Monte-Carlo Simulation of Neutron Phase-Space Evolution in the UCNτ Experiment and Corrections on the Neutron Lifetime*

The UCNτ experiment utilizes ultracold neutrons (UCNs) and field-trapping techniques to measure the neutron lifetime with high precision. In analyzing the lifetime dataset in 2022, in which the neutrons are counted using a horizontally segmented detector, we discovered that the neutron lifetime varies across the detector segments, indicating a systematic effect associated with phase space evolution. A comprehensive Monte-Carlo model for UCN simulation within the magneto-gravitational trap will help us gain valuable insights into understanding this systematic effect. In this talk, we will present the results of Monte-Carlo simulations and show how we optimized parameters (related to the initial neutron distributions and neutron interactions with detectors) to agree with experimental observables. The optimized model is then used to study multiple systematic effects in the UCNτ experiment, such as aluminum absorption probabilities and how different detector geometries, including the 2022 segmented detector, coupled to measured spatial variations in detector response and lead to biases in neutron lifetime measurement.