Deepak Sathyan: Thesis Defense

Mon, Jul 22, 2024 3:00 pm - 4:00 pm
PSC 3150


Candidate: Deepak Sathyan

Title: Unifying Searches for New Physics with Precision Measurements of the W Boson Mass

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The Standard Model (SM) ofparticle physics has been extremely successful in describing the interactionsof electromagnetic, weak nuclear, and strong nuclear forces. Yet, there areboth unexplained phenomena and experimentally observed tensions with the SM,motivating searches for new physics (NP).

Collider experiments typicallyperform two kinds of analyses: direct searches for new physics and precisionmeasurements of SM observables. For example, experimental collaborations usecollider data to search for NP particles like the heavy superpartners of the SMparticles, whose observation would be clear evidence of supersymmetry (SUSY).These direct searches often consider kinematic regions where the SM backgroundis small. This strategy is unable to probe regions of the NP parameter spacewhere the SM background is dominant.

The same collaborations also measurethe masses of SM particles, which not only serve as consistency tests of theSM, but can also probe effects of NP. In 2022, the Collider Detector atFermilab (CDF) collaboration published the most precise measurement of the Wboson mass: mW = 80433.5 ± 9.4 MeV. This measurement is in 7𝜎significance tension with the SM prediction via the electroweak (EW) fit, mWpred.= 80354 ± 7 MeV. Many extensions to the SM can affect the prediction of mWwith indirect effects of heavy NP. However, in 2023, the ATLAS re-measurementof the W boson mass, mW = 80360 ± 16 MeV, was found to be consistentwith the SM prediction. Both collaborations found a high-precision agreementbetween the measured kinematic distributions and the SM prediction of thekinematic distributions for their corresponding extracted mW

We propose using the precisionmeasurements of mW to directly probe NP contributing to the samefinal state used to measure mW: a single charged lepton (l) andmissing transverse energy (MET). This strategy is independent of modifying theEW fit, which tests indirect effects of NP on the predicted value of mW.Any NP producing l+MET which modifies the kinematic distributions used toextract mW can be probed with this method. An important point ofthis strategy is that since these distributions are used to search for NP whilemeasuring mW, a simultaneous fit of NP and SM parameters, thusunifying searches and measurements. This simultaneous fitting can induce a biasin the measured mW, but only to a limited extent for our consideredmodels.

We consider three categories of NPwhich can be probed: (i) modified decay of W bosons; (ii) modified productionof W bosons; and (iii) l+MET scenarios without an on-shell W boson. We alsoshow that models whose signals extend beyond the kinematic region used tomeasure mW can be probed in an intermediate kinematic region. Ourresults highlight that new physics can still be directly discovered at the LHC,including light new physics, via SM precision measurements. Additionally,anticipated improvements in precision SM measurements at the High LuminosityLHC further enables new searches for physics Beyond the Standard Model (BSM).