High-flying Experiments Tackle the Mysteries of Cosmic Rays

Cosmic rays are not rays at all, but highly energetic particles that zoom through space at nearly the speed of light.  The particles range in size, from subatomic protons to the atomic nuclei of elements such as carbon and boron. Scientists suspect that the particles are bits of subatomic shrapnel produced by supernovae, but could also be signatures of other cataclysmic phenomena.

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Destabilized solitons perform a disappearing act

When your heart beats, blood courses through your veins in waves of pressure. These pressure waves manifest as your pulse, a regular rhythm unperturbed by the complex internal structure of the body. Scientists call such robust waves solitons, and in many ways they behave more like discrete particles than waves. Soliton theory may aid in the understanding of tsunamis, which—unlike other water waves—can sustain themselves over vast oceanic distances.

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Crossing the quantum-chaotic divide

Chaos is all around us, a fact that weather forecasters know all too well.

Their job is notoriously difficult because small changes in air pressure or temperature, which ultimately drive winds and weather systems, can have huge consequences on a global scale. This sensitivity to tiny differences is commonly called the butterfly effect, and it makes weather patterns chaotic and hard to predict.

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Stacked nanocrystals offer a new twist on handedness

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Chirality is another name for the asymmetry that we see between our left and right hands. This handedness crops up nearly everywhere in nature, from galaxy spirals down to the quantum mechanical properties of fundamental particles. Life itself boasts some of the most well-studied examples of chirality, including DNA's famed helical staircase.

Recently, some scientists have turned their attention to chirality found in the inorganic world of rocks, minerals and other non-biological materials. University of Maryland physics professor Min Ouyang leads a team that has been working to control chirality in tiny inorganic arrangements called nanocrystals. Their goal is to understand how atoms can cooperate to form larger, superchiral objects, analogous to how assemblies of carbon atoms stack into DNA. To achieve this, the researchers developed a new recipe for both engineering and analyzing chiral nanostructures from the bottom-up. This research was published recently in the open access journal Nature Communications.

Ouyang, whose work has been inspired by biological chirality, sees this research as an exciting step, with applications to a broad range of inorganic structures with tunable chirality. "Our technique is general, and does not depend on the material we used," Ouyang says. "Using inorganic molecules, we could mimic natural assemblies, perhaps building a DNA-like strand from the ground-up and study its chiral dependent physical properties, step by step."

The recipe begins in a flask containing a carefully crafted solution of cinnabar and penicillamine, both of which are chiral and play different roles in guiding the growth of nanostructures. Cinnabar, a red compound found naturally in volcanos, is readily synthesized in the laboratory. Here, it acts as an atomic building block, seeding the growth of chiral lattices. The mercury and sulfur atoms that make up cinnabar line up according to the handedness of the initial seeds. Penicillamine, an organic drug molecule, provides a helping hand for guiding the larger cinnabar assemblies. Smaller bits of cinnabar take their cue from the penicillamine's orientation, left or right, and stack into even bigger shapes, eventually completing a full nanocrystal.

Such crystal growth techniques have been used before, but Ouyang's group modified the process so that they could, for the first time, independently manipulate chirality of both the atomic building blocks and the larger crystals.

The researchers assembled crystals that ranged in size from tens of nanometers to a few hundred nanometers—roughly the size of viruses. They adjusted both the final nanocrystal size and their handedness by tweaking the different growth conditions. To show that they could control chirality the team purposefully constructed other, non-chiral shaped nanocrystals, like rods and cubes.

chiral image ouyang 2017 01Caption: Electron microscope images of achiral and chiral shapes. The dotted line on the right chiral nanostructures outlines the twists that are the visual signature for handedness. (Images from paper, courtesy of authors)Ouyang and his collaborators used complementary methods to measure the chirality of their nanocrystals. They employed a high-power electron microscope to take snapshots of their creations, providing the first direct image of cinnabar's handedness. The images (inset) show diamond-like crystals with a pronounced s-shaped line where the shape is twisting. This twist, left or right, is the visual signature of the final product's chirality. In the case of the non-chiral structures, no 's' is present.

The researchers also shined light onto samples and looked at how they responded. Using this standard probe, they saw that, as expected, asymmetric structures absorbed light waves according to their handedness. More importantly, they used the optical absorption measurement to monitor the chirality at each growth stage, from molecules to lattices to nanocrystals. To complete the study, the team developed a mathematical model for understanding this observed chiral evolution, one that was adaptable to the different length scales.

Taken together, Ouyang and colleagues have demonstrated a powerful method for systematically examining nanoscale chirality, which includes flexible synthesis, direct visualization, and a tested computational tool. Bottom-up experiments, like this work, may enable a better understanding of chirality in inorganic materials, such as those used for novel quantum spin-based technologies.

"Cooperative expression of atomic chirality in inorganic nanostructures,"Peng-peng Wang, Shang-Jie Yu, Alexander O Govorov & Min Ouyang Nature Communications 8, 14312 (2017) doi:10.1038/ncomms14312: http://www.nature.com/articles/ncomms14312
Related videos:
Side view of 3D reconstruction of a twisted triangular bipyramid HgS nanocrystal.  A movie shows the 3D electron tomography reconstruction of a single nanocrystal, confirming assignment of geometric morphology. 
Top view of 3D reconstruction of a twisted triangular bipyramid HgS nanocrystal. A movie shows the 3D electron tomography reconstruction of a single nanocrystal, confirming its three-fold structural symmetry.


As dark matter eludes scientists, they plan a more extensive search

carter imageThis empty honeycomb array in the LUX experiment now holds photomultiplier tubes, which are ready to catch light signals caused by dark matter. Photo by Carlos Faham.

For decades, scientists have suspected that the universe contains more matter than we can see.  They point to clues in the cosmos, such as irregularities in the radiation left over from the early universe and the way light bends around galaxies. By studying the rotation rates of galaxies, pioneering astrophysicists like the late Vera Rubin determined that about 85% of the mass in the universe doesn't betray its presence by emitting any light. For this reason, they named this mysterious mass dark matter.

Researchers from the University of Maryland (UMD), led by Associate Professor Carter Hall, have been blazing the trail in the search for dark matter. Their involvement with the Large Underground Xenon (LUX) experiment—the most sensitive detector of its kind—has placed the most stringent limits yet on the properties of dark matter. And now that LUX has finished taking data, the focus is shifting to the next generation of experiments. An even more sensitive detector called the LUX-ZEPLIN (LZ) experiment is already under construction, and Hall will soon begin a term as the spokesperson for the collaboration.

Since 2013, the LUX collaboration, which includes scientists from the United Kingdom, Portugal and across the United States, has been looking for dark matter in the form of weakly interacting massive particles, or WIMPs. WIMPs are a leading candidate for this mysterious matter, and LUX has been hunting them from an old gold mine in the Black Hills of South Dakota, nearly a mile underground.

The experiment, which consists of a tank with 375 kilograms of ultra-pure liquid xenon and a layer of xenon gas, tries to spot the telltale signs of a WIMP knocking into ordinary matter. Theory predicts that when a WIMP collides with a xenon nucleus, the nucleus will recoil and collide with other nearby xenon atoms. This sets off a chain reaction, liberating electrons from the atoms and releasing ultraviolet light in the process.

LUX sits and waits for this light. Photomultiplier tubes, which act like simple cameras, line the top and bottom of the tank, eyes peeled for quick flashes. An electric field corrals the free electrons toward the top of the tank, and they produce more light when they leap into the layer of xenon gas. These signals tell the researchers where in the detector the WIMP collision occurred. The researchers chose to use xenon in the LUX and LZ experiments because the xenon nucleus is very likely to interact with WIMPs. "It's also not too expensive," says Hall. "You can buy many tons of it."

The experiment must reject false signals, which requires exquisite control. For example, the team worked hard to shield the experiment from cosmic rays—the steady stream of particles from outer space that pass through your body 30 times every second. The mile of bedrock above the mine shields the experiment from these particles, as does a 6-meter- high tank of ultra-pure water surrounding the xenon. They also take care to use detector materials that don't spit out WIMP impostors like neutrons through radioactive decay.  

Hall and his UMD collaborators played crucial roles in the LUX collaboration. They provided a source of tritium, a radioactive isotope of hydrogen, to help calibrate the detector. They also built a system to screen the detector material—liquid xenon— for impurities. Richard Knoche, one of Hall's graduate students, recently defended his Ph.D. thesis on how to analyze data from LUX given that its electric field is not uniform.  

Different dark matter theories predict different properties for WIMPs, and no one knows how massive WIMPs are or how likely they are to react with ordinary matter. But every dark matter detector scientists build is more sensitive than its predecessor, and progress means putting stricter and stricter limits on the possible masses and interactions strengths a WIMP could have.

After an initial experimental run in 2013, the LUX team has continued to update the experiment, making it four times more sensitive to most WIMP masses. The collaboration did not identify any nuclear recoils due to WIMPs in their 2016 data, which confirms the findings reported by other dark matter searches.

The lack of observed WIMP events makes the continuing work in dark matter detectors even more important, and the next-generation LZ experiment will be 100 times more sensitive than LUX. It will be so sensitive, in fact, that it should be able to detect the constant stream of tiny and nearly massless neutrinos streaming toward us from the sun. Hall says that detecting this neutrino wind will be compelling evidence that LZ is functioning properly.

If LZ sees the neutrino wind and still doesn't spot any WIMPs, it could mean that a new approach to detecting dark matter is needed. Scientists have proposed other theories of what dark matter is, including another particle called the axion, and they may shift toward exploring those theories more seriously if LZ exhausts the search for WIMPs. The lack of discoveries from the Large Hadron Collider that would support the WIMP hypothesis has also caused scientists to take a step back and re-examine their theories on dark matter.

But if researchers are getting close to finishing the search for WIMPs, then LZ will play an important role in taking that those last steps. "There's another, at least, ten years of WIMP experiments that are now planned to be carried out," Hall says. The LZ detector, which is scheduled to start taking data in 2020, may provide the final word on many lingering questions surrounding dark matter.

More information about the LUX and LZ experiments can be found at http://luxdarkmatter.org/ and http://lz.lbl.gov/, respectively.

Written by Erin Marshall, UMD Physics

Research article reference:

"Results from a Search for Dark Matter in the Complete LUX Exposure," D. S. Akerib et al. (LUX Collaboration)
Phys. Rev. Lett. 118, 021303, (2017); APS Physics Viewpoint: http://physics.aps.org/articles/v10/3

Selected popular news accounts on this research:

- https://www.scientificamerican.com/article/new-techniques-could-target-more-exotic-dark-matter/

- https://www.sciencenews.org/article/latest-dark-matter-searches-leave-scientists-empty-handed