The Nab Experiment: A Precision Measurement of Unpolarized Neutron Beta Decay*

Neutron beta decay is one of the most fundamental processes in nuclear physics and provides 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, $\lambda$ = $G_A$/$G_V$ , through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter $a$ with a precision of $\delta a/a$ = 10$\times 10^{-3}$ and the Fierz interference term $b$ to $\delta b$ = 3$\times 10^{-3}$ in unpolarized free neutron beta decay. These results aim to deliver an independent and competitive determination of $\lambda$ that will allow an evaluation of $V_{ud}$ and sensitively test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long asymmetric spectrometer that guides the decay electron and proton to two large area silicon detectors in order to precisely determine the electron energy and proton momentum. The Nab apparatus is under installation on the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Lab. We present the motivation, goals, and principles of the Nab experiment.