Semiclassical Simulation of Spin Evolution in the UCNτ Experiment

The free neutron lifetime τ_n is a β-decay observable used in Big Bang Nucleosynthesis predictions of light element abundances, and along with other β-decay observables allows testing of the unitarity of the CKM matrix. The goal of the UCNτ experiment is to measure τ_n with a maximum uncertainty of 0.01%. The experiment uses a bowl-shaped, permanent magnet Halbach array within a vacuum jacket to hold low-field-seeking, ultracold neutrons (UCN). To achieve a high precision, UCN must not leave the trap for reasons other than β-decay, such as depolarization. Depolarization is when UCN become high-field-seeking and get sucked into the magnets rather than repelled by them. To minimize depolarization, the vacuum jacket is surrounded by coils that produce a magnetic holding field. Two simulations were developed to model UCN spin dynamics in the trap; one uses a Monte Carlo Wave Function (MCWF) approach and the other integrates the Bloch equations to evolve the expectation value of the spin. These were first applied in modeling depolarization rate dependence on holding field strength. Calculations will be presented using the semiclassical approach and compared to results of the MCWF approach and empirical data.