Simulations and Fitting Methods in the Nab Experiment

The Nab collaboration at the Spallation Neutron Source (SNS) aims to measure the electron-neutrino correlation parameter $a$ with a relative uncertainty of about $10^{-3}$, and the Fierz interference term $b$ with absolute uncertainty of $3\times 10^{-3}$. Nab will use a novel time-of-flight magnetic field spectrometer to guide the charged decay products to two segmented Si detectors. In the Nab experiment, $a$ is determined by combined precise determinations of the electron energy and the proton time-of-flight. In this talk, we will present a fitting method to analyze $a$, as well as specific techniques developed to increase the simulation speed while keeping the required precision in the Geant 4 simulation.