Speaker
Description
An intriguing anomaly has emerged in our understanding of the weak mixing of quarks, described by the Cabibbo Kobayashi Maskawa (CKM) matrix. Thanks to major strides in both theory and experiment, improved precision in determinations of the first row of matrix elements has revealed disagreement with the expectation of unitarity. The Nab experiment at the Spallation Neutron Source is designed to improve precision of the extraction of the first matrix element $V_{ud}$ and shed light on experimental tensions within the neutron beta decay dataset. Nab’s asymmetric spectrometer allows coincident reconstruction of the decay proton and electron energies, which are used to determine the electron-neutrino correlation coefficient, and thus (with the neutron lifetime) determine $V_{ud}$. This unique approach provides a more comprehensive view of neutron beta decay, including a first observation of the full phase space of the decay above 100 keV electron energy. This talk will present preliminary results from Nab’s first data collection runs and an outlook for its sensitivity in tests of CKM unitarity and to new physics beyond the Standard Model.
