In my research, I use electron scattering to learn about the structures that protons and neutrons form inside nuclei, and the structures and quarks and gluons form inside protons and neutrons.
The Origin of the EMC Effect
In 1983, the European Muon Collaboration released startling findings that the quark momentum distributions in protons and neutrons in iron nuclei were significantly different than expected from measuring quarks in free protons. Quarks are probed with gigaelectron volts of energy (GeV) in deep inelastic scattering reactions. Meanwhile, protons are only bound in iron by an average of 0.008 GeV of energy. It is bizarre that the environment of the iron nucleus somehow affects deeply bound quarks. But it does. And the term EMC Effect was born.
There have been many proposed explanations, but there is one that intrigues me in particular: maybe quarks in most protons don’t change their momentum distribution, and only protons (and neutrons) in short-range correlated pairs are significantly affected. I argue there’s good evidence that support this hypothesis. However, we need experimental confirmation.
I’m leading an experiment at Jefferson Lab, in Newport News, Virginia, that will look at quarks in fast-moving correlated protons by tagging on a correlated spectator neutron partner. To detect these neutrons, I helped build a new detector, called BAND, for the Backward Angle Neutron Detector (pictured above). We collected our first data in early 2019, and we’ll get more at the end of the year!
Short-Range Correlations Between Nucleons
In all but the very lightest nuclei, about 20% of the protons and neutrons are moving surprisingly fast, with momenta that exceed the nuclear Fermi momentum, which one might expect to be an upper limit. These high-momentum nucleons are the result of short-range correlations, fluctuations in which two nucleons get very close together and interact strongly. And while these short-range correlated nucleons are a minority, they have an outsized influence in several areas. They carry the majority of the nucleus’s internal kinetic energy, they may be the cause of the EMC Effect (see above), and they may even play a role in the properties of neutron-star matter.
I’m interested in learning how these correlated nucleons can tell us about the fundamental nuclear force between protons and neutrons. Short-range correlated nucleons interact much more strongly with each other than with the rest of the nucleus, and they may open a window into learning about the behavior of the nuclear forces at very short range. I’m leading an experiment at Jefferson Lab that will study electron-induced nucleon knockout from a wide-range of nuclei to study short-range correlated pairing, the properties of these pairs, and help determine the nuclear force at the shortest distance scales.
Two-Photon Exchange
One of the great things about electron scattering as a tool is that the electromagnetic force is weak: a scattering reaction can be accurately modeled by an interaction in which one virtual photon is exchanged—the probability of exchanging a second photon is often small enough to neglect.
But sometimes two-photon exchange matters. In fact, a serious discrepancy between two different techniques for determining the proton’s form factors (cross section measurements and polarization asymmetries) may be the result of neglecting two-photon exchange.
I worked on an experiment, OLYMPUS, (pictured above) which measured how much two-photon exchange (TPE) contributes to elastic electron-proton scattering. The results: not much, given the amount of momentum transfer in the collisions we were able to record.
I’m working designing a follow up experiment called TPEX, which aims to make an unambiguous test of whether or not TPE is the cause of the proton’s form factor discrepancy. We’re kicking off with calorimeter tests at the DESY facility in Hamburg, Germany, in Fall 2019.