Magnetometry Methodology of the Nab Experiment

The Nab experiment aims to measure the neutron beta decay electron-neutrino correlation coefficient $a$   to a relative uncertainty of  $10^{-3}$  in order to extract $ \lambda$, the ratio of axial to vector coupling constant, at roughly the same precision level as the vector coupling determined from superallowed decays. An additional measurement of $b$ with an uncertainty of $3 \times 10^{-3}$ would provide a sensitivity that could access new physics beyond the standard model. Nab uses an asymmetric magnetic spectrometer and two large-area highly segmented Si detectors to extract $a$ from the proton momentum and electron energy and $b$ from the electron energy spectrum. The design of the magnet spectrometer aligns the proton momentum perpendicularly to the Si detectors. Our measurement of the proton momentum depends strongly on the path length of the charged particle in the spectrometer, so a detailed mapping and analysis of the magnetic field is needed. This talk will discuss our method of mapping the magnetic field within the spectrometer and how the analysis of this field influences our measurement of $a$.