Fundamental Tests of P and CP Symmetries Using Octet Baryons at the $J/\psi$ Threshold

TL;DR

This paper explores testing P and CP symmetries via angular distributions in $J/a$ decays at BESIII and future facilities, providing new bounds on weak mixing angles and baryon electric dipole moments, with implications for fundamental symmetry violations.

Contribution

It introduces a novel method to measure weak mixing angles and baryon electric dipole moments using $J/a$ decays, achieving unprecedented sensitivity and setting new bounds.

Findings

Parity-violating asymmetries are around 10^{-4}, barely observable at current data levels.

Current data allows measurement of the weak mixing angle with an uncertainty of about 0.09.

Future facilities could improve the bounds on the weak mixing angle to 0.005 and baryon EDMs by two to three orders of magnitude.

Abstract

We systematically investigate tests of the parity P and the combined parity and charge-conjugate CP symmetries from differential angular distributions of $J/\psi$ decaying into the lowest-lying baryon pairs at BESIII and the next-generation super tau-charm facilities (STCFs). Large corrections from $Z$ and $W$ exchange induced parity violating effects are found for $J/\psi$ decays with large logarithms resummed up to $\mathcal{O}(\alpha_s)$. The parity-violating asymmetries on the production and the decay sides of $J/\psi$ are both estimated to be of $\mathcal{O}(10^{-4})$, thus barely observable with the 10 billion $J/\psi$ events currently collected at BESIII. Nevertheless, these asymmetries utilizing the current BESIII data already permit a measurement of the weak mixing angle with an absolute uncertainty $\delta s_w^2\approx{0.09}$, corresponding to the first determination of $s_w^2$ at the $J/\psi$ threshold. In the future, STCFs are estimated to improve this bound by a factor of $\sim\,20$ to $\delta s_w^2\approx{0.005}$ within one year based on luminosity rescaling. We also obtain the 95% confidence level upper bounds on the electric dipole moments of the octet baryons, which are of $\mathcal{O}(10^{-18})\,e{\rm\,cm}$ for BESIII and $\mathcal{O}(10^{-19})\,e{\rm\,cm}$ for STCFs. These bounds are improved by two to three orders of magnitude in comparison with the only existing one on $\Lambda$ from Fermilab. The method discussed in this work also paves a way for a first and direct measurement of the $\Xi$ and $\Sigma$ electric dipole moments.