# Experimental Upper Bound and Theoretical Expectations for Parity-Violating Neutron Spin Rotation in 4He

## TL;DR

This paper reports an experiment measuring neutron spin rotation in helium-4 to detect parity violation, finding results consistent with zero and aligning with theoretical predictions of weak nucleon interactions.

## Contribution

It provides the first experimental upper bound on neutron spin rotation in helium-4 and discusses systematic analysis methods for future precision measurements.

## Key findings

- Measured neutron spin rotation angle consistent with zero
- Results agree with theoretical estimates of weak nucleon-nucleon interactions
- Developed analysis tools for systematic uncertainty quantification

## Abstract

Neutron spin rotation is expected from quark-quark weak interactions in the Standard Model, which induce weak interactions among nucleons that violate parity. We present the results from an experiment searching for the effect of parity violation via the spin rotation of polarized neutrons in a liquid $^{4}$He medium. The value for the neutron spin rotation angle per unit length in $^{4}$He, $d\phi/dz =(+2.1 \pm 8.3 (stat.) \pm 2.9 (sys.))\times10^{-7}$ rad/m, is consistent with zero. The result agrees with the best current theoretical estimates of the size of nucleon-nucleon weak amplitudes from other experiments and with the expectations from recent theoretical approaches to weak nucleon-nucleon interactions. In this paper we review the theoretical status of parity violation in the $\vec{n}+^{4}$He system and discuss details of the data analysis leading to the quoted result. Analysis tools are presented that quantify systematic uncertainties in this measurement and that are expected to be essential for future measurements.