The Nab Experiment: Present Status

The Nab experiment, currently taking data at the Spallation Neutron Source, will precisely measure the neutron beta decay correlation coefficients $a$, relating the momenta of the produced electron and antineutrino, and $b$, the Fierz interference term. Precise extraction of these two parameters can probe physics beyond the Standard Model by investigating CKM unitarity and the existence of scalar and tensor currents in the weak interaction. The Nab experiment uses the world's largest cryogen-free superconducting magnet to guide electrons and protons from neutron decays into one of two silicon detectors. The electron energy and the difference in time of flight between electrons and protons in coincident decays can be used to determine $a$ and $b$. This talk will describe Nab's principles of operation and introduce some of the systematic considerations required for a precision measurement. We will discuss Nab's ongoing commissioning, and will provide a first look at neutron decay data taken during the most recent campaign.