The Nab experiment at the Spallation Neutron Source*

Neutron beta-decay, in which a neutron decays to a proton, electron and antineutrino, is governed by the weak interaction.  This process is extremely precisely characterized (at about 2 parts in 10⁴) in the standard model of particle physics.  Because the weak force results from the exchange of remarkably massive particles (in this case the W- boson with a mass-energy of 80.4 GeV, about 80 times of the mass of a proton), high precision measurements can probe for new interactions due to even more massive particles, above the particle production scales reachable even by the Large Hadron Collider.  At present, there are numerous signs that something doesn't "fit" with many related weak decay processes for tau particles, kaons and nuclei, collectively referred to as the Cabibbo Anomaly. This anomaly strongly motivates improving our knowledge of neutron beta decay, with an especially pronounced need for improved measurements of the angular distribution of the decay products.  The Nab experiment, based at the Spallation Neutron Source in Oak Ridge Tennessee, is designed to determine decay angular correlations through measurements of the emitted electron and the recoiling proton in coincidence.  Combining measurements (for each decay) of the electron energy with the time-of-flight of the proton, the Nab experiment will determine the "beta-neutrino" angular correlation parameter, "a_βν".  The precision goal for this parameter is 0.1%, making it by far the most precise measurement the a_βν coefficient in neutron decay and providing critical data concerning the Cabibbo Anomaly.  The motivation, principles of measurement, and the current status of the Nab experiment will be presented.