Physics Overview of the Nab Experiment*

From the beta decay of unpolarized free neutrons, the Nab experiment at Oak Ridge National Lab measures the electron-antineutrino angular correlation coefficient (a) and the Fierz interference term (b), as an avenue for stringent tests of the Standard Model, specifically testing the Cabibbo Kobayashi Maskawa matrix unitarity, and searching for exotic Scalar and Tensor couplings. The Nab experiment utilizes a 7 m long asymmetric time-of-flight spectrometer to reconstruct the kinematics of the decay particles and map out the allowed phase-space of the free neutron beta decay for the extraction of physics observables, a and b. The measurement principle and the operation of the experimental apparatus has been successfully demonstrated in the 2023 commissioning run, and after an 8-month upgrade of the main detector system, the Nab experiment is embarking on the data taking campaign.

In this talk, I will present the principles of measurement and extraction of the physics observables, and the preliminary result of the 2024 data taking run.