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Atomic physics study sets new limits on hypothetical new particles

University of Nevada, Reno team findings to be published in Physical Review Letters article

In a forthcoming Physical Review Letters article, a group of physicists at the University of Nevada, Reno are reporting a refined analysis of experiments on violation of mirror symmetry in atoms that sets new constraints on a hypothesized particle, the extra Z-boson.

Andrei Derevianko, an associate professor in the College of Science's Department of Physics, who has conducted groundbreaking research to improve the time-telling capabilities of the world's most accurate atomic clocks, is one of the principals behind what is believed to be the most accurate to-date low-energy determination of the strength of the electroweak coupling between atomic electrons and quarks of the nucleus.

"It is remarkable that the low-cost atomic precision experiments and theory are capable of constraining new physics at the level competitive to colliders," Derevianko said. He has been able to define new limits without needing something like a $6 billion Large Hadron Collider, an enormous particle accelerator in Europe that is not yet fully operational.

"This is like David and Goliath, we are just a small group of people able to better interpret the data on violation of mirror symmetry in atoms. Our work indicates less of a possibility for extra Z-bosons, potential carriers of the fifth force of is possible the LHC will be able either to move the mass limit higher or discover these particles," he said.

Derevianko and his colleagues have determined the coupling strength by combining previous measurements made by Dr. Carl Wieman, a Nobel laureate in physics, with high-precision calculations in a cesium atom.

The original work by Wieman on violation of mirror symmetry in atoms used a table-top apparatus at the University of Colorado in Boulder, Colo. The Boulder team monitored a "twinge" of weak force in atoms, which are otherwise governed by the electromagnetic force. The Standard Model of elementary particles, developed in the early 1970s, holds that heavy particles, called Z-bosons, carry this weak force. In contrast to the electromagnetic force, the weak force violates mirror symmetry: an atom and its mirror image behave differently. This is known to physicists as "parity violation."

The Boulder group's experiment opened the door to new inquiry, according to Derevianko. "It pointed out a discrepancy, and hinted at a possibility for new physics, in particular, extra Z-bosons," he said.

Interpretation of the Boulder experiment requires theoretical input. The analysis requires detailed understanding of the correlated motion of 55 electrons of cesium atom. This is not an easy task as the number of memory units required for storing full quantum-mechanical wavefunctions exceeds the estimated number of atoms in the Universe. Special computational tools and approximations were developed. Compared to previous analyses, reaching the next level of accuracy required a factor of 1,000 increase in computational complexity.

The paper represents a dramatic improvement as researchers have struggled to develop a more precise test of the Standard Model. Derevianko's group, which included Dr. S. Porsev and a number of students, has worked on the analysis of the Boulder experiment for the past eight years.

"Finally, the computer technology caught up with the number-crunching demands of the problem and we were able to attack the problem," says Derevianko. "I have greatly benefited from collaborations in this complex problem. A fellow co-author, Kyle Beloy, for example, has recently been recognized as an Outstanding Graduate Researcher by the University."

In contrast to previous, less accurate interpretations of the Boulder experiment, Derevianko's group has found a perfect agreement with the prediction of the Standard Model. This agreement holds important implications for particle physics.

"Atomic parity violation places powerful constraints on new physics beyond the Standard Model of elementary particles," Derevianko said. "With this new-found precision, we are doing a better job of 'listening' to the atoms."

By refining and improving the computations, Derevianko said there is potential for a better understanding of hypothetical particles (extra Z-bosons) which could be carriers of a so-far elusive fifth force of nature. For years, physics researchers have grappled with experiments to prove or disprove the possibility of a fifth force of Nature.

There are four known fundamental forces of Nature. In addition to gravity, electromagnetism creates light, radio waves and other forms of radiation. Two other forces operate only on an atomic level: These are the strong force, which binds particles in the nucleus, and the weak force, which reveals itself when atoms break down in radioactive decay, or as in the Boulder experiment, through the parity violation.

The possibility of a fifth force could dispute the long-held belief that the force of gravity is the same for all substances.

"New physics beyond the Standard Model is the next frontier," Derevianko said, "and it's the theoretical motivation for much of this research."

To read Derevianko's paper co-authored with S. Porsev and K. Beloy, go to:

Below is a summary of Derevianko's paper, which is entitled, "Precision determination of electroweak coupling from atomic parity violation and implications for particle physics":

Atomic parity violation places powerful constraints on new physics beyond the Standard Model of elementary particles. The measurements are interpreted in terms of the nuclear weak charge, quantifying the strength of the electroweak coupling between atomic electrons and quarks of the nucleus. We report the most accurate to-date determination of this coupling strength by combining previous measurements by the Boulder group with our high-precision calculations in cesium atom. Our result is in a perfect agreement with the prediction of the Standard Model.

In combination with the results of high-energy collider experiments, our work confirms the predicted energy dependence (or ``running'') of the electroweak interaction over an energy range spanning four orders of magnitude (from ~10 MeV to ~100 GeV) and places new limits on the masses of extra Z bosons (Z'). Our raised bound on the Z' masses carves out a lower-energy part of the discovery reach of the Large Hadron Collider. At the same time, a major goal of the LHC is to find evidence for supersymmetry (SUSY), one of the basic, yet experimentally unproven, concepts of particle physics. Our result is consistent with the R-parity conserving SUSY with relatively light (sub-TeV) superpartners. This raises additional hopes of discovering SUSY at the LHC.

Mike Wolterbeek | EurekAlert!
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