The intermediate energy group research is supported by the U. S. National Science Foundation under grant PHY 0098642 (C. Glashausser, R. Ransome, R. Gilman). We generally work in collaborations of 50 - 100 physicists from 10 - 20 institutions.

For much of the past decade, the group has focussed on the structure of nucleons, and the structure of few-body nuclei, particularly the interplay between quark and hadronic descriptions of nuclei and nuclear reactions. Our particular area of expertise has been in spin physics, measuring the spin orientation of protons. We were largely responsible, with colleagues from William & Mary and elsewhere, for building the focal plane proton polarimeter in Jefferson Lab Hall A, 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

Following is a list of projects in which we are or have been involved. Publications can be found from the individual group member pages.

Future experiments:

• We are involved in developing a proposal for investigating the nucleon with neutrino beams at Fermilab, with unprecedented precision. The experiment is expected to run from about 2006 - 2009. See the MINERVA home page.
• We are involved in the Fermilab experiment 906, expected to run 2008 - 2011. This experiment uses the ``Drell-Yan'' process to investigate the anti-u and anti-d quark distributions in the proton.
• We are spokespeople of several approved Jefferson Lab experiments, two of which are scheduled to run in the next few years. Unscheduled experiments have a good chance to run starting in late 2005.
  • The deep virtual Compton scattering experiment (expected to run 2004) investigates the description of the nucleon in terms of quarks in an exclusive process, in which all final state particles are measured - this has been largely impossible until recent theoretical developments.
  • The G_Eⁿ experiment (expected to run 2005) investigates the charge structure of the uncharged neutron.
  • The transversity experiment investigates the quark distributions of the neutron, when the neutron's spin orientation is perpendicular to the momentum direction.
  • The ⁴He polarization transfer experiment investigates the issue of whether it is sensible to consider nucleon properties to be modified in the nuclear medium.
  • The ³He photodisintegration experiment, in addition to our previous deuteron photodisintegration results, investigates the quark structure of light nuclei.
  • The electron-deuteron elastic scattering experiment is intended to put effective field theories derived from QCD, or relativity in conventional hadronic theories, on a firmer basis by resolving some questions in existing data.

• 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 arealso 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. We are developing new experiments to explore this possibility.
• 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.) At present there are several contending quark models, all in approximate agreement with existing data. To further stress these models, we took additional polarization data during 2002, which are under analysis. We also have an approved experiment on ³He photodisintegration. 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.

Gerfried Kumabartzki maintains online documentation for the spectrum analysis code SA .