1.
Structure of the Nucleon
a. Nucleon Spin Structure (S. Kuhn coordinator of 6 GeV and 12 GeV run goups)
In Hall B at Jefferson Lab we use polarized electrons incident on polarized NH3 and ND3 targets to study the spin structure of the nucleon. We are interested in the inclusive double polarization asymmetry, which allows us to extract the spin structure functions g1(x,Q2) and A1(x,Q2). We also study exclusive and semi-inclusive asymmetries where an additional hadron is detected in the final state. By covering an enormous kinematic range, we can determine these observables and their moments in the resonance region and part of the DIS region, for very low to moderately high momentum transfers Q2. A series of experiments have been completed or are underway, and one additional one is planned for the future:
b. BONuS Experiment (E12-06-113; S. Bueltmann and S. Kuhn co-spokespersons)
We built a novel slow proton recoil detector (a "Radial
Time Projection Chamber") using Gas Electron Multipliers (GEMs) to
detect slow spectator protons in the reaction d(e,e'pS)X. By tagging
the scattered electron with a low-momentum (down to 70 MeV/c)
"spectator" proton moving backwards relative to the momentum transfer,
we can ensure that the reaction took place on a nearly on-shell neutron
in deuterium. Experiment E03-012
took data in 2005 and allowed us to extract the free neutron structure
function at moderately high x
and in the resonance region, for the first time undisturbed by Fermi
motion and off-shell effects. The data have been published in 5 papers. A similar detector was also used for
another series of experiments, to study exotic meson production
(E07-009) and Deeply Virtual Compton Scattering (DVCS) on Helium 4
(E08-024).
In 2020, we ran BONuS again (RG-F) with 11 GeV electrons, which
enables us to extract the ratio of the neutron to proton F2
structure function up to x
~ 0.8, which is particularly interesting since it depends on the ratio
of d to u quarks in the nucleon and there are several very different
predictions for this ratio as x goes to 1.
This project was recently featured in ODU's "Inside" magazine.
(S. Bueltmann, G. Dodge, M. Hattawy, S. Kuhn).
Deeply virtual Compton scattering is a powerful tool for studying the structure of the nucleon. In this process an electron is scattered from the proton or deuteron and a real photon is emitted and detected in the final state. We have studied this process in several Hall A and Hall B experiments, including the case in which the electron and target are polarized (EG1-DVCS). The upgrade of the beam energy at Jefferson Lab to 12 GeV enables us to study this process in a kinematic regime where we probe the structure of the nucleon very cleanly. E12-06-114 is one of the commissioning experiments in Hall A and ran in 2015-17. E12-13-010 will run in Hall C. We also study time-like Compton scattering and J/psi production in Hall B.
d. Two Photon Exchange (E07-005; L.B. Weinstein co-spokesperson)
Recent measurements have shown a discrepancy between proton form factors measured using the Rosenbluth method and using the polarization transfer method. One explanation for this discrepancy is the possibility of two photon exchange between the scattered electron and the target proton. We took data in Hall B to measure this effect by scattering both positrons and electrons from proton targets. In order to do this we created a beam of mixed positrons and electrons in Hall B. The results were recently published.
3. Short Range Correlations
Due to the strong interaction and short distances between nucleons in nuclei, there is a significant probability for nucleon wave functions to overlap, resulting in short range nucleon-nucleon correlations (SRC) in nuclei. These correlations are responsible for the high momentum components in nuclei. In addition, because the nucleons overlap, the local density can be several times larger than the average nuclear density, providing an extreme environment for the study of nucleons. We have also shown a relationship between the EMC effect and short range correlations in nuclei, which is an exciting new development. We are currently pursuing the following experimental program:
4. Physics Beyond the Standard Model (L. Weinstein, S. Stepanyan)
We are involved in the Heavy Photon Search (HPS) experiment in Hall B, which has begun to run in 2015. The goal of the experiment is to look for new heavy vector boson(s), aka `heavy photons' or `dark photons' or `hidden sector photons', in the mass range of 20 MeV/c2 to 1000 MeV/c2. If they exist, heavy photons mix with the ordinary photon through kinetic mixing, which induces their weak coupling to electrons of ~10-3e, where e is the charge of the electron. Heavy photons in this mass/coupling range are expected on very general theoretical grounds, and motivated by recent astrophysical evidence suggesting they might mediate dark matter annihilations and/or interactions with ordinary matter. Since they couple to electrons, heavy photons are radiated in electron scattering and can subsequently decay into e+e-, which is one of the signatures that HPS be able to detect.
5. EIC (C. Hyde)
Our
group plays a major role in developing the science case and detectors
for the future flagship facility in high energy nuclear and hadronic
physics, the Electron Ion Collider (EIC). This project has been given
high priority in the latest Nuclear Science Advisory Committee Long
Range Plan, and will be built at the Brookhaven National Lab on Long Island, NY.
Here is our most
recent grant renewal proposal to the U.S. Department of Energy