# Evaluating $V_{ud}$ from neutron beta decays

## TL;DR

This paper reviews the status and progress of experimental efforts to precisely measure neutron beta decay parameters, aiming to determine the CKM matrix element V_{ud} and test Standard Model predictions.

## Contribution

It provides an overview of current neutron decay experiments, their precision levels, and upcoming improvements for independent V_{ud} determination.

## Key findings

- Current neutron decay measurements constrain V_{ud}
- Experimental techniques are advancing for higher precision
- Future experiments aim to reduce uncertainties significantly

## Abstract

Although well studied, the neutron still offers a unique laboratory for precise tests of Standard Model (SM) predictions. Neutron decay is free of nuclear structure corrections present in nuclear beta decays, and, with a $10^8$ times larger branching ratio than the theoretically cleaner pion beta decay, it is more readily accessible to experimental study than the latter. Measurements at sufficient precision of the neutron lifetime, and of correlations in free neutron beta decay, offer several stringent tests of the SM, including the weak quark couplings (quark-lepton universality), and certain extensions beyond the standard $V-A$ weak interaction theory. This paper focuses on the long-running free neutron beta decay experimental program aimed at obtaining an independent determination of the Cabibbo-Kobayashi-Maskawa (CKM) mixing matrix element $V_{ud}$. We discuss the present state of precision achieved in this program and briefly review the currently active projects, as well as the expected near-term improvements in the field.