The Nab Experiment: Examining Unpolarized Neutron Beta Decay Correlations

Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, λ = G_A / G_V, through multiple decay correlations. The Nab experiment will make measurements of the electron-neutrino correlation parameter a with a relative precision of δa / a ≈ 10^-3 and the Fierz interference term b, to a few x10^-3 in unpolarized free neutron beta decay. These results aim to deliver an independent determination of the ratio λ that will sensitively test CKM unitarity, as well as probe exotic electroweak scalar and tensor currents. Nab utilizes a novel, long asymmetric electromagnetic spectrometer that guides the decay products to two large area silicon detectors in order to precisely determine the electron energy and proton momentum. The Nab apparatus is finishing installation and beginning commissioning on the Fundamental Neutron Physics Beamline at the SNS at ORNL. We will present an overview and updates of the Nab experiment.