Enhancing Neutron Beta-Asymmetry Precision: Developments in Geant4 Simulations for the UCNA+ Experiment*

The UCNA+ experiment at Los Alamos National Laboratory is an upgrade to the original UCNA experiment, aiming to measure the neutron beta asymmetry A₀ with a precision of 0.2% or better. A measurement at this precision will permit a determination of the axial coupling constant for neutron decay which is competitive to the most precise to date, and push the global neutron dataset closer to a definitive, nuclear structure-independent value for the CK matrix element Vud. A reliable value for this parameter is one of the critical ingredients required to unravel the source of the Cabbibo Anomaly, one of the strongest challenges to  high precision analysis of the standard model through weak decays. UCNA+ incorporates enhancements such as a higher density ultracold neutron (UCN) source, a detector design which eliminates almost all ”dead layers” on the detector package and SiPM readout which should improve light yield and stability, benchmarked electron backscattering corrections.

In this study, we present results from GEANT4 simulations of the UCNA+ experimental setup, focusing on necessary backscattering corrections. We investigate the impact of the dead layer on angular effects, and explore the influence of different detector thresholds and analysis windows on these corrections. Our analysis includes a comparison of these corrections between UCNA and UCNA+, providing a benchmark for the simulation code. These improvements aim to significantly reduce systematic uncertainties, advancing the precision of neutron beta asymmetry measurements.