Precision Magnetometry and Systematic Effects in the Nab Experiment

The Nab experiment will determine the electron-neutrino correlation parameter $a$ with a precision of $\delta a / a = 10^{-3}$ and the Fierz interference term $b$ to $\delta b = 3\times10^{-3}$ in unpolarized neutron $\beta$ decay. A long asymmetric spectrometer is optimized to achieve fast proton momentum longitudinalization and the required narrow proton momentum response function. A reliable relation of the measured proton TOF to $a$ requires detailed knowledge of the effective proton pathlength, which imposes requirements on the precision of the magnetic fields in the Nab spectrometer. The Nab magnetometry goals, associated systematics, and some initial results will be discussed.