Limit on the Fierz Interference Term b from a Measurement of the Beta Asymmetry in Neutron Decay

TL;DR

This paper sets a new experimental limit on the Fierz interference term in neutron decay by analyzing beta asymmetry with polarized neutrons, providing a cleaner systematic approach and improving previous bounds.

Contribution

It introduces a novel measurement method for the Fierz term using neutron beta asymmetry, achieving a fourfold improvement over prior limits.

Findings

Limit on Fierz term: b = 0.017(21) at 68.27% confidence level

Method offers cleaner systematics despite lower statistical sensitivity

Significantly tighter constraints on scalar and tensor interactions in neutron decay

Abstract

In the standard model of particle physics, the weak interaction is described by vector and axial-vector couplings only. Non-zero scalar or tensor interactions would imply an additional contribution to the differential decay rate of the neutron, the Fierz interference term. We derive a limit on this hypothetical term from a measurement using spin polarized neutrons. This method is statistically less sensitive than the determination from the spectral shape but features much cleaner systematics. We obtain a limit of b = 0.017(21) at 68.27 C.L., improving the previous best limit from neutron decay by a factor of four.