Analysis of an Asymmetric Field-Expansion Magnetic Spectrometer for Free Neutron Beta Decay Products

Neutron beta decay, one of the most fundamental processes in subatomic physics, offers significant insights into the electroweak sector of the Standard Model (SM) and its potential extensions. The experimental study of neutron beta decay is crucial for various applications in the theory of weak interactions, particularly in astrophysics. Precise measurements of neutron beta decay parameters are vital for the ongoing search for new physics.

The Nab experiment at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) aims to achieve the most precise measurements to date (10^-3 precision) of the electron-neutrino correlation parameter a and the Fierz term b. Central to this endeavor is a custom cryogenic spectrometer designed to collect protons, guide them, and correlate their time-of-flight with their momentum through adiabatic longitudinalization. To minimize the variance in the trajectories of individual protons, the magnetic field intensifies to approximately 4 Tesla immediately after the decay region.

Our goal is to computationally model the magnetic field with error margins at or below 0.1% when compared with mapping measurements. This poster presents finite-element calculations of the Nab magnetic field using COMSOL Multiphysics and compares these calculations with analytical calculations and measurements conducted by the Nab collaboration.