Progress towards a precision measurement of neutron decay correlations with the Nab experiment

Correlations between the products of free neutron decay provide insights into the underlying weak interaction. The Nab spectrometer is designed to measure the electron-neutrino correlation term a from a neutron beam at the Spallation Neutron Source with unprecedented precision, which, in combination with a precise measurement of the neutron lifetime, can test CKM-matrix unitarity. In addition, Nab is sensitive to the Fierz interference term b, which might indicate new tensor couplings beyond the Standard Model of particle physics.

We recently demonstrated that Nab is capable of fully reconstructing the neutron decay kinematics, a prerequisite to determine a, and are now transitioning to taking precision data. This presentation will introduce the principles of the spectrometer and provide an update on the progress towards a precision measurement.