Improving Limits on the Spin Evolution Systematic for UCNτ+

The loss of ultracold neutrons (UCN) from magnetic traps as a result of spin dynamics is a systematic effect for experiments like UCNτ that measure the free neutron mean lifetime τ_n using magnetic field gradients to confine UCN without material interactions. This loss mechanism is expected to generate a correction to τ_n over an order of magnitude smaller than the ~0.01% precision targeted for probes of beyond the Standard Model physics. But because the effect is small, it is challenging to measure the loss rate directly. For UCNτ, an upper bound is determined via measurement of the trap lifetime as a function of the polarization-preserving magnetic "holding field" strength. This bound, currently 0.008% of τ_n, needs to be refined for the upgraded UCNτ+ experiment to ensure a negligible contribution to the error budget. We will present improvements in modeling combined with plans for a dedicated detector that will better constrain this systematic effect.