Our Group’s Research Questions
How do short-range correlations form between protons and neutrons in nuclei?
Protons and neutrons are not stationary within a nucleus; rather they move around and can even be grouped into orbitals, much like those of electrons in atoms. Most protons and neutrons move rather slowly, feeling the attractive force of all of the other protons and neutrons in the nucleus. But a small fraction of protons and neutrons find themselves extremely close to a partner and form a “short-range correlation,” and get a big kick in momentum. These correlations seem to form universally: in the lightest nuclei with only a couple of nucleons all the way up to the heaviest. Our group is curious about what happens in these correlations and why.
One of our favorite experiments to do is to use a highly accelerated electron to knock-out a proton from the nucleus. By detecting the scattered electron, we can figure out how much momentum was transferred to the proton. And by measuring the proton, and subtracting the transferred momentum, we have an estimate of how much momentum it had before the collision, and determine if it was part of a correlated pair. Even more interesting is that its correlated partner nucleon can also emerge from the nucleus heading the other way! Our group is currently collecting a new high-statistics data set on a wide range of nuclei using the CLAS12 spectrometer at Jefferson Lab, and our hope is that these data can be analyzed to answer a wide range of questions about correlations.
We are also trying a new technique for the first time: high-energy photo-production. We’ll collide a high-energy (≈ 8 GeV) photon against a proton or neutron in a short-range correlation, boosting it into a short-lived excited configuration, which will decay by emitting one of several different kinds of meson particles. This method of identifying short-range correlations hasn’t been tried until now. We are conducting an experiment using the GlueX spectrometer at Jefferson Lab in 2021.
What is 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 we now give for this bizarre finding is the “EMC Effect.”
There have been many proposed explanations, but there one that we find particularly intriguing: maybe quarks in most protons don’t change their momentum distribution, and only protons (and neutrons) in short-range correlated pairs are significantly affected. There is good evidence supporting this hypothesis. However, we need direct experimental confirmation.
Our group is leading a pair of experiments at Jefferson Lab that will look for quarks in fast-moving correlated protons and neutrons by tagging on a correlated partner nucleon. The BAND Experiment uses a Backward Angle Neutron Detector (pictured above) to detect spectator neutrons. First data were collected in 2019, and analysis is under way. The LAD Experiment, to run in the near future, will use the Large-Acceptance Detector to detect spectator protons in the next few years. The results from these experiments may confirm or rule out the short-range correlations hypothesis.
What are the short-lived excited states of quark matter?
One of the peculiar properties of quarks is that they can’t be found in isolation. They either come in a bound state of three, called a baryon, such as the familiar proton and neutron, or in a quark + anti-quark pair, called a meson. A whole panoply of short-lived excited baryon and mesons states have been observed, but nevertheless, state-of-the-art theoretical calculations suggests we should be seeing even more. Why haven’t we? It’s possible that we haven’t built the right type of experiment to observe them or to separate faint signals from background. It’s possible that in certain regimes, quarks don’t act as individuals and that we need to be thinking more about meson-meson molecules or other exotic structures. Whatever the cause, our group and many others are seeking to better understand how these excited states form, how they decay, and what their properties are. We conduct experiments both using CLAS12 and GlueX to produce these excited particles and detect the stable particles they decay into.
How often does two-photon exchange happen?
Electron scattering can be well described by thinking of the reaction proceeding through the exchange of a single virtual photon. However, a puzzling discrepancy in measurements of the proton’s form factors—its distributions of electric charge and magnetism—could be a sign of two-photon exchange. We would like to measure the two-photon exchange effect directly and see if that is the case.
Our group is working to realize a future experiment called TPEX that will hopefully run at the DESY facility, in Hamburg, Germany, and compare the relative elastic scattering cross sections for positrons colliding with protons and electrons colliding with protons. In a one-photon exchange framework, they have to be equal, so any deviations reveal the magnitude of the two-photon exchange effect.