Simulating the UCN$\tau$ Neutron Lifetime Experiment

The UCN$\tau$ experiment at Los Alamos National Laboratory measures the neutron lifetime by storing ultracold neutrons (UCNs) in a magneto-gravitational trap for variable holding times, which can be longer than the neutron’s lifetime. Systematic effects can occur due to changes in the UCN phase space distribution between different holding times, potentially exposing neutrons to loss mechanisms besides the fundamental neutron decay rate, or changing the detection efficiency of UCN in the trap. In order to minimize and understand possible sources of loss, we have utilized a Monte Carlo simulation of UCN trajectories. 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 modifying characteristics of the simulated trap, such as heating amount or cleaning height, we can look for effects on the measured lifetime. We will present results of these simulations as part of an effort to reduce UCN$\tau$’s total uncertainty to about 0.2 seconds.