From Commissioning to Precision Data-Taking: Resolving Operational Challenges in the Nab Detector Systems

TL;DR

The Nab experiment aims to resolve discrepancies in the CKM matrix by precisely measuring neutron beta decay parameters, with recent detector upgrades enhancing data quality for high-precision physics analysis.

Contribution

This paper reports on operational improvements and detector upgrades that enable high-precision measurements of neutron decay parameters to test CKM matrix unitarity.

Findings

Improved detector robustness and stability for proton detection

First observation of full neutron decay momentum phase space

Enhanced limits on exotic neutron decay states

Abstract

Our understanding of the weak mixing of quarks, described by the Cabibbo Kobayashi Maskawa (CKM) matrix, currently presents an anomaly. Thanks to major strides in both theory and experiment, improved precision in determinations of the first row of matrix elements has revealed disagreement with the expectation of unitarity. The Nab experiment at the Spallation Neutron Source is designed to precisely extract the first matrix element $V_{ud}$ and shed light on experimental tensions within the neutron beta decay dataset. Nab's asymmetric spectrometer allows coincident reconstruction of the decay proton and electron energies, which will be used to determine the electron-neutrino correlation coefficient, and thus (with the neutron lifetime) determine $V_{ud}$. This unique approach has provided a more comprehensive view of neutron beta decay, including a first observation of the full momentum phase space of the decay above detector thresholds and limits on exotic neutron states. Recent upgrades to the Nab detector system have improved the robustness and stability of the detector performance in terms of proton detection efficiency, noise performance, and detector segment availability, setting the stage for high precision physics data-taking.