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Breaking symmetries of quarks—and publishing in 'Nature'

  • Looking at quarks
    Looking at quarks
    The electron-quark scattering experiment was carried out in Jefferson Lab's Experimental Hall A. Two high resolution spectrometers are shown with their shield house doors (white) open.
    Courtesy of Jefferson Lab

From matching wings on butterflies to the repeating six-point pattern of snowflakes, symmetries echo through nature, even down to the smallest building blocks of matter. Since the discovery of quarks, the building blocks of protons and neutrons, physicists have been exploiting those symmetries to study quarks' intrinsic properties and to uncover what those properties can reveal about the physical laws that govern them.

A recent experiment carried out at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Lab) to study a rare instance of symmetry breaking in electron-quark scattering has provided a new determination of an intrinsic property of quarks that's five times more precise than the previous measurement. The result  of the Parity-Violating Deep Inelastic Scattering  (PVDIS) experiment was published in the Feb. 6 edition of Nature.

Jefferson Lab and the University of Virginia were the lead institutions on the experiment, which included four William & Mary physicists among its collaborators. Among the co-authors on the Nature article are David Armstrong, Chancellor Professor of Physics and chair of the department, and Wouter Deconinck, assistant professor of physics, along with post-doctoral researchers Jeong Han Lee and Bo Zhao.

Armstrong said he was among the advocates for publishing results of the experiment in Nature, which has a broader readership than Physical Review Letters, which although prestigious, has little lay readership.

“Most people would say that the two major scientific journals are Science and Nature,” Armstrong said. “The perception is that they both tend to be dominated by the health sciences and other fields. You can count on the fingers of one hand the number of nuclear-physics papers that have been published in Nature or Science in the past ten years.”

The experiment probed properties of the mirror symmetry of quarks. In mirror symmetry, the characteristics of an object remain the same even if that object is flipped as though it were reflected in a mirror.

In Jefferson Lab's Experimental Hall A, experimenters measured the breaking of the mirror symmetry of quarks through the process of deep-inelastic scattering. A 6.067 GeV beam of electrons was sent into deuterium nuclei, the nuclei of an isotope of hydrogen that contain one neutron and one proton each (and thus an equal number of up and down quarks).

"When it's deep-inelastic scattering, the momentum carried by the electron goes inside the nucleon and breaks it apart," said Xiaochao Zheng, an associate professor of physics at the University of Virginia and a spokesperson for the collaboration that conducted the experiment.

Armstrong said the PVDIS experiment had similarities as well as differences from the Qweak experiment, also conducted at JLab. Qweak, had a more substantial involvement of William & Mary scientists.

“The ultimate goal of both experiments was to try to measure a quantity that was well predicted—robustly predicted—within the Standard Model,” Armstrong said. In the case of Qweak, the goal was to measure the weak charge of the proton. Both experiments aimed to verify aspects of the Standard Model, the inventory of particles and interactions that make up the universe as we know it. But Armstrong said that experimental physicists want to do more than fill in observationally the blank spots that remain in the Standard Model; they also want to find areas beyond the current theoretical confines. 

“Finding physics beyond the Standard Model is the holy grail of all subatomic physics,” Armstrong said. He said precise-enough measurements in experiments such as QWeak and the PVDIS study can be signposts indicating possible routes to get beyond Standard Model-physics or signposts that effectively say “this road ends” at other theoretical intersections.

Both Qweak and the PVDIS experiments used electron scattering techniques to get subatomic particles to reveal their secrets. Armstrong noted that there were differences between the two studies, as well.

“In Qweak, we were looking for a property of the proton as a whole—the weak charge of the proton. So, we tickled the proton. We went in with electrons just gently scattering off the proton,” Armstrong explained. “In PVDIS, we slammed electrons into protons and neutrons, very hard. Now, we weren’t just looking at the proton as a whole, we were looking at the quarks inside them.”

To study the quarks, Armstrong said that the PVDIS experimenters used deuterium, a heavy isotope of hydrogen as a target. The target choice is another difference from Qweak, which used regular hydrogen. An atom of hydrogen, the lightest element, contains only a single proton, which itself is made up of  two up quarks and one down quark. The odd number of quarks wouldn’t work for PVDIS.

“Deuterium of course has equal numbers of protons and neutrons,” he explained, “which means it contains equal numbers of up quarks and down quarks.”

The experimenters found an asymmetry, or difference, in the number of electrons that interacted with the target when they were spinning in one direction versus the other. This asymmetry is due to the weak force between the electron and quarks in the target. The weak force experienced by quarks has two components. One is analogous to electric charge and has been measured well in previous experiments. The other component, related to the spin of the quark, has been clearly isolated for the first time in the Jefferson Lab experiment.

The last experiment to access this coupling combination was E122 at DOE's Stanford Linear Accelerator Center (now SLAC National Accelerator Laboratory). Data from that experiment were used to establish the newly theorized Standard Model more than 30 years ago.

The good agreement between the PVDIS result and the Standard Model also indicates that experimenters must reach higher energy limits in order to potentially find new interactions beyond the Standard Model with respect to the violation of mirror symmetry due to the spin of the quarks. The new limits, 5.8 TeV and 4.6 TeV, are within reach of the Large Hadron Collider at CERN, but the spin feature provided by this experiment cannot be identified cleanly in collider experiments.

Researchers plan to extend this experiment in the next era of research at Jefferson Lab. In a bid to further refine the knowledge of quarks' mirror-symmetry breaking, experimenters will use Jefferson Lab's upgraded accelerator to nearly double the energy of the electron beam, reducing their experimental errors and improving the precision of the measurement by five to ten times the current value. The experiment will be scheduled following completion of the upgrade in 2017.

The experiment was funded by the DOE Office of Science, the National Science Foundation Division of Physics and the Jeffress Memorial Trust, as well as with support provided to individual researchers by their home institutions. Nearly 100 researchers from more than 30 institutions collaborated on the experiment, including two DOE National Labs, Jefferson Lab and Argonne National Lab.

Jefferson Science Associates, LLC, a joint venture of the Southeastern Universities Research Association, Inc. and PAE Applied Technologies, manages and operates the Thomas Jefferson National Accelerator Facility, or Jefferson Lab, for the U.S. Department of Energy's Office of Science.