Beta spectrum of unique first-forbidden decays as a novel test for fundamental symmetries

TL;DR

This paper proposes analyzing the electron energy spectrum in unique first-forbidden beta decays as a new method to test fundamental symmetries of the weak interaction, potentially revealing physics beyond the Standard Model.

Contribution

It introduces the energy spectrum of unique first-forbidden beta decays as a novel observable sensitive to weak interaction symmetries, offering a new experimental approach to BSM physics.

Findings

Energy spectrum sensitive to weak interaction symmetries

Modest energy resolution (~20 keV) can constrain BSM tensor interactions

Provides constraints on right and left-handed coupling deviations

Abstract

Within the Standard Model, the weak interaction of quarks and leptons is characterized by certain symmetry properties, such as maximal breaking of parity and favored helicity. These are related to the $V-A$ structure of the weak interaction. These characteristics were discovered by studying correlations in the directions of the outgoing leptons in nuclear beta decays. These days, correlation measurements in nuclear beta decays are intensively studied to probe for signatures for deviations from these symmetries, which are an indication of Beyond Standard Model physics. We show that the structure of the energy spectrum of emitted electrons in unique first-forbidden $\beta$-decays is sensitive to the symmetries of the weak interaction, and thus can be used as a novel probe of physics beyond the standard model. Furthermore, the energy spectrum gives constraints both in the case of right and left coupling of the new symmetry currents. We show that a measurement with modest energy resolution of about 20 keV is expected to lead to new constraints on beyond the standard model interactions with tensor symmetry.