Speaker
Description
Exchange of two neutrinos and other light fermions generates a parity-violating dispersion interaction between electrons and quarks. Although the long-range part of this interaction scales as $G_F/r^5$, its highly singular short-distance behaviour produces a correction of order $G_F^2 M_Z^2\sim \alpha G_F$, comparable to standard electroweak radiative corrections. This makes the effect relevant for precision tests of the Standard Model in atomic parity violation and low-energy electron scattering.
We calculate the effective short-distance parity-violating potential generated by fermion-loop diagrams and express its contribution as a correction to the nuclear weak charge. Including loops of neutrinos, charged leptons, and quarks lighter than the $Z$ boson gives an effective fermion-loop enhancement, leading to a correction of approximately −0.8% to weak charges in medium and heavy atoms. In cesium, this correction is larger than the present experimental uncertainty and shifts the Standard Model prediction into agreement with the measured parity-violating amplitude, removing the existing $2\sigma$ discrepancy. The corresponding extracted weak mixing angle becomes
$$
\sin^2θ_W=0.2375(19)
$$
at $q^2\approx 0$, consistent with the Standard Model value.
The same mechanism gives a larger relative correction, about 3%, to the proton weak charge, relevant for parity-violating electron–proton scattering. We also discuss implications for searches for new physics, including improved limits on an additional $Z'$ boson and a low-energy constraint on the Peskin–Takeuchi oblique parameter, $S=−0.32(53)$. These results show that dispersion parity-violating interactions must be included in future high-precision analyses of atomic parity violation and low-energy electroweak observables.
| I am the presenting author | Yes |
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