Precision determination of electroweak coupling from atomic parity violation and implications for particle physics

TL;DR

This paper presents a high-precision calculation of atomic parity violation in cesium, confirming the Standard Model predictions and constraining new physics beyond it, including limits on extra Z-boson masses.

Contribution

It provides the most accurate measurement of the weak charge of cesium and confirms the running of the electroweak force over a wide energy range.

Findings

Agreement with Standard Model within uncertainties

Confirmation of electroweak force running from low to high energies

Stronger constraints on new physics, including Z-boson mass limits

Abstract

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.