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Potential fifth force of nature, outcome of LHC

May 1st, 2009

The Large Hadron Collider is an huge particle accelerator whose 27 kilometres tunnel beneath the Franco-Swiss border. A group of physicists at the University of Nevada, Reno has analyzed data from the accelerator that could ultimately prove or disprove the possibility of a fifth force of nature. Their refined analysis sets new limits on a hypothesized particle, the extra Z-boson, carving out the lower-energy part of the discovery reach of the LHC.

Andrei Derevianko, from 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.

We carry out high-precision calculation of parity violation in cesium atom, reducing theoretical uncertainty by a factor of two compared to previous evaluations. We combine previous measurements with our calculations and extract the weak charge of the 133Cs nucleus, Q_W = -73.16(29)_exp(20)_th. The result is in agreement with the Standard Model (SM) of elementary particles. This is the most accurate to-date test of the low-energy electroweak sector of the SM. In combination with the results of high-energy collider experiments, we confirm the energy-dependence (or “running”) of the electroweak force over an energy range spanning four orders of magnitude (from ~10 MeV to ~100 GeV). Additionally, our result places constraints on a variety of new physics scenarios beyond the SM. In particular, we increase the lower limit on the masses of extra $Z$-bosons predicted by models of grand unification and string theories.

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