Improved limits on Fierz Interference using asymmetry measurements from the UCNA experiment

TL;DR

This paper reports improved experimental limits on the Fierz interference term in neutron beta decay by analyzing asymmetry data from the UCNA experiment, enhancing constraints on possible new physics beyond the Standard Model.

Contribution

The study provides the first combined analysis of multiple UCNA datasets to set tighter limits on the Fierz interference term in neutron decay.

Findings

Measured $b_{n} = 0.066 \, extpm \, 0.041_{\text{stat}} \, \pm \, 0.024_{\text{syst}}$

Established a 90% confidence interval for $b_{n}$ as $-0.012$ to $0.144$

Improved constraints on physics beyond the Standard Model in neutron decay

Abstract

The Ultracold Neutron Asymmetry (UCNA) experiment was designed to measure the $\beta$-decay asymmetry parameter, $A_0$, for free neutron decay. In the experiment, polarized ultracold neutrons are transported into a decay trap, and their $\beta$-decay electrons are detected with $\approx 4\pi$ acceptance into two detector packages which provide position and energy reconstruction. The experiment also has sensitivity to $b_{n}$, the Fierz interference term in the neutron $\beta$-decay rate. In this work, we determine $b_{n}$ from the energy dependence of $A_0$ using the data taken during the UCNA 2011-2013 run. In addition, we present the same type of analysis using the earlier 2010 $A$ dataset. Motivated by improved statistics and comparable systematic errors compared to the 2010 data-taking run, we present a new $b_{n}$ measurement using the weighted average of our asymmetry dataset fits, to obtain $b_{n} = 0.066 \pm 0.041_{\text{stat}} \pm 0.024_{\text{syst}}$ which corresponds to a limit of $-0.012 < b_{n} < 0.144$ at the 90% confidence level.