissertation Committee Chair: Professor Christopher J. Lobb
Committee:
Dr. Benjamin S. Palmer
Professor Frederick C. Wellstood
Professor Steven Anlage
Professor Thomas Murphy, Dean’s Representative
Abstract:
This dissertation describes three main projects focused on characterizing and improving superconducting transmon qubits operating nominally at temperatures of 20 mK. The first topic I discuss is characterization of ground state fidelity of a passively cooled 3D transmon qubit using two techniques. The first technique was counting the number of false counts when performing single-shot read-out measurements of the weak resonator signal using a nearly quantum limited traveling wave parametric amplifier. Over about a million shots, only 772 counts were found with the system in the excited state, corresponding to a residual excited state population of Pe = 0.083%. The second technique used was performing detailed Rabi oscillations between the first and second excited state levels of the 3D transmon qubit. By fitting the measured data, the residual excited state population was shown to be Pe = 0.088% +- 0.018%. These state of the art low values for the infidelity of the ground state suggest that the effective temperature of the transmon qubit with fundamental transition frequency of 3.6 GHz is T < 25 mK.
The second topic I discuss is improvements in the coherence of our planar transmon qubits such that I was able to measure energy relaxation times and coherence times up to tens of microseconds. There were two main improvements that I achieved during the course of this research. First, I identified a significant loss mechanism in the way that the planar transmon qubits were packaged. This was identified by measuring the internal quality factors (Qi) for a series of thin-film Al quarter-wave resonators with fundamental resonant frequencies varying between 4.9 and 5.8 GHz. By utilizing resonators with different widths and gaps, I sampled different electromagnetic energy volumes for the resonators affecting Qi. When the backside of the sapphire substrate of the resonator device was adhered to a Cu package with a conducting silver glue, a monotonic decrease in the maximum achievable Qi was found as the electromagnetic sampling volume is increased. My simulations and modeling show that the observed dissipation is a result of induced currents in large surface resistance regions underneath the substrate. By placing a hole underneath the substrate and using superconducting material for the package, I was able to decrease the Ohmic losses and increase the maximum Qi by an order of magnitude for the larger size resonators. The second improvement I made to achieve improvements to our planar transmon qubits was developing a new fabrication process to improve the quality of the interface between the substrate and the superconducting shunting capacitor of the transmon. For this new process, the large features (> 1 um) of the thin superconducting film are subtractively defined by etching the film. The small Al/AlOx/Al junction is added in a second step by defining the junction in electron-beam lithography, an ion mill step, and standard double-angle evaporation.
Finally, the last studies I discuss are studies of the recovery time after injecting quasipacticles in several transmon qubits. For these studies, quasiparticles across the transmon junction were created by applying a large-amplitude microwave pulse resonant with the readout resonator. Immediately after generating the quasiparticles, a significant decrease of the energy relaxation time and dephasing time of the transmon qubit is measured. By performing T1 and T2 measurements over the course of the several milliseconds I tracked the recovery T1 and T2, which was used as a metric of the quasiparticle density at the junction. By fitting the recovery data with a numerical model involving differential equations, I extracted the quasiparticle trapping rates around 1 ms^-1 and recombination rates around 1 / 25 ns^-1 at the sites of a few transmon qubits. I measured transmons that were either galvanically connected or isolated, fabricated with superconducting aluminum (gap 200uV) or tantalum (gap 600uV) for the shunting capacitor. Finally, with a larger quasiparticle injection power and by measuring two transmons on the same chip, I observed a phenomenon that's consistent with “phonon-assisted quasiparticle poisoning”. I quantitatively modelled such data, in addition to discussing how this presents challenges to further improving the coherence times of transmon qubits.
Sohitri Ghosh - July 13, 2023
Dissertation Title: Quantum Enhanced Impulse Measurements and Their Applications in Searches for Dark Matter
Date and Time: Thursday, July 13, 10:15 am
Location: PSC 3150
Dissertation Committee Chair: Peter Shawhan
Committee:
Jacob M. Taylor, Co-Chair
Daniel Carney
Ronald L. Walsworth
Christopher J. Lobb
Abstract:
Optomechanical systems have enabled a variety of novel sensors that transduce an external force on a mechanical sensor to an optical signal which can be read out through different measurement techniques. Based on recent advances in these sensing technologies, we suggest that heavy dark matter candidates around the Planck mass range could be detected solely through their gravitational interaction. After our understanding of the possibility of direct gravitational detection of dark matter, a coalition of researchers came together to form a lasting collaboration — Windchime — to explore the potential implementation and impact of this approach. With this ultimate goal in mind, the Windchime collaboration is developing the necessary techniques, systems, and experimental apparatus using arrays of optomechanical sensors that operate in the regime of high-bandwidth force detection, i.e., impulse metrology. One of the key challenges in achieving more sensitive measurements is to mitigate the noise which arises due to the fundamental uncertainty principle while trying to precisely measure the variable of interest. Today’s state-of-the-art sensors can be limited by this added noise due to the act of measurement itself. One of the techniques to go beyond this limit include squeezing of the light used for measurement. The other technique is using backaction evading measurements by estimating quantum non-demolition operators — typically the momentum of a mechanical resonator well above its resonance frequency.
In our work, we have explored various backaction evading techniques based on this principle. In the first part of the thesis, we present the impulse metrology task in the context of gravitational detection of dark matter. In the next part, we present a practical way to achieve backaction evading measurements in the optical domain. In the subsequent part, we analyze the theoretical limits to noise reduction while combining different quantum enhanced readout techniques for these mechanical sensors. In the last part of the thesis, we explore the possibility of a microwave domain readout for maximizing the energy efficiency and noise reduction while having a scalable system for our dark matter detection purpose.
Saurav Das - July 7, 2023
Dissertation Title: Exploring Beyond Standard Model Physics with Cosmological and Terrestrial Probes
Date and Time: Friday, July 7, 12:00 pm
Location: PSC 1136
Dissertation Committee Chair: Prof. Anson Hook
Committee:
Prof. Kaustubh Agashe
Prof. Zackaria Chacko
Prof. Kara Hoffman
Prof. Richard Wentworth
Abstract:
The standard model for particle physics has been extremely successful as a description of nature. Despite this success, there remain many unsolved puzzles both observationally and theoretically. In this thesis we explore a few ideas in search of beyond the standard model physics, especially we focus on the Higgs mass, magnetic monopole and vector dark matter.
In the first part of the thesis, we show that the Goldstone bosons of discrete symmetry can be parametrically lighter than otherwise expected. While non-linear realizations of continuous symmetries feature derivative interactions and have no potential, non-linear realizations of discrete symmetries feature non-derivative interactions and have a highly suppressed potential. These Goldstone bosons of discrete symmetries have a non-zero potential, but the potential generated from quantum corrections is inherently very highly suppressed. We explore various discrete symmetries and to what extent the potential is suppressed for each of them.
In the second part, we showed that in the early universe, evaporating black holes heat up the surrounding plasma and create a temperature profile around the black hole that can be more important than the black hole itself. As an example, we demonstrate how the hot plasma surrounding evaporating black holes can efficiently produce monopoles via the Kibble-Zurek mechanism. In the case where black holes reheat the universe, reheat temperatures above $\sim 500$ GeV can already lead to monopoles overclosing the universe.
In the last part of the thesis, we showed that vector Dark Matter (VDM) that couples to lepton flavor ($L_e$, $L_{\mu}$, $L_{\tau}$) acts similarly to a chemical potential for the neutrino flavor eigenstates and modifies neutrino oscillations. VDM imparts unique signatures such as time and directional dependence with longer baselines giving better sensitivity. We use the non-observation of such a signal at Super-Kamiokande to rule out the existence of VDM in a region of parameter space several orders of magnitude beyond other constraints and show the projected reach of future experiments such as DUNE.
Arushi Bodas - July 5, 2023
Dissertation Title: Hunting inflationary fossils in primordial inhomogeneities
Date and Time: Wednesday, July 5, 2:00 pm
Location: PSC 3150 and Zoom
Dissertation Committee Chair: Professor Raman Sundrum (advisor)
Committee:
Professor Zackaria Chacko
Professor Anson Hook
Professor Peter Shawhan
Professor Richard Wentworth (Dean’s representative)
Abstract:
Cosmological observables such as the Cosmic Microwave Background (CMB) allow us to probe the early universe at extremely high energies far beyond the reach of any particle collider on Earth. In the inflationary paradigm, small perturbations in the energy distribution across space can be directly linked to the quantum fluctuations of an ''inflaton'' field that drives inflation. Using these perturbations, it is therefore possible to learn about physics at energies as high as 1013 GeV. In this thesis, we exploit this powerful connection and explore novel mechanisms to hunt for previously unexplored inflationary dynamics.
During inflation, particles with masses larger than the inflationary Hubble scale Hinf < 1013 GeV are produced due to an accelerating spacetime. If coupled to the inflaton, these particles could imprint distinct oscillatory features in higher moments of the density perturbations. In Chapter 2, we explore a mechanism that can boost spin-0 particle production by mining the kinetic energy of the inflaton. This leads to an enhancement of the oscillatory features, which can extend the observability of heavy particles by two orders of magnitude beyond Hinf.
In the final part of the thesis, spanning Chapters 3 and 4, we explore the viability of gravitational wave backgrounds (GWB) as data sources for new inflationary physics. It was recently shown that a GWB from a first-order phase transition must exhibit fluctuations, much like the CMB. Despite the close analogy, it is possible for fluctuations of the GWB to differ significantly in their detailed pattern from those of the CMB, which would imply the existence of a second light field during inflation in addition to the inflaton. Such a GWB could thus unlock a wealth of new information about multi-field inflation. In Chapter 3, we elaborate on this point with an example. We show that there may exist signals that cannot be extracted using standard cosmological probes such as the CMB and galaxy surveys, but can in principle be detected within GWB with upcoming and proposed gravitational wave experiments. Lastly, in Chapter 4, we focus on the detectability of GWB itself. We discuss a cosmological mechanism that can enhance the strength of the gravitational wave signal from phase transitions, thereby increasing their detection prospects significantly.
Ron Belyansky - June 26, 2023
Dissertation Title: Quantum Simulation and Dynamics with Synthetic Quantum Matter
Date and Time: Monday, June 26, 2:30 pm
Location: ATL 3100A
Dissertation Committee Chair: Professor Zohreh Davoudi
Committee:
Professor Alexey Gorshkov, Advisor, Co-chair
Professor Andrew Childs, Dean’s Representative
Professor Alicia Kollar
Professor Nathan Schine
Abstract:
Significant advancements in controlling and manipulating individual quantum degrees of freedom have paved the way for the development of programmable strongly-interacting quantum many-body systems. Quantum simulation emerges as one of the most promising applications of these systems, offering insights into complex natural phenomena that would otherwise be difficult to explore. Motivated by these advancements, this dissertation delves into several analog quantum simulation proposals spanning different fields, including high-energy and condensed matter physics, employing various synthetic quantum systems. A primary objective is the theoretical investigation of the dynamical phenomena that can be effectively studied using these simulation approaches.
The first part of the dissertation focuses on quantum simulation utilizing superconducting circuits. We demonstrate that this platform can natively realize several intriguing models including the massive Schwinger model (quantum electrodynamics (QED) in 1+1 dimensions) and various strongly interacting quantum impurity models. By studying high-energy scattering of quark and meson states within the Schwinger model, we reveal a wealth of rich phenomenology encompassing inelastic particle production, hadron disintegration as well as dynamical string formation and breaking. Furthermore, we demonstrate how the presence of a single impurity (artificial atom) can profoundly modify the properties of light-matter interactions in a waveguide, leading to anomalous transport of a single photon, strong photon decay, and the emergence of atom-photon bound states.
The second part of the dissertation focuses on quantum simulation with atomic, molecular,
and optical (AMO) systems. Leveraging the tunable and long-range interactions available in platforms such as cavity-QED and trapped ions, we explore exotic regimes of quantum information dynamics. On the one hand, we demonstrate that the combination of simple and uniform all-to-all interactions together with chaotic short-range interactions can induce fast scrambling, a central feature associated with quantum black holes. On the other hand, we investigate how short-range yet non-local Rydberg interactions can strongly suppress atom tunneling in an optical lattice, resulting in frozen dynamics and Hilbert-space fragmentation. Finally, we propose a method of sympathetic cooling of neutral atoms using state-insensitive Rydberg interactions, potentially enabling longer quantum simulations and computations with this platform.
Wen-Chen Lin - June 12, 2023
Dissertation Title: High Magnetic Field Studies of Unconventional Superconductor UTe2: Unveiling the Phase Diagram and Unconventional Hall Effect
Date and Time: Monday, June 12, 2:00 pm
Location: Toll 2219
Dissertation Committee Chair: Prof. Johnpierre Paglione
Committee:
Prof. Nicholas Butch
Prof. Steven Anlage
Prof. Min Ouyang
Prof. Efrain Rodriguez
Abstract:
The recent discovery of superconductivity in UTe2 has drawn strong attention owing to a fascinating list of properties -- including the absence of magnetic order at ambient pressure, extremely high upper critical fields – that have led to proposals of spin-triplet pairing, and multiple superconducting phases.
The first part of this dissertation unveils the comprehensive (H, T, P) phase diagram of UTe2 under magnetic fields reaching 41 T along the crystallographic b-axis, combined with applied pressures of up to 18 kbar. Utilizing magnetoresistance and tunnel diode oscillator measurements, we investigated the pressure-induced evolutions of multiple phases. Our findings indicate that the superconductivity in UTe2 is encapsulated within two orthogonal magnetic phases, specifically, a field-induced polarized phase and a pressure-induced magnetic ground state. This establishes the boundaries of triplet superconductivity in this system.
In the latter part of the thesis, we delve into an unconventional Hall effect in UTe2 that is induced by an in-plane magnetic field. An extensive study was conducted on the field and angle dependence of the Hall response under in-plane fields, confirming its field-odd symmetry nature, which aligns with the recently identified in-plane Hall effect. We examined the field dependence of the Hall response up to 33 T, and observed a remarkable enhancement, indicating a significant linkage to the well-identified metamagnetic transition. In addition, we will discuss the symmetry restrictions on the in-plane Hall response, and evidence of symmetry lowering in our system.
Joseph Murray - June 6, 2023
Dissertation Title: Low Temperature Scanning Tunneling Microscopy of Topological Materials and Magnetic Structures
Date and Time: Tuesday, June 6, 10:30 am
Location: PSC 2148
Dissertation Committee Chair: Prof. Chris Lobb
Committee:
Dr. Robert Butera
Dr. Michael Dreyer
Prof. Johnpierre Paglione
Prof. John Cumings (Dean’s representative)
Abstract:
Scanning tunneling microscopy (STM) provides an opportunity to study the physical and electromagnetic properties of surfaces at the atomic scale. When performed at low temperatures, in high magnetic fields, and with a variety of different probes, it offers a wide range of methods by which novel materials of great practical and theoretical interest can be evaluated, characterized, and even fabricated with atomic precision.
This thesis describes three independent STM studies performed at cryogenic temperatures. In the first, we examined oxygen-doped aluminum films with anomalously high kinetic inductance. A suggested explanation was the migration of oxygen to the grain boundaries, forming a percolation network separated by Josephson links. To determine the coupling between grains, the films were studied using milliKelvin STM performed with a superconducting tip.
In the second study, transport measurements indicated the possible presence of a topological Hall effect in thin films of Cr2Te3, induced by the presence of topologically non-trivial magnetic textures called magnetic skyrmions. In order to provide more decisive evidence, we studied the films using spin-polarized STM at 4K.
Finally, we present an in-situ modification to our 4K STM which permits us to current-bias our samples during STM operation. The modification can be used to study non-equilibrium effects such as spin accumulation induced by a current through a spin Hall material and the spin-momentum locking which is present at the surface of topological insulators.
Tamoghna Barik - May 25, 2023
Dissertation Title: Tunneling properties in Fe-based superconductor, FeTe.55Se.45 (FTS)
Date and Time: Thursday, May 25, 10:30 AM
Location: ATL 4402
Dissertation Committee Chair: Prof. Jay D Sau
Committee:
Prof. Sankar Das Sarma
Prof. Victor Yakovenko
Prof. Maissam Barkeshli
Prof. Ichiro Takeuchi (Dean’s representative)
Abstract:
The Fe-based chalcogenide compound, FeTe1-xSex (FTS), has recently attracted attention as a potential candidate for a readily available platform for topological superconducting (TSC) phase on its surface. The co-existence of the strong topological insulating (TI) phase and cylindrical Fermi sheets provide the two necessary ingredients for the TSC phase on its surface.
The strong TI phase in FTS depends on the relative Te/Se composition. The widely studied FTS sample, FeTe.55Se.45, interestingly, lies close to the topological-trivial phase boundary which is estimated to be close to x = .5 by a recent experiment. Additionally, a typical sample of FTS suffers from fluctuations in Te/Se composition at multiple length scales due to its alloy nature. Thus, in a topological FTS sample such fluctuations can give rise to regions where the phase is driven out of its topological nature. Such trivial domains would be scattered throughout the sample. After carefully exploring the effects of such topological domain disordered phase in an effective model of FTS, we conclude that its resultant signatures can be distinguished in a scanning tunneling spectroscopy (STS) measurement of the topological surface states (TSS) density.
Another possibility of an exotic phase arises on the TSC surface when a linear defect forms a Josephson junction (JJ) characterized by a phase shift of 𝜋 for the SC order parameter (OP). Such a 𝜋-JJ induces exotic helical Majorana modes - signatures of which has been observed by a recent tunneling experiment on the surface of FTS in the density of states (DOS) within the SC gap at a crystalline domain wall (DW) associated with half-unit-cell-shift (HUCS). Observation of such signatures naturally poses question about the origin of such 𝜋-JJ which is yet to be fully understood. We propose a mechanism that stabilizes a 𝜋-junction at the HUCS DW when the intrinsic superconducting pairing is of s± character which is the case for bulk FTS. We argue that if the DW induced inter-pocket transmission between the Γ and M pockets in FTS is strong enough, the resultant enhancement of the two OPs of opposite sign stabilizes the 𝜋-junction.
Naren Manjunath - May 24, 2023
Dissertation Title: Classification and characterization of crystalline topological invariants in quantum many-body states
Date and Time: Wednesday, May 24, 10:30 am
Location: ATL 4402 (CMTC conference room)
Dissertation Committee Chair: Professor Maissam Barkeshli (advisor)
Committee:
Professor Jay Deep Sau
Professor Sankar Das Sarma
Professor Johnpierre Paglione
Professor Mohammad Hafezi (Dean’s Representative)
Abstract:
The theory of topological phases of matter, now a major direction in condensed matter physics, is framed around two complementary problems. The first is to mathematically classify topological states of matter given various symmetries, in terms of suitable topological invariants. The second is to characterize a given state by numerically or experimentally extracting the topological invariants associated with it. Although remarkable progress has been made for topological states with internal symmetries, major open questions remain in the case of many-body systems with crystalline symmetries, especially those with a nonzero Chern number and a magnetic field.
The first part of this thesis develops a theory of crystalline topological response in (2+1) dimensions based on the idea of crystalline gauge fields and their effective actions, which we derive using topological quantum field theory. We use this to obtain a complete classification of topological states with U(1) charge conservation, discrete magnetic translation, and point group rotation symmetries, finding several new invariants. We separately consider symmetry-enriched topological states of bosons, which admit anyonic excitations with fractional statistics, and invertible fermionic states, which do not.
The second part of this thesis focuses on numerically extracting these invariants from many-body invertible states. First, we study two quantized invariants, the discrete shift, and a charge polarization which is quantized by rotational symmetries. We show how to extract these invariants in multiple different ways, which include the fractional charge bound to lattice defects, as well as the angular and linear momentum of magnetic flux. Thereafter, we obtain a complete characterization of the theoretically predicted invariants, by studying the expectation value of the ground state under partial rotation operators.
An immediate application of these ideas is to fully characterize the celebrated Hofstadter model of spinless free fermions on a square lattice. Although the Chern number and filling were first computed in this model in 1982, our theory predicts seven nontrivial invariants, including four new invariants which depend on the crystalline symmetry. We compute these numerically and obtain several additional colorings of Hofstadter's butterfly.
Saurabh Vasant Kadam - May 23, 2023
Dissertation Title: Theoretical developments in lattice gauge theory for applications in double-beta decay processes and quantum simulation
Date and Time: Tuesday, May 23, 12:00 pm
Location: PSC 2136
Dissertation Committee Chair: Zohreh Davoudi
Committee:
Paulo Bedaque
Zackaria Chacko
Manuel Franco Sevilla
Konstantina Trivisa, Dean’s Representative
Abstract:
Nuclear processes have played, and continue to play, a crucial role in unraveling the fundamental laws of nature. They are governed by the interactions between hadrons, and in order to draw reliable conclusions from their observations, it is necessary to have accurate theoretical predictions of hadronic systems. The strong interactions between hadrons are described by quantum chromodynamics (QCD), a non-Abelian gauge theory with symmetry group SU(3). QCD predictions require non-perturbative methods for calculating observables, and as of now, lattice QCD (LQCD) is the only reliable and systematically improvable first-principles technique for obtaining quantitative results. LQCD numerically evaluates QCD by formulating it on a Euclidean space-time grid with a finite volume, and requires formal prescriptions to match numerical results with physical observables.
This thesis provides such prescriptions for a class of rare nuclear processes called double beta decays, using the finite volume effects in LQCD framework. Double beta decay can occur via two different modes: two-neutrino double beta decay or neutrinoless double beta decay. The former is a rare Standard Model transition that has been observed, while the latter is a hypothetical process whose observation can profoundly impact our understating of Particle Physics. The significance and challenges associated with accurately predicting decay rates for both modes are emphasized in this thesis, and matching relations are provided to obtain the decay rate in the two-nucleon sector. These relations map the hadronic decay amplitudes to quantities that are accessible via LQCD calculations, namely the nuclear matrix elements and two-nucleon energy spectra in a finite volume. Finally, the matching relations are employed to examine the impact of uncertainties in the future LQCD calculations. In particular, the precision of LQCD results that allow constraining the low energy constants that parameterize the hadronic amplitudes of two-nucleon double beta decays is determined.
Lattice QCD, albeit being a very successful framework, has several limitations when general finite-density and real-time quantities are concerned. Hamiltonian simulation of QCD is another non-perturbative method of solving QCD that, by its nature, does not suffer from those limitations. With the advent of novel computational tools, like tensor network methods and quantum simulation, Hamiltonian simulation of lattice gauge theories (LGTs) has become a reality. However, different Hamiltonian formulations of the same LGT can lead to different computational-resource requirements with their respective system sizes. Thus, a search for efficient formulations of Hamiltonian LGT is a necessary step towards employing this method to calculate a range of QCD observables. Toward that goal, a loop-string-hadron (LSH) formulation of an SU(3) LGT coupled to dynamical matter in 1+1 dimensions is developed in this thesis. Development of this framework is motivated by recent studies of the LSH formulation of an SU(2) LGT that is shown to be advantageous over other formulations, and can be extended to higher-dimensional theories and ultimately QCD.
En-Jui Kuo - May 16, 2023
Dissertation Title: Quantum Simulation of Bosonic Systems and Applications of Machine Learning
Date and Time: Tuesday, May 16, 11:00 am
Location: PSC 3150
Dissertation Committee Chair: Mohammad Hafezi
Committee:
Alexey Gorshkov
Maissam Barkeshli
Victor Albert
Amin Gholampour
Abstract:
First, we introduce the notion of "generalized bosons," whose exchange statistics resemble those of bosons, but the local bosonic commutator $[a_i,a_i^{\dagger}]=1$ is replaced by an arbitrary single-mode operator that is diagonal in the generalized Fock basis. Examples of generalized bosons include boson pairs and spins. We consider the analogue of the boson sampling task for these particles and observe that its output probabilities are still given by permanents, so the results regarding the difficulty of sampling carry over directly. Finally, we propose implementations of generalized boson sampling in circuit-QED and ion-trap platforms.
In the rest of the thesis, we move on to different topics. Firstly, we incorporate machine learning techniques in quantum information. We use machine learning to classify rational two-dimensional conformal field theories (CFTs). We first use the energy spectra of these minimal models to train a supervised learning algorithm. In contrast to conventional methods that are typically qualitative and involve system size scaling, our method quantifies the similarity of the spectrum of a system at a fixed size to candidate CFTs. Such an approach allows us to correctly predict the nature and value of critical points of several strongly correlated spin models using only their energy spectra. Our results are also relevant for the ground-state entanglement Hamiltonian of certain topological phases of matter described by CFTs. Remarkably, we achieve high prediction accuracy by only using the lowest few Rényi entropies as the input. Finally, using autoencoders, an unsupervised learning algorithm, we find a hidden variable that has a direct correlation with the central charge and discuss prospects for using machine learning to investigate other conformal field theories, including higher-dimensional ones.
Next, we demonstrate how machine learning techniques, especially unsupervised learning algorithms, can be used to study Symmetry-Protected Topological (SPT) phases of matter. SPT phases are short-range entangled phases of matter with a non-local order parameter that are preserved under a local symmetry group. Here, we use an unsupervised learning algorithm, namely diffusion maps, to differentiate between symmetry-broken phases and topologically ordered phases and between non-trivial topological phases in different classes. Specifically, we show that phase transitions associated with these phases can be detected in various bosonic and fermionic models in one dimension, including the interacting SSH model, the AKLT model and its variants, and weakly interacting fermionic models. Our approach provides a cost-effective computational method for detecting topological phase transitions associated with SPT systems, which can also be applied to experimental data obtained from quantum simulators.
Batoul Banihashemi - May 16, 2023
Dissertation Title: Thermodynamics of quantum gravitational ensembles
Date and Time: Tuesday, May 16, 10:30 am
Location: PSC 1136
Dissertation Committee Chair: Theodore Jacobson
Committee:
Alessandra Buonanno
Christopher Jarzynski, Dean’s Representative
Raman Sundrum
Brian Swingle
Abstract:
The discovery of black hole thermodynamics and its extension to cosmological horizons demonstrated a deep connection between thermodynamics and the nature of spacetime as a quantum system. It is then of great importance to properly understand the statistical mechanics of gravitational systems with horizon from first principles. While employing a partition function and the gravitational “Euclidean path integral” produces the expected physical result for entropy, a number of fundamental questions about the underlying analysis persist. This dissertation sharpens and resolves some puzzles regarding statistical mechanics of gravitational ensembles and the gravitational path integral, with a focus on cosmological horizon and de Sitter space.
The main questions addressed in this dissertation are: how is the entropy of de Sitter space derived in absence of any boundary on which the statistical ensemble can be properly defined? What is the correct interpretation of the first law of de Sitter horizon, according to which the horizon area shrinks upon adding matter in de Sitter static patch? And finally, how can entropy of horizon-bounded systems be derived from a Hamiltonian approach and phase space path integral, without the trickery of the gravitational Euclidean path integral? The first two questions are answered by introducing an artificial boundary in the system on which a gravitational ensemble can be properly defined. Once the ensemble is defined, the semiclassical approximation of the statistical partition function yields the entropy, and the interpretation of the de Sitter first law becomes clear by identifying the system energy as the quasilocal energy defined on the boundary. To tackle the last question, the real-time phase space path integral is utilised in the Hamiltonian formulation which maintains connection to the Hilbert space of the system, and it is found that the horizon entropy is derived from a nearly Lorentzian stationary point.
Gong Cheng - May 16, 2023
Dissertation Title: Quantum information scrambling and protection in many-body systems
Date and Time: Tuesday, May 16, 2:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Maissam Barkeshli
Committee:
Prof. Brian Swingle (Co-Chair)
Prof. Paulo Bedaque
Prof. Victor Albert
Prof. Xiaodi Wu (Dean’s representative)
Abstract:
This work focuses on two topics in quantum information theory: the scrambling of quantum information and the preservation of quantum information in large degrees of freedom. The primary object I investigate in this topic is the Out-of-time-order correlator (OTOC), which probes the dynamics of quantum information as it spreads from localized degrees of freedom to those that are distributed throughout the system. Meanwhile, the goal of studying quantum information protection is to construct a system that can preserve quantum information for a sufficiently long time when coupled to a finite-temperature environment.
The many-body systems analyzed in this work belong or are related to a class of strongly interacting systems known as holographic quantum models. The standard examples in this class are believed to be equivalent to gravitational theory in spacetime that is one-dimensional higher than that the quantum model lives in. Therefore, the results may also provide insights into topics in quantum gravity.
The first part of the thesis explores the scrambling dynamics close to a critical point where conformal symmetry emerges. The second case deals with the scrambling dynamics with conservation law constraints in holographic quantum field theory. The result also clarifies how conserved charges influence the dynamics in the bulk dual.
The third part of the thesis presents a matrix model with a large matrix rank N that belongs to the class of approximate quantum error correction codes. We investigate its thermal stability by coupling it to a thermal bath and demonstrate that it behaves as a self-correcting quantum memory at finite temperature. The coherent memory time scales polynomially with the system size N.
Troy Sewell - May 15, 2023
Dissertation Title: Variational Algorithms and Resources for Near-Term Quantum Simulation
Date and Time: Monday, May 15, 3:00 pm
Location: PSC 3204
Dissertation Committee Chair: Jay Sau
Committee:
Stephen Jordan, Advisor / Co-chair
Brian Swingle
Maissam Barkeshli
Andrew Childs, Dean’s Representative
Abstract:
The difficulty of efficiently simulating quantum many-body systems was one of the first motivations for developing quantum computers and may also be one of the first applications to find practical computational advantage on real quantum hardware. With the relatively recent advent of publicly available quantum technologies, we have now entered the era of noisy intermediate-scale quantum (NISQ) computing. The capabilities of these technologies are evolving rapidly, and with them the computational affordances to which we have access. The time is now ripe to test the capabilities of existing quantum hardware and leverage them to the best of our ability toward achieving a practical quantum computational advantage.
In this dissertation, we address these aims by benchmarking a class of variational multi-scale quantum circuits for state preparation which are locally robust against noise. We demonstrate the advantages of these multi-scale circuits compared to a purely local circuit ansatz using the critical transverse-field Ising model to optimize circuit parameters, numerically test the noise resilience of observables and customized error mitigation techniques using a local gate noise model, and demonstrate the robustness of local subregion preparation on an existing ion-trap quantum computer. We then show how the ground state optimized circuit can be simply extended to an ansatz for thermal state preparation using the separation of energy scales afforded by the multi-scale circuit structure.
Additionally, we evaluate the quantum resources needed for some quantum simulation tasks. We estimate the gate complexity of the site-by-site algorithm for fault-tolerant ground state preparation, which we extend to the case of degenerate Hamiltonians. Using matrix product states we evaluate the non-stabilizer quantum resources needed to represent thermalized subregions of a chaotic Potts model, which we use to address the feasibility for classical simulation of quantum hydrodynamics.
Stefano Antonini - May 15, 2023
Dissertation Title: Holographic Cosmological Models and the AdS/CFT Correspondence
Date and Time: Monday, May 15, 10:00 AM
Location: PSC 3150
Dissertation Committee Chair: Theodore Jacobson
Committee:
Zackaria Chacko
Christopher Jarzynski (Dean’s Representative)
Raman Sundrum
Brian Swingle (Academic Advisor)
Abstract:
The formulation of a quantum theory of gravity is a central open problem in theoretical physics. In recent years, the development of holography---and in particular the Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence---provided a new framework to investigate quantum gravity and led to consistent advancement. However, how to describe cosmology within holography remains an unanswered question whose solution could determine whether holography is able to capture physics in our universe.
This dissertation describes a new proposal for embedding cosmological physics in the holographic paradigm. This is articulated in two different but related approaches, both involving time-symmetric Big Bang-Big Crunch cosmologies with negative cosmological constant Λ .
In the first approach, the cosmological universe is given by a four-dimensional end-of-the-world brane moving in a five-dimensional AdS black hole spacetime. The proposed holographic dual description is given by a boundary conformal field theory. Under specific conditions, gravity is localized on the brane and effectively four-dimensional: an observer living on the brane is unaware of the existence of the extra dimension. In this dissertation, I show how these conditions can be met in an AdS-Reissner-Nordström background while retaining a holographic dual description.
The second approach focuses on spatially flat Λ<0 cosmologies which analytically continue to Euclidean wormholes connecting two asymptotic AdS boundaries. The proposed dual theory is given by two holographic 3D CFTs coupled by non-holographic 4D degrees of freedom on a strip. A different analytic continuation of the Euclidean wormhole leads to a Lorentzian traversable wormhole. After discussing the general features of these holographic cosmologies, I describe how the traversable wormhole can be reconstructed from the dual theory and how the existence of the former constrains the latter. Finally, I show that these Λ<0 cosmologies can undergo phases of accelerated expansion and match observational data for the scale factor evolution.
Sanket Doshi - May 10, 2023
Dissertation Title: Dark Matter and Neutrino Masses from a Composite Hidden Sector
Date and Time: Wednesday, May 10, 1:30pm
Location: PSC 2204
Dissertation Committee Chair: Dr. Zackaria Chacko
Committee:
Dr. Raman Sundrum
Dr. Kaustubh Agashe
Dr. Massimo Ricotti (Dean’s Representative)
Dr. Christopher Palmer
Abstract:
Despite the remarkable success of the Standard Model in explaining the interactions of the elementary particles, there is now indisputable evidence that it is incomplete. Although the Standard Model predicts that neutrinos are massless, over the last few decades experiments have established that the masses of the neutrinos, although very small, are nonvanishing. Furthermore, cosmological and astrophysical observations have established that about 80% of the matter in the universe is composed of some form of non-luminous dark matter, but there is no particle in the Standard Model that can play this role. Any explanation of the origin of neutrino masses and the nature of dark matter therefore requires physics beyond the SM. In this thesis, we present a novel class of models that can explain both the origin of neutrino masses and the observed abundance of dark matter.
In these models, the particle that constitutes dark matter arises as the composite state of a strongly coupled hidden sector that couples to the Standard Model through the neutrino portal. A discrete symmetry ensures that the dark matter particle is stable and does not decay. The hidden sector is in thermal equilibrium with the Standard Model in the early universe. The abundance of dark matter is set by its annihilation into final states containing neutrinos. The neutrino portal coupling also gives rise to small Majorana masses for the neutrinos through the inverse seesaw mechanism, with the role of the singlet neutrinos being played by composite states. The Standard Model neutrinos mix with the singlet neutrinos, and so the Standard Model neutrinos are partially composite in this framework. The dynamics of the hidden sector is taken to be approximately conformal in the ultraviolet, and a relevant deformation leads to breaking of the conformal symmetry in the infrared. Since the hidden sector is uncharged under the Standard Model gauge groups, the compositeness scale can lie below the weak scale, leading to striking experimental signals.
We employ the AdS/CFT correspondence to construct a holographic dual of this scenario. This takes the form of a Randall Sundrum model with two branes. Within this framework we explore the signals of these models at various current and future experiments. These include searches for lepton flavor violation in and conversion experiments, direct and indirect detection of dark matter and searches at colliders and beam dumps. We determine the current bounds on this scenario and show that future experiments can significantly expand the reach.
John Collini - April 28, 2023
Dissertation Title: HIGH PRESSURE DRIVEN EVOLUTION OF CHARGE AND
STRUCTURAL ORDER IN NEMATIC SUPERCONDUCTOR, Ba1−xSrxNi2As2
Date and Time: Friday, April 28, 1:30 pm
Location: Toll 0360. QMC Conference Room
Dissertation Committee Chair: Johnpierre Paglione
Committee:
Richard Green
Nicholas Butch
Ichiro Takeuchi
Jeffrey Lynn
Abstract:
The desire for a complete understanding of high temperature unconventional superconductivity
has illustrated a necessity for the study of non-magnetic sources of superconducting
enchantment, such as nematically driven fluctuations and charge order fluctuations. BaNi2As2,
a non-magnetic counterpart to high Tc superconductor BaFe2As2, shows a six-fold superconducting
enhancement neighboring charge and nematic orders, positioning it as an excellent candidate
for studying the interactions between charge order, nematic order, and enhanced superconductivity.
In this thesis, I will present X-ray diffraction and electrical transport evidence for the
development of complex charge order within the system as functions of isovalent chemical
substitution via Ba1−xSrxNi2As2 and applied hydrostatic pressure. The discovery of three
separate charge order will be detailed: an incommensurate charge order at Q = 0.28
and two commensurate charge orders at Q = 0.33 and Q = 0.5. X-ray diffraction
measurements of the Q = 0.28 charge order will be used to show a strong correlation
between it and a previously established nematic order for Ba1−xSrxNi2As2. Applied
pressure of BaNi2As2 up to 10.4 GPa will detail the development of all three charge orders and be used to show a correlation between pressure and isovalvent substitution in BaNi2As2. The critical substitution of 71% Sr and the critical pressure of 9 ± 0.5 GPa will be directly compared by X-ray measurements of their lattice parameters, revealing a collapsed tetragonal phase. This phase is shown to be analogous to the collapsed
tetragonal phase of the Fe-pnictide superconductors, likely playing a key role seen at
the critical substitution and pressure of BaNi2As2
Rahul Gaur - April 14, 2023
Dissertation Title: Optimization of high-beta fusion devices against linear instabilities
Date and Time: Friday, April 14, 9:45 AM
Location: ERF 1207 Large conference room, Energy Research Facility
Dissertation Committee Chair: Prof. William Dorland
Committee:
Dr. Ian Abel
Dr. Matt Landreman
Prof. Alexander Philippov
Prof. Thomas Antonsen (Dean’s representative)
Abstract:
Magnetic confinement fusion is a technique in which a strong magnetic field is used to contain a hot plasma, which enables nuclear fusion. Regarding overall energy efficiency, the two most promising magnetic confinement concepts are tokamaks (axisymmetric devices) and stellarators (nonaxisymmetric devices). The power P produced by a magnetically confined nuclear fusion device is proportional to , where V is the device's volume, β is the plasma pressure - magnetic pressure ratio, and B is the magnetic field strength. Most tokamaks and stellarators currently in operation are low-β devices. Broadly speaking, there are three ways to increase P, one may increase the operating β (as in Spherical Tokamaks), the magnetic field (as in the SPARC and EAST), or the volume of the device (as in ITER and W7X). The cost of these devices is proportional to V, making large devices extremely expensive. Similarly, a large magnetic field (>10T) requires superconducting magnets that, even after the recent innovations in HTS (High-Temperature Superconductors), are expensive to operate.
In addition, more research needs to be done on the effect of neutron flux on the lifecycle of HTS magnets. High-β devices are an attractive idea for producing fusion energy efficiently. However, a high β generally also implies a large gradient in plasma pressure that can be a source of numerous magnetohydrodynamic (MHD) and kinetic instabilities. If fusion devices could be optimized against such instabilities, high-β operation would become attractive compared to high-field or large-volume reactors. Therefore, this thesis examines the stability of the high-β tokamak and stellarator equilibrium equilibria.
We will start by investigating the stability of high-β tokamaks and stellarator equilibria
against the infinite-n, ideal ballooning mode, an important pressure-driven magnetohydrodynamic (MHD) instability. We optimize these equilibria for stability against the ideal-ballooning mode. To achieve this, we formulate a gradient-based adjoint technique and demonstrate its speed and effectiveness by stabilizing these equilibria. We also formulate how this technique can be easily extended to low-n ideal-MHD modes in both tokamaks and stellarators.
After demonstrating the process of stabilizing against ideal MHD unstable modes, we analyze the kinetic stability of high-β tokamak and stellarator equilibria by numerically solving the δf gyrokinetic model – a simplified version of the Vlasov-Maxwell model. These kinetic instabilities are driven by temperature and density gradients. To better understand these instabilities, we scan multiple values of the plasma β, temperature and density gradients, and plasma boundary shapes, discovering useful relationships between equilibrium-dependent quantities and growth rates of these instabilities.
From the microstability study, we find that electromagnetic effects are important for high-β devices. Hence, we use the numerical tools and knowledge derived from the previous chapters to write an optimization framework that searches for axisymmetric equilibria that are stable to electromagnetic kinetic instabilities. Due to the similarity between axisymmetry and quasisymmetry – a hidden symmetry in stellarators – we extend the microstability optimizer to search for high-β quasisymmetric stellarator equilibria.
Rui Zhang - April 11, 2023
Dissertation Title: Lattice Quantum Chromodynamics (QCD) Calculations of Parton Physics with Leading Power Accuracy in Large Momentum Expansion
Date and Time: Tuesday, April 11, 2:00 pm
Location: PSC 3150
Dissertation Committee Chair: Xiangdong Ji
Committee:
Thomas Cohen
Drew Baden
Zohreh Davoudi
Alice Migenery
Abstract:
Parton distributions describing how momenta of quarks and gluons are distributed inside a hadron moving at the speed of light, are important inputs to the standard model prediction of collider physics. Their non-perturbative nature does not allow traditional perturbative calculations from quantum field theory. Besides a global fitting to experimental data, it is also possible to calculate parton physics from lattice-QCD, a first principle non-perturbative Monte Carlo simulation of the strong interaction on super computers. Among the different strategies to extract information for parton physics, the large momentum effective theory, based on a large momentum expansion of non-local Euclidean correlation functions, allows us to directly calculate the momentum fraction x-dependence. When matching the lattice-QCD calculations to the physical parton physics in the large momentum expansion, there are unavoidable power corrections in the expansion parameter Lambda/Pz, which is determined by the QCD characteristic non-perturbative scale Lambda~300 MeV and the hadron momentum Pz, and the leading term appears as order Lambda/2xPz due to the linear divergent self-energy of Wilson line in the Euclidean lattice correlators. For current lattice calculations of Pz~2 to 3 GeV, this correction can be as large as 30% at small x, dominating the uncertainties in the calculation. Achieving power accuracy in linear order of Lambda/Pz is thus crucial for a high precision calculation of the parton physics from lattice.
In this dissertation, I summarize our work to eliminate this linear correction by consistently define the renormalization for the linear divergence in lattice data and the resummation scheme of the factorially growing infrared-renormalon series in the perturbative matching. We show that the method significantly reduces the linear uncertainty by a factor of 3 to 5 and improves the convergence of the perturbation theory. We then apply the strategy to the calculation of pion distribution amplitude, which describes the pion light-cone wave function in a quark-antiquark pair. The method improves the short distance behavior of the renormalized lattice correlations, which is now consistent with the prediction of the short distance operator product expansion, showing a reasonable value for the moments of pion distribution amplitude. We also develop the first strategy to resum the large logarithms in the matching to physical pion distribution amplitude when the momentum of quark or antiquark in the pion are small, that could improve the accuracy of the prediction near the endpoint regions. After extracting the x-dependence from the large momentum expansion in mid-x region, we complete the endpoint regions by fitting to the short distance correlations. Then a complete x-dependence is obtained for the pion distribution amplitude, which suggests a broad distribution compared to previous lattice calculations or model predictions.
Yuxun Guo - April 6, 2023
Dissertation Title: UNRAVELING THE NUCLEON 3D STRUCTURE FROM EXPERIMENT, LATTICE AND GLOBAL ANALYSIS
Date and Time: Thursday, April 6, 11:00 am
Location: PSC 2136
Dissertation Committee Chair: Prof. Xiangdong Ji
Committee:
Professor Kaustubh Agashe
Professor Zackaria Chacko
Professor Thomas Cohen
Professor Da-Lin Zhang
Abstract:
Nucleon 3D structure has been one of the most important goals in modern nuclear physics, which will provide, among other insights, an intuitive understanding of how the fundamental properties of the nucleon, such as its mass and spin, arise from the underlying quark and gluon degrees of freedom. I will present studies the 3D structure of nucleon via generalized parton distributions (GPDs) with inputs from experiments, lattice and global fits. I will discuss various exclusive measurements at HERA, JLab that can be used to probe the nucleon 3D structures as well as progresses in lattice QCD calculation related to nucleon 3D structures. I will also introduce a global analysis program that combines both experiment and lattice inputs and generate the state-of-art GPDs, which will shed a refreshing new light on the problem of proton structure and confinement.
DinhDuy Vu - March 29, 2023
Dissertation Title: Topology, localization, and spontaneous symmetry breaking in nonequilibrium many-body systems
Date and Time: Wednesday,March 29, 10:30 am
Location: ATL 4402 (CMTC conference room)
Dissertation Committee Chair: Prof. Sankar Das Sarma
Committee:
Prof. Jay Deep Sau
Prof. Maissam Barkeshli
Prof. Alicia Kollar
Prof. Christopher Jarzynski (Dean's Representative)
Abstract:
Exotic many-body phenomena are usually associated with the ground state of a time-independent Hamiltonian. It is natural to ask whether these physics can survive in a dynamic setting. Under a generic drive, the steady equilibrium state is most likely an infinite-temperature featureless thermal state. However, there exist exceptional cases where thermalization either does not happen or is delayed for a sufficiently long time, called nonequilibrium many-body systems. In this thesis, we study mechanisms that can generate nonequilibrium dynamics: many-body localization, prethermalization, and projective measurements. We then demonstrate that the resulting quantum states can host a wide variety of many-body phenomena similar to the groundstate, focusing on three aspects: topology, localization, and spontaneous symmetry breaking.
Kaustubh Wagh- March 27, 2023
Dissertation Title: REGULATING GENE EXPRESSION: THE ROLE OF TRANSCRIPTION FACTOR DYNAMICS
Date and Time: Monday, March 27, 12:30 PM
Location: Conference Room, 1116, Institute for Physical Science and Technology (IPST) Building
Dissertation Committee Chair: Prof. Arpita Upadhyaya
Committee:
Dr. Gordon L. Hager
Dr. Michelle Girvan
Dr. Wolfgang Losert
Dr. Helim Aranda-Espinoza
Abstract:
The genetic information encoded within our DNA is converted into RNA in a process called transcription. This is a tightly regulated process where multiple proteins act in concert to activate appropriate gene expression programs. Transcription factors (TFs) are key players in this process, with TF binding being the first step in the assembly of the transcriptional machinery. TFs are sequence-specific DNA binding proteins that bind specific motifs within chromatin. How TFs navigate the complex nuclear microenvironment to rapidly find their target sites remains poorly understood. Technological advances over the past 20 years have enabled us to follow single TF molecules within live cells as they interact with chromatin. Most TFs have been shown to exhibit power law distributed residence times, which arise from the broad distribution of binding affinities within the nucleus. This blurs the line between specific and non-specific binding and renders it impossible to distinguish between different binding modes based on residence times alone.
In this dissertation, I combine single molecule tracking (SMT) with statistical algorithms to identify two distinct low-mobility states for chromatin (histone H2B) and bound transcriptional regulators within the nucleus. On our timescales, the TF mobility states represent the mobility of the piece of chromatin that they are bound to. Ligand activation results in a dramatic increase in the proportion of steroid receptors in the lowest mobility state. Mutational analysis revealed that only chromatin interactions in the lowest mobility state require an intact DNA-binding domain as well as oligomerization domains. Importantly, these states are not spatially separated as previously believed but in fact, individual H2B and chromatin-bound TF molecules can dynamically switch between them. Single molecules presenting different mobilities exhibit different residence time distributions, suggesting that the mobility of a TF is intimately coupled with their temporal dynamics. This provides a way to identify different binding modes that cannot be detected by measuring residence times alone. Together, these results identify two unique and distinct low-mobility states of chromatin that appear to represent common pathways for transcription activation in mammalian cells.
Next, I demonstrate how SMT can complement genome wide assays to paint a complete picture of gene regulation by TFs using two case studies: corticosteroid signaling and endocrine therapy resistance in breast cancer. Finally, I conclude with a roadmap for future work on examining the role of mechanical cues within the cellular microenvironment (such as stiffness and topography) in regulating TF dynamics and gene expression.
Yipeng Sun- March 1, 2023
Dissertation Title: MEASUREMENT OF R(D(∗)) IN SEMILEPTONIC B DECAYS AND UPGRADE OF THE LHCB UPSTREAM TRACKER
Date and Time: Wednesday,March 1, 2023, 1:00 pm
Location: PSC3204
Dissertation Committee Chair: Prof. Manuel Franco Sevilla
Committee:
Dr. Abohassan Jawahery
Dr. Zakaria Chacko
Dr. Christopher Palmer
Dr. Alice Mignerey
Abstract:
The LHCb experiment at the Large Hadron Collider provides a unique opportunity to study flavor physics with high luminosity. One topic in flavor physics is lepton flavor universality (LFU), a property of the standard model (SM) which requires the three generations of leptons (e,µ, τ) couple to gauge bosons of the eletroweak interactions with) couple to gauge bosons of the eletroweak interactions with the same strength. It is an important probe for testing the validity of the SM and possibly providing hints to new physics beyond the SM. This thesis presents a preliminary framework for the measurement of R(D(∗)), a proxy to test LFU, defined as the ratio of branching fractions B(B → D(∗)τ −ντ)/B(B → D(∗)µ−νµ), with LHCb 2016 data. Another topic of the thesis is the upgrade of the LHCb Upstream Tracker (UT) which greatly increases the readout rate of the detector and removes limitations due to hardware trigger, paving the way for future precision measurements with even higher luminosity.
Zishuo Yang - February 20, 2023
Dissertation Title: Experimental study of semitauonic B_c decays and development of the Upstream Tracker electronics for the LHCb upgrade
Date and Time: Monday, February 20, 9:00 am
Location: PSC 3150
Dissertation Committee Chair: Prof. Hassan Jawahery
Committee:
Dr. Thomas Cohen
Dr. Sarah Eno
Dr. Manuel Franco Sevilla
Dr. Richard Mushotzky
Abstract:
The LHCb experiment at the Large Hadron Collider is designed for studying the properties of heavy quarks and CP violation to indirectly search for new physics beyond the Standard Model. The first topic of this dissertation is a study of semitauonic B_c meson decays at LHCb to test the universality of the couplings of charged leptons in electroweak interactions, which is known as lepton flavor universality in the Standard Model. The second topic of this dissertation is the development of readout electronics for a new silicon-strip tracking detector, the Upstream Tracker, to upgrade the LHCb detector. The upgraded LHCb detector will collect much more data in the upcoming runs of the Large Hadron Collider.