Refining the Limit on Depolarization Loss Rate in the UCNτ Magneto-gravitational Trap*

By counting the number of polarized, very low energy "ultracold" neutrons (UCN) surviving confinement in a magnetic plus gravitational potential, the UCNτ collaboration has completed a measurement of the free neutron lifetime τ_n with 0.04% relative precision. An advantage of our approach is that the expected UCN loss rate due to interaction with the magnetic part of the trapping potential is negligible relative to the ∼0.01% precision targeted for probes of beyond the Standard Model physics. The actual depolarization rate in the as-built apparatus is empirically constrained via measurement of the trap lifetime as a function of the polarization-preserving "holding field" strength. We will present results from our spin-tracking simulation effort that incorporate a higher-fidelity field model in order to study the scaling of depolarization rates for smaller holding field values, better constraining the nominal loss rate.