Experimental Intermediate Energy Nuclear Physics Group
The intermediate energy group (R. Ransome and R. Gilman) research is supported by the U. S. National Science Foundation under grant PHY 06-52713. We investigate strong QCD (mostly the structure of the proton and neutron), nuclear structure, and physics beyond the standard model. We are mainly involved in experiments at the Thomas Jefferson National Accelerator Facility (JLAB) 6 GeV accelerator, CEBAF, in Virginia (where I am currently the chair of the Users Group Board of Directors) and at Fermilab, in Batavia Illinois. Our JLab research is centered on studies of spin physics, studying nucleons and subnucleonic effects in light nuclei using polarized electron beams, targets, and recoil polarimeters. Our research at Fermilab uses neutrino beams. We generally work in international collaborations of 50 - 100 physicists from 10 - 20 institutions.
Our largest construction project, which has led to a significant portion of our research program from the late 1990s through now, was building the focal plane proton polarimeter in Jefferson Lab Hall A, with colleagues from William & Mary and elsewhere. The prject ws funded by the U. S. National Science Foundation, grants PHY 9213864 (C. F. Perdrisat, College of William & Mary, et al.), and PHY 9213869 (R. Gilman, Rutgers University, et al.). See Ron Gilman's Focal Plane Polarimeter for CEBAF Hall A Home Page. We are now involved in a much bigger project to build the MINERνA experiment at Fermilab.
Following is a list of ongoing projects (most recent work first):
- We are now in the construction phase for a major experiment investigating the nucleon and nuclei with neutrino beams at Fermilab. The experiment is expected to run from about 2010 - 2012. See the MINERVA home page.
- Fermilab experiment 906, expected to run 2010 - 2011, will use the ``Drell-Yan'' process (q + antiquark -> μ+μ-) on hydrogen, deuterium, and other targets to investigate the anti-u and anti-d quark distributions in the proton, hadronization of quarks, etc.
- We are currently involved in detector upgrades for the upcoming (late 2008-early 2009) ``Big Family'' round of polarized 3He experiments in Jefferson Lab Hall A. One example is the transversity experiment investigates the quark spin distributions of the neutron, when the neutron's spin orientation is perpendicular to the momentum direction. The experiments inve.tigate a wide range of physics, including the structure of 3He, two-photon exchange effects, the color polarizibility of the neutron, etc
- In May 2008 we will run a new low Q2 proton form factor experiment in Jefferson Lab Hall A, which will determine the form factors with unprecedented precision, investigating a range of issues including proposed fine structures in the form factors, the isoscalar and isovector form factors, the u and d quark form factors, the role of strange quarks in the proton, the electric vs. magnetic radii of the proton, and theoretical uncertainties in the hydrogren hyperfine splitting.
- We are currently running a series of experiments in Jefferson Lab Hall C to investigate the proton electric and magnetic distributions at high momentum transfer, to put experimental constraints on the possible two-photon corrections to these measurements, and to study high energy real Compton scattering on the proton, which provides a complementary set of data on the quark distributions in the proton.
- The Jefferson Lab Hall A 3He / two-proton photodisintegration experiment (ran mid 2007), in combination with our previous deuteron photodisintegration results, investigates the quark structure of light nuclei and the reaction mechanism in the difficult transition region, where there are at present no good methods of solving QCD. (There is recent promising work from ads/CFT.) The analysis is approaching final.
- The Jefferson Lab Hall A Li/B elastic scattering experiment investigates the radii of these light nuclei. They are needed to anchor isotope differences measured with atomis physics techniques, for comparison to modern ab initio nuclear structure calculations.
- The Jefferson Lab Hall A 4He polarization transfer experiment (most recent run in late 2006) investigates the issue of whether it is sensible to consider nucleon properties to be modified in the nuclear medium. The analysis is underway.
- The Jefferson Lab Hall A electron-deuteron elastic scattering experiment (ran mid 2006) is intended to put effective field theories derived from QCD, or relativity in conventional hadronic theories, on a firmer basis by resolving some issues in existing data sets. Analysis is progressing.
- Measurements of low-energy deuteron photodisintegration, run in 2006 in Jefferson Lab Hall A, show the limits in our ability to describe reactions using hadronic degrees of freedom. The results are nearly final.
Following is a list of important experimental programs from the past several years.
- Proton charge distribution : The probability of elastically scattering an electron from the proton depends on the proton charge and magnetic distributions. These distributions also lead to an orientation of the proton's spin, if the electron spin is oriented along its momentum direction. Previously, the best experiments, based on scattering probabilities, indicated the charge and magnetic distributions of the proton are essentially the same. Our measurements of the spin orientations are a superior technique, and show a clear difference between the two distributions. (Jones et al., Gayou et al., or see the E93-027 home page) A new higher precision scattering probability measurement in which we are also involved is in agreement with earlier data, indicating some difference in the techniques. Both experiments are based on the idea that the electron and proton interact by exchanging a single photon. There is now excitement about the possibility of explaining the discrepancy between the techniques from two photon exchange. Two experiments are currently running, the GEp-III/GEp-2g experiments. The first pushes measurements to a much higher momentum transfer / much finer resolution, while the second checks for possible two-photon exchange effects.
- The proton charge distribution, in the nucleus : From the perspective that quarks make up the nucleon, the nucleon-nucleon interactions in the nucleus modify the quark distributions, and thus the charge and magnetic distributions, of the nucleon. To some degree these effects are already incorporated in the nucleon-nucleon force, and to date experiments have largely put limits on possible effects; claims of modifications have been controversial. In what are perhaps the cleanest measurements to date, we have compared measurements of charge and magnetic distributions of the proton in the nucleus to those of the free proton, using the proton spin technique. The most complete theory suggests only about 2-3 % conventional effects, but our data show about 8 % effects. The 5 % difference is consistent with estimates of changes in the charge and magnetic distributions. (Strauch et al.)
- Deuteron photodisintegration experiments : High-energy deuteron photodisintegration probes the quark structure of the deuteron. Over the past decade, we have shown that cross sections are consistent with simple scaling rules expected from the quark picture. (Bochna et al., Schulte et al.) We have also performed the highest energy polarization measurements, which further suggest the appropriateness of quark models. (Wijesooriya et al., and Jiang et al.) At present there are several contending quark models, all in approximate agreement with existingcross section data. Only two seem to have qualitative predicting power for the polarization observables. To further stress these models, we measureed two proton disintegration in 3He. See the 1999 experiments home page or the 2002 experiment home page or the Gilman and Gross review article.
- Spin structure of the neutron (in 3He): There has been a series of polarized 3He experiments, looking at different aspects of the neutron spin structure (since the two protons in 3He are largely coupled to spin 0). Two examples follow.
- The extended Gerasimov-Drell-Hearn sum rule on the neutron (3He) was studied at moderate Q2 in 1998, followed by a very low Q2 measurement during 2003. The sum rule (at Q2 = 0) relates the difference of total inelastic spin-dependent scattering to the anomalous magnetic moment, and can be predicted from fundamental theories based on QCD over almost the entire range of Q2 = 0 to infinity. We performed the first measurements in the difficult region of 0.1 to 1.0 GeV2, which show a smooth variation from effective field theory region to the perturbative QCD region. See the E94-010 home page . (Amarian et al.) The data are also being used to determine the proton polarizibility, the deformation due to the applied electromagnetic fields, and other quantities of interest.
- The A1n asymmetry and g2n were measured during 2001. The first measurement gives the alignment of quark spins to the nucleon spin. We found for the first time that A1n is positive, the quark and nucleon spins are aligned, when the proton u quark has a large fraction of the nucleon's momentum. However, the proton d quark remains anti-aligned to the nucleon's spin. (X. Zheng et al.)
- Elastic electron deuteron scattering : These experiments measure the charge distributions of the deuteron. Quantum mechanically, since the deuteron has spin 1, it has three charge distribution, the electric, magnetic, and quadrupole distributions. The JLab Hall C t20 experiment and the JLab Hall A cross section experiment allowed these quantities to be determined with good accuracy to high momentum transfer. The experiments present clean indications that understanding nuclear structure at large momentum requires special relativisitic effects. Unlike the photodisintegration measurements, the electron elastic scattering is not sensitive to the underlying quarks.
Experimental Nuclear Structure Physics Group
Gerfried Kumbartzki maintains online documentation for the spectrum analysis code SA .