# Improved determination of the $\beta$-$\overline{\nu}_e$ angular correlation coefficient $a$ in free neutron decay with the $a$SPECT spectrometer

## TL;DR

This paper reports a highly precise measurement of the neutron beta-decay electron-antineutrino angular correlation coefficient using the $a$SPECT spectrometer, improving the accuracy of fundamental weak interaction parameters.

## Contribution

The study presents the most precise measurement of the neutron $a$ coefficient, utilizing a novel spectrometer setup and comprehensive systematic error analysis.

## Key findings

- Measured $a = -0.10430(84)$, the most precise to date.
- Derived $| ext{lambda}| = 1.2677(28)$ from the $a$ coefficient.
- Confirmed consistency with the Standard Model predictions.

## Abstract

We report on a precise measurement of the electron-antineutrino angular correlation ($a$ coefficient) in free neutron beta-decay from the $a$SPECT experiment. The $a$ coefficient is inferred from the recoil energy spectrum of the protons which are detected in 4$\pi$ by the $a$SPECT spectrometer using magnetic adiabatic collimation with an electrostatic filter. Data are presented from a 100 days run at the Institut Laue Langevin in 2013. The sources of systematic errors are considered and included in the final result. We obtain $a = -0.10430(84)$ which is the most precise measurement of the neutron $a$ coefficient to date. From this, the ratio of axial-vector to vector coupling constants is derived giving $|\lambda| = 1.2677(28)$.