Measurement of Transverse Polarization of Electrons Emitted in Free Neutron Decay

TL;DR

This paper reports precise measurements of the transverse polarization components of electrons emitted in free neutron beta decay, testing fundamental symmetries and constraining new physics beyond the Standard Model.

Contribution

It provides the most accurate measurements to date of the transverse polarization components, improving limits on scalar and tensor couplings in weak interactions.

Findings

The T-odd correlation coefficient R is consistent with zero, supporting time reversal invariance.

The correlation coefficient N agrees with Standard Model predictions.

New constraints are placed on imaginary scalar and tensor couplings in weak interactions.

Abstract

The final analysis of the experiment determining both components of the transverse polarization of electrons ($\sigma_{T_{1}}$, $\sigma_{T_{2}}$) emitted in the $\beta$-decay of polarized, free neutrons is presented. The T-odd, P-odd correlation coefficient quantifying $\sigma_{T_{2}}$, perpendicular to the neutron polarization and electron momentum, was found to be $R=$ 0.004$\pm0.012\pm$0.005. This value is consistent with time reversal invariance, and significantly improves both earlier result and limits on the relative strength of imaginary scalar couplings in the weak interaction. The value obtained for the correlation coefficient associated with $\sigma_{T_{1}}$, $N=$ 0.067$\pm0.011\pm$0.004, agrees with the Standard Model expectation, providing an important sensitivity test of the experimental setup. The present result sets constraints on the imaginary part of scalar and tensor couplings in weak interaction. Implications for parameters of the leptoquark exchange model and minimal supersymmetric model (MSSM) with R-parity violation are discussed.