Measurement of neutron beam polarization at SNS*

Accurate measurement of beta decay observables offers an opportunity to explore physics beyond the standard model. Specifically, studying the beta-neutrino correlation coefficient (a) in neutron beta decay can provide valuable information related to the unitarity condition of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. To achieve this, the Nab experiment, based at the Spallation Neutron Source (SNS) in Oak Ridge National Lab, aims to measure beta neutrino correlation co-efficient and the Fierz interference term (b) in an unpolarized free neutron decay with a relative uncertainty of ∆a/a < 10⁻³ and ∆b/b < 3×10⁻³, respectively. When making this measurement, it's essential to account for the unwanted polarization of the neutron beam, which is a significant source of systematic uncertainty. To meet the precision goal of Nab, we need to ensure that the unwanted polarization of the beam is restricted to below ∆P ≤ 2 × 10⁻⁵.

 

To achieve the small beam polarization experimentally, we utilize a ³He cell to determine the polarization of the SNS beam and an Adiabatic Fast Passage (AFP) neutron spin flipper to reduce any net remaining beam polarization. During August 2022, we conducted the initial beam polarization measurements utilizing a ³He cell, without using the spin flipper. In this presentation I will discuss the outcomes of this measurement.