In a recent Physical Review Letters paper, Chien-Yeah Seng—a visiting scholar at the University of Washington, a visiting assistant professor at FRIB, and an FRIB Theory Alliance Fellow—explains how physicists see new hopes to resolve an outstanding puzzle in low-energy nuclear physics. The puzzle is known as the first-row Cabibbo-Kobayashi-Maskawa (CKM) matrix unitarity deficit. It may be solved via experimental measurements of nuclear charge distributions (or how protons distribute themselves in a nucleus).  

The anomaly concerns the determination of a quantity known as Vud. Vud governs the probability of a proton bounded in a nucleus to decay weakly into a neutron. The extracted value of Vud from nuclear beta decays seems to be significantly smaller than what required by the Standard Model, the commonly-acknowledged best theory for elementary particle physics. This observed anomaly stimulates discussions of possibilities of new physics discovery.

To study its origin, physicists investigated the so-called nuclear weak distribution. The nuclear-weak distribution describes the distribution of “active” protons in a nucleus eligible to transition weakly into a neutron. They found that the current understanding of this distribution is based on some oversimplified modelings, which assume non-interacting nucleons inside a nucleus.

Furthermore, they showed that this distribution may be determined model-independently from measurements of nuclear charge distributions. Upon analyzing existing data, they found that the value of Vud moved closer to the Standard Model prediction. Future measurements of nuclear charge distribution, for example at FRIB, may provide further evidence towards the eventual resolution of the unitarity deficit.

This material is based upon work supported by the U.S. Department of Energy Office of Science Office of Nuclear Physics.

Michigan State University (MSU) operates the Facility for Rare Isotope Beams (FRIB) as a user facility for the U.S. Department of Energy Office of Science (DOE-SC), supporting the mission of the DOE-SC Office of Nuclear Physics. User facility operation is supported by the DOE-SC Office of Nuclear Physics as one of 28 DOE-SC user facilities.

The U.S. Department of Energy Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of today’s most pressing challenges. For more information, visit energy.gov/science.