Constraining P and CP violation in the main decay of the neutral Sigma hyperon

TL;DR

This paper investigates the potential for observing CP violation in the decay of the neutral Sigma hyperon by analyzing angular asymmetries, concluding that current experimental limits are too low to detect such effects, implying any observation would indicate new physics.

Contribution

The study provides theoretical constraints on CP violation in Sigma hyperon decay, linking it to neutron EDM limits and highlighting the challenges for future experimental detection.

Findings

Current upper limits on asymmetry are below experimental resolution.

Any observed CP asymmetry would suggest physics beyond the Standard Model.

Theoretical bounds are derived from flavor SU(3) symmetry and neutron EDM constraints.

Abstract

On general grounds based on quantum field theory the decay amplitude for $\Sigma^0 \to \Lambda \gamma$ consists of a parity conserving magnetic and a parity violating electric dipole transition moment. Because of the subsequent self-analyzing weak decay of the $\Lambda$ hyperon the interference between magnetic and electric dipole transition moment leads to an asymmetry in the angular distribution. Comparing the decay distributions for the $\Sigma^0$ hyperon and its antiparticle gives access to possible C and CP violation. Based on flavor SU(3) symmetry the present upper limit on the neutron electric dipole moment can be translated to an upper limit for the angular asymmetry. It turns out to be far below any experimental resolution that one can expect in the foreseeable future. Thus any true observation of a CP violating angular asymmetry would constitute physics beyond the standard model, even if extended by a CP violating QCD theta-vacuum-angle term.