Studies of the Short-Range Nuclear Interaction in Deuterium

This award supports research aimed at understanding how the nuclear interaction works. We know that elements are made from protons and neutrons bound together and when they are extremely close, they must experience a very strong repulsive force that keeps normal matter from collapsing. Understanding how that repulsive part of the nuclear interaction balances with the attractive part has far-reaching implications such as in understanding the collapse of very dense neutron stars at extreme temperatures. In a similar way to when a curious child purposely breaks a toy to figure out how it works, nuclear physics experiments, carried out by Dr. Yero and his team at The Catholic University of America (CUA), seek to advance the current understanding of the repulsive part of the nuclear force at Jefferson Laboratory (JLab) by using electrons to break apart the deuteron nucleus into a proton and a neutron. Both graduate and undergraduate students supported by this award will gain valuable knowledge and technical expertise with regards to data analysis as well as presentation skills, necessary for creating a capable scientific workforce of a younger generation of scientists at the frontlines of potential scientific discovery. This research program will focus on the data analysis of a recently completed deuteron experiment that probes the repulsive core and on the development of other deuteron experiments at Jefferson Lab (JLab). The recent experiment took data at unprecedented high relative nucleon momenta (up to ~ 1.2 GeV/c) corresponding to sub-Fermi inter-nucleon distances where the repulsive part becomes significant. The remaining experiments will explore various aspects of the nuclear interaction such as understanding the behavior of final state interactions (FSI), as well as investigating the onset of color transparency in the deuteron. Each of these experiments will probe the deuteron in a unique way and will help push the frontiers in the current understanding of the strong nuclear force. The very preliminary results of the repulsive core experiment show a deviation of the data from all theoretical calculations with relative momenta above 700 MeV/c. This discrepancy occurs in a region where relativistic effects become increasingly important, the description of the deuteron in terms of the conventional neutron-proton picture is expected to break down, and the onset of non-nucleonic components is expected. New theoretical calculations that include relativistic effects are currently underway, and will be compared against data. In addition, FSIs become more significant with increasing missing momenta, and as a result, Dr. Yero has recently proposed to the JLab Program Advisory Committee a dedicated experiment that will investigate and quantify the effect of FSI at very high missing momenta (> 500 MeV/c). By tackling the problem of the repulsive core with various complementary experiments, Dr. Yero and his research team, in collaboration with JLab, aim to address a fundamental problem that has persisted in Nuclear Physics since the discovery of the deuteron: what is the origin of the repulsive part of the nuclear force? 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.