NSF Award
2402275
Neutrinos, the universe's lightest particles, are notoriously hard to detect but nonetheless important. Their unknown properties, including their masses, their interactions with other as yet unobserved particles, and whether they are their own antiparticles could explain why the universe contains so little antimatter. Experiments to observe a process called neutrinoless double-beta decay, in which two neutrons inside an atomic nucleus change into protons while emitting no neutrinos (in contrast to the usual two) can help us learn; the decay can occur only if neutrinos are their own antiparticles, and its rate reflects the properties both of neutrinos themselves and of undiscovered particles they might interact with. What we learn will be limited, however, without a better understanding of what happens inside the nucleus that hosts the decay. This understanding, at a quantitative level with a good estimate of uncertainty, is the FRHTP's goal. The project connects theorists across the country to achieve it, and at the same time provides postdocs with intensive training in several areas at the interface of nuclear and particle physics. It also supplies experiments and information to K-12 students across the country, helping them understand what it's like to work on a cutting-edge scientific problem.<br/><br/>The effect of the nucleus on the decay can be summarized in a "matrix element" between the initial and final states of a decay operator. Both determining the operator and reliably computing its matrix element are challenges. The operator depends on the particle physics causing the decay, and on the behavior of quarks and gluons inside nucleons. The operator's matrix element depends on the structure of the nucleus, which is made up of those nucleons. Thus, an accurate and precise calculation of the matrix element (a restatement of the Hub's goal) requires coordinated work in particle phenomenology, quantum chromodynamics, and nuclear-structure theory, as well as the use of effective field theory to bridge the energy scales associated with new particles, nucleons, and nuclei. It also requires statistical expertise to combine uncertainties at each of the scales and from several computational methods into an overall error estimate. The Hub uses lattice quantum chromodynamics, three different ab initio nuclear-structure methods, chiral effective field theory, and Bayesian model mixing to produce matrix elements for isotopes used in experiments that are vastly more accurate than anything presented earlier. Bayesian statistical techniques result in a reliable uncertainty estimate, something that has never been achieved before.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
