Toward attaining near zero neutron beam polarization in the Nab experiment at SNS

High precision measurement of beta decay observables presents means to look for beyond standard model physics. Specifically, measurement of beta-neutrino correlation coefficient(a), in neutron beta decay can provide input for Cabibbo-Kobayashi-Maskawa (CKM) matrix unitarity condition. In this regard, the Nab experiment stationed at Spallation Neutron Source (SNS) in Oak Ridge National Lab aims to measure 'a' and Fierz interference term(b) in an unpolarized free neutron decay with the relative uncertainty of ∆a/a < 10⁻³ and ∆b/b < 3×10⁻³, respectively. One of the major sources of systematic uncertainty in such a measurement is the degree of unwanted polarization of the neutron beam. To reach the precision goal of Nab we must be able to restrict the unwanted polarization of the beam below 3×10⁻⁵, i.e, ∆P ≤ 3 × 10⁻⁵. To achieve the small beam polarization experimentally, we employ a ³He cell to measure the polarization of the SNS beam, and an Adiabatic Fast Passage(AFP) neutron spin flipper to reduce the beam polarization to the Nab requirements. In this talk, we present details on the process of doing this measurement and the results from the last measurement.