The Neutron Beta Decay Experiment (Nab) Overview

Neutron beta decay is a fundamental nuclear process that provides a means to perform precision measurements that test the limits of our present understanding of the weak interaction described by the Standard Model of particle physics. The Nab experiment which is currently taking data at Oak Ridge National Lab, measures kinematic properties of decay products observed in unpolarized free neutron decay in order to measure ‘a’, the electron-neutrino angular correlation parameter to a relative uncertainty of △a/a∽10^-3 and ‘b’, the Fierz interference term to an absolute uncertainty of △b = 3×10^-3. Precision measurements of these quantities can be used to probe the unitarity of the Cabibbo-Kobayashi-Maskawa matrix, address tensions in the neutron lifetime puzzle and to search for Scalar and Tensor couplings which extend beyond the Standard Model. The Nab experiment uses an asymmetric time-of-flight spectrometer and a pair segmented silicon detectors to reconstruct the kinematic observables of neutron beta decay products, effectively sampling the allowed phase space for free neutron beta decay from which quantities a and b are extracted. This talk will cover the principles of measurement with the Nab spectrometer, the methods of extraction of physical observables, ‘a’ and ‘b’, and preliminary results from data taking.