- By:
- Broussard, Leah J; Baessler, Stefan F; Benoit, Mathieu ; Ericson, Milton N; Fabris, Lorenzo ; Gonzalez, Francisco M; Hollander, Sean T; Mathews, David G; Ramsey, John C; Reed, Frederick K; Saunders, Alexander ; Schreyer, Wolfgang S
- Journal Name:
- EPJ Web of Conferences
- Page Number:
- 4001
- Volume:
- 378
- Publication Date:
- September 16, 2026
- Conference Name:
- 29th International Nuclear Physics Conference (INPC 2025)
- Conference Location:
- Daejeon, South Korea
- Conference Sponsor:
- Center for Exotic Nuclear Studies (CENS) and the Center for Underground Physics (CUP) at the Institute for Basic Science (IBS) and the Center for Extreme Nuclear Matters (CENuM) at Korea University.
- View DOI Listing:
- https://doi.org/10.1051/epjconf/202637804001
Abstract
Our understanding of the weak mixing of quarks, described by the Cabibbo-Kobayashi-Maskawa (CKM) matrix, currently presents an anomaly. 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 precisely extract the first matrix element Vud 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 will be used to determine the electron-neutrino correlation coefficient, and thus (with the neutron lifetime) determine Vud. This unique approach has provided a more comprehensive view of neutron beta decay, including a first observation of the full momentum phase space of the decay above detector thresholds and limits on exotic neutron states. Recent upgrades to the Nab detector system have improved the robustness and stability of the detector performance in terms of proton detection efficiency, noise performance, and detector segment availability, setting the stage for high precision physics data-taking.
