# Final results for the neutron $\beta$-asymmetry parameter $A_0$ from the UCNA experiment

## TL;DR

The UCNA experiment measured the neutron beta-asymmetry parameter $A_0$ with high precision using polarized ultracold neutrons, providing a key test of the Standard Model of particle physics.

## Contribution

This paper reports the final results of the UCNA experiment, achieving a 0.67% precision measurement of $A_0$, which is the most accurate to date for this parameter.

## Key findings

- Measured $A_0$ with 0.67% precision
- Summarized results from multiple run periods
- Confirmed consistency with Standard Model predictions

## Abstract

The UCNA experiment was designed to measure the neutron $\beta$-asymmetry parameter $A_0$ using polarized ultracold neutrons (UCN). UCN produced via downscattering in solid deuterium were polarized via transport through a 7 T magnetic field, and then directed to a 1 T solenoidal electron spectrometer, where the decay electrons were detected in electron detector packages located on the two ends of the spectrometer. A value for $A_0$ was then extracted from the asymmetry in the numbers of counts in the two detector packages. We summarize all of the results from the UCNA experiment, obtained during run periods in 2007, 2008--2009, 2010, and 2011--2013, which ultimately culminated in a 0.67\% precision result for $A_0$.