Precision Experiments with Cold Neutrons

University Of Virginia Main Campus, Charlottesville VA

Investigators

Abstract

This grant supports a program of precise experiments with low energy (cold) neutrons. With the first project, the Nab and pNab experiments, the PI and his team continue a new study of neutron beta decay. The goals of this project tackle two of three items in a list of priorities listed in the Weak Interaction section of the new Long-Range Plan for Nuclear Science. The main purpose of the measurements is to shed light on recent evidence for a failure of the Standard Model of Elementary Particle Physics. The discovery of Beyond-Standard-Model physics would have important implications for High Energy Physics and Cosmology. With the second project, the PI and his team study states of cold neutrons with the goal to detect new short-range forces. Furthermore, the project has a valuable impact on higher education: Most of the work is done by students at undergraduate and graduate levels, and these students will earn valuable skills and experiences for their future career. The redundancy inherent in the Standard Model description of the neutron beta decay process allows uniquely sensitive checks of the model’s validity and limits: The neutrino-electron-correlation coefficient “” will allow an extraction of “ud”, the upper left element of the CKM matrix, with the purpose to test the matrix’s unitarity. A recent re-evaluation of radiative corrections made it appear that the CKM unitarity is violated. A non-unitarity CKM matrix is not possible within the SM. It would signal that either physics at a scale of several TeV affects how CKM matrix elements are computed from observables, or that we have more than three quark generation even if those extra quarks have masses that makes them undetectable directly. The other observable, the Fierz term, allows verifying hints for non-SM effective scalar or tensor interactions at low energies, in a complementary way to what is expected from LHC. Measurements of electron and proton asymmetry with a polarized beam with pNab add precision and redundancy. The studies are performed by the Nab collaboration, of which the PI is the experiment director. The grant funds the PI’s group’s specific contributions to the Nab and pNab project in leadership, simulation, data analysis, neutron beam polarization and in the characterization of the proton detection efficiency. With the whispering gallery experiment, spectroscopy of gravitationally bound quantum states of ultracold neutrons will be performed. The results are sensitive to new short- range interactions at a length scale of nanometers which could be mediated by so far undiscovered pseudoscalar bosons (axion-like particles). Those would contribute to the answer to the question about the nature of dark matter. The PI is strongly committed to attracting members of underrepresented groups and is leading a diverse research group. The PI will try to obtain funding for a REU program at UVa. 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.