A Precision Measurement of Zero Beam Polarization in the Nab Experiment at the SNS*

The Nab Experiment at the Fundamental Physics Beamline (FnPB) at the Spallation Neutron Source (SNS) aims to precisely measure the electron-neutrino correlation coefficient a,  and the Fierz interference term b, associated with the beta decay of free unpolarized neutrons. This measurement provides a cross-check and independent measurement of λ, the ratio of vector to axial-vector coupling constants in the Standard Model. A serious potential source of systematic error in this experiment stems from unwanted residual polarization in the incident neutron beam. We must be able to hold the polarization of the beam below 3*10^-5. The experimental approach to measuring and mitigating this small polarization uses a polarized ³He spin filter, and an Adiabatic Fast Passage (AFP) neutron spin flipper. Here we present several details on the process of taking this measurement and some preliminary measurement data.