Radiative Beta Decay for Studies of CP Violation

TL;DR

This paper explores how radiative beta decay can be used to detect CP violation beyond the standard model by analyzing a specific triple-product correlation sensitive to new physics effects.

Contribution

It calculates the expected asymmetry in neutron radiative beta decay due to a pseudo-Chern-Simons term and assesses its potential to reveal new sources of CP violation.

Findings

Asymmetry size computed in chiral effective theory

Background from standard-model interactions evaluated

Potential to constrain new physics scenarios

Abstract

A triple-product correlation in the radiative \beta-decay rate of neutrons or of nuclei, characterized by the kinematic variable \xi\equiv (\vec{l}_\nu\times\vec{l}_e)\cdot\vec{k}, where, e.g., n(p) \to p(p') + e^-(l_e) + \bar{\nu}_e(l_\nu) + \gamma(k), can be generated by the pseudo-Chern-Simons term found by Harvey, Hill, and Hill as a consequence of the baryon vector current anomaly and SU(2)_{L} x U(1)_Y gauge invariance at low energies. The correlation probes the imaginary part of its coupling constant, so that its observation at anticipated levels of sensitivity would reflect the presence of sources of CP violation beyond the standard model. We compute the size of the asymmetry in n\to p e^- \bar\nu_e \gamma decay in chiral effective theory, compare it with the computed background from standard-model final-state interactions, and consider the new physics scenarios which would be limited by its experimental study.