Overview

The focus of our program is the understanding of the strong interaction in the nuclear medium. Our emphasis is the study of the nucleon and few-body nuclei with electromagnetic probes. In the past, we have complemented these efforts with key measurements with hadronic probes (pion scattering from few-body nuclei, radiative pion capture on the proton) and/or on very heavy nuclei (photofission on the actinides and pre-actinides) as well, but most of these results are now published or submitted for publication, and this aspect of our program is occupying a smaller fraction of our effort as time goes on.

We are now beginning to reap the fruits of our experimental buildup at Jefferson Lab (JLab) that has taken place over the course of many years. We now find ourselves in possession of data that are unprecedented in both quantity and quality, and which require large-scale analytical and computational efforts to understand and interpret. These efforts are now well under way, and our first results are in the publication-review stage.

Although our primary activity has shifted from data acquisition to data analysis, we continue to think of new experiments that explore the potential implications of our present results. Even though the central focus of our research remains at JLab, we also are looking towards new horizons at other facilities. The closure of the fixed-target program at the AGS at BNL brings to an end our experimental work there, but the Crystal Ball detector array, with its high efficiency for detecting multiphoton events, particularly useful for studying the decay of the excited states of the neutron, has found a new home at the Mainz Microtron (MAMI). The advent of the High-Intensity Gamma Source (HIGS), the superb new polarixed photon-beam facility at Duke University, provides us with a modern replacement for the old, sometimes closed facilities (such as SAL) in the low-to-medium energy range, and constitutes a modern complement to the high-energy photon beams at JLab, particularly useful for studies of few-body nuclei. New problems in photoabsorption and photofission have arisen from our work at SAL and JLab, which we propose to address at HIGS as well. Co-PI Briscoe is providing leadership for our proposed new initiative at MAMI; co-PI Feldman is doing so for our proposed new initiative at HIGS.

At Jefferson Lab, the successful commissioning of our goniometer there and the first polarized-photon beams obtained with it have ushered in a new era of polarized-photon experiments in the GeV range, for which we are now engaged in providing a photon-beam polarimeter. We also are playing a major role in the program of nucleon studies with a polarized hydrogen/deuterium target at JLab; we have joined with our colleagues at Catholic and Arizona State Universities in a program of photoproduction of scalar mesons, and we have succeeded in obtaining approval for our new proposal for photopion studies. Finally, we have joined our colleagues from Halls A and C in three new experiments – two newly approved experiments on medium effects in few-body nuclei in Hall A and one on nucleon form factors that just completed data-taking in Hall C.

During the past year, we have completed the analysis of our data on photofission from Jefferson Lab (the g5 experiment) and SAL. We have published a long paper on this subject, and we have submitted another on its theoretical implications. We have completed the analysis of our data on photodisintegration of the three-body nuclei from SAL, and we also have helped to analyze our complementary data from Lund. We have data completed our work and published both experimental and theoretical papers on pion scattering from ³H and ³He, the for which we obtained earlier at LAMPF. We have completed the analysis of our data on pion radiative capture (inverse pion photoproduction on the neutron) from BNL.

Turning to our experimental work with the CLAS in Hall B at Jefferson Lab, we note that the CLAS Collaboration has finally produced a growing number of physics publications, to some of which (particularly those done with real photons) we have made considerable contributions, and we are about to produce several of our own. Analyses that are in the CLAS review stage include two-p ⁰ photoproduction on the proton (g1 data), strangeness photoproduction on the neutron (g2), and three-body photodisintegration on ³He (g3). Analyses that are mature and approaching submission are helicity-dependent p ^{+p -} photoproduction on the proton (g1) and coherent pion photoproduction on ²H (g2). Several other analyses are well underway, in particular many reaction channels on ³He and ⁴He (g3), including strangeness photoproduction, two- and three-body photodisintegration, coherent p ⁺ photoproduction, r ⁰ photoproduction, D ⁺⁺ knockout, and polarization asymmetries for p ^{+p -} and pion-proton channels.

Although our staff has grown over the years to the point where our group can indeed proceed along several lines at once, still it is not so large that we can accommodate these major new efforts without a significant increase in manpower, particularly at the postdoctoral level. We are happy to report that both JLab and HIGS have offered to share new postdoc positions with us at those laboratories (a second one for JLab), and MAMI has guaranteed a substantial amount of beam time for the Crystal Ball program. These offers will help us enormously to exploit the new opportunities at all three of these laboratories.

At the same time, we have been building up our computational resources and expertise, so that now we have at our command a substantial array of modern workstations and PCs. We now have developed a massively parallel computational installation capable of analyzing the several terabytes of data that we have obtained in our Hall-B experiments. We now also have the use of the multi-terabyte GW data-storage silo. Hand in glove with this buildup has been the establishment of the GW Data Analysis Center, as part of our Center for Nuclear Studies, at our Virginia Campus.

We continue to receive superb support from the University, as well as from Jefferson Lab. We have just moved into new and refurbished office-and-conference quarters at our Foggy Bottom Campus, and we are about to gain substantial space at our Virginia Campus as well.