Understanding the UCNτ Neutron Lifetime Experiment Through Simulations of Trapped Neutrons

The UCNτ experiment at Los Alamos National Lab measures the neutron β-decay lifetime by storing ultracold neutrons (UCNs) in a magneto-gravitational trap for holding times longer than the neutron's lifetime. Systematic effects can stem from changes in the UCN phase space distribution between different holding times, and potential exposure of neutrons to additional loss mechanisms. We have utilized a Monte Carlo simulation of UCN trajectories to understand this evolution, and constrain possible sources of loss. Additionally, this simulation allows us to model the UCN capture efficiency of the in-situ dagger detector. The simulation uses Indiana University's Big Red II supercomputer to symplectically integrate neutrons in a magnetic potential, derived from an analytic expression for the trap's field. By adjusting simulated trap characteristics such as heating amount or cleaning height, we can look for shifts in the measured lifetime. We will present results of these simulations, as part of an effort to reduce UCNτ's total uncertainty to about 0.2 seconds.