Laboratory Profile: Jefferson Lab Scientific Motivation and Research Program (Nuclear Physics News)
Scientific Motivation and Research Program
Jefferson Lab's accelerator has its roots in the decades-long tradition of exploiting the advantages of electron beams (their point-like structure and their well-understood interaction) for the investigation of nucleon and nuclear structure. It has been recognized since the 1960s that high-energy electron beams (probing spatial scales down to a fraction of the nucleon's size) with 100% duty factor (which facilitates the extension of electromagnetic interaction studies to a broad range of coincidence reactions) would provide a unique and powerful new tool. This perception was bolstered by the scientific successes of the "second generation" of electron accelerators (typified by the machines at MIT, NIKHEF, and Saclay, with ~1 GeV, 1% duty factor beams), which have made many important contributions to our understanding of the nucleus and the strong interaction.
The construction of a 100% duty factor electron accelerator with an energy in the 1-2 GeV range became the highest construction priority for U.S. nuclear science with the issuance of the very first NSAC Long Range Plan (in 1979). By the mid-1980s, as planning for the accelerator advanced, it became clear that an energy of order 4 GeV, with straightforward upgrade capabilities, was preferred. This change came from the growing perception that the crucial mission of the facility would be to understand the transition from the nucleon-based description of nuclei to the underlying quark-based description.
In the intervening years our understanding of the problem to be solved has deepened. Quantum chromodynamics now appears to provide an exact theory of the strong interaction, but has only been tested accurately at very high energies (corresponding to very short distance scales) where the interaction becomes weak and perturbative calculations are feasible. It is at larger distance scales, where QCD becomes truly strongly interacting, that startling and still only very poorly understood phenomena occur: the quarks become confined; the gluonic fields appear to freeze out; and quarks cluster into the nucleons of standard nuclear physics, interacting via the residual interactions between these color-neutral clusters. Jefferson Lab's principal scientific goal is to use the power of the electromagnetic interaction to investigate the transition region between the "asymptotically free" high-energy regime and the strongly interacting regime where the phenomena of the world around us are determined.
The approved nuclear physics research program ranges from measurements of nucleon and meson form factors and sum rules, studies of N* and meson properties, and investigations of the properties of nuclei and of changes of nucleon properties in the nuclear environment, to studies of the role of strange quarks in the nucleon. Polarized electron beams and targets and neutron, proton and deuteron polarimeters will play an important role in the program. Roughly half of the experiments involve the measurement of spin variables, either as the fundamental observable or as an experimental tool to enhance sensitivity to the fundamental quantity of interest. The brief discussion of some typical planned experiments provided here gives a sample of the research program that is just underway.
The first experiment to be completed is a measurement of the photodisintegration of the deuteron for energies up to 4 GeV. According to simple constituent counting rules, the photodisintegration cross section should scale as s-11, where s is the Mandelstam variable, once the quark degrees of freedom are dominating the reaction. As can be seen in Figure 1 the preliminary deuteron data (which have systematic errors estimated to be under 30%) show this scaling behavior at 90o. Data taken at 37o appear to show evidence for the onset of scaling at the highest photon energy measured, 4 GeV. A transition occurs in both cases at a transverse momentum of ~1 GeV. It would be quite interesting to extend the 37o data to higher photon energies to verify this behavior. It would also be interesting to test the prediction of QCD that the polarization of the nucleon emitted in this reaction goes to zero in the "true" perturbative scaling regime. The polarization at energies below 0.75 GeV, where data exist, is large. Measuring the polarization in the 1-4 GeV region should provide additional insights into the transition from nucleon to quark behavior.
Another example of an experiment probing the transition region is the planned study of the q2 evolution of the nucleon spin structure function. Heretofore, the study of the spin structure functions has focused on the short-distance behavior of the nucleon, where the measurements can be interpreted simply in terms of the parton sub-structure. Experiments planned at Jefferson Lab will measure the q2 evolution of the spin structure function sum rules from low q2, where they are dominated by the resonances of the naive quark model, to high q2, where they will join onto existing measurements indicating the existence of a "spin crisis". An example of the anticipated data is shown in Figure 2.
A major research program at the laboratory will be the investigation of the quark-gluon structure of the nucleon. The CEBAF Large Acceptance Spectrometer (CLAS) in Hall B will carry out a detailed study of its spectrum of excited states. As in atomic physics, this spectrum contains vital information on the nature of the nucleon's constituents and the forces between them. It is not understood why the naive constituent quark model is so successful in explaining the particle spectrum discovered so far. Also puzzling is the fact that many of the states this model predicts have not been observed in pN reactions, the classical tool for their study. Several of these "missing" states are predicted to couple strongly to photons, and may thus be observable with the CLAS. Planned experiments will either confirm this model by discovering the complete pattern of states it predicts or, more likely, will reveal its shortcomings.
High-precision studies of the elastic, inelastic, and weak structure of the nucleon will also be important. Planned experiments in Halls A and C will complement the CLAS data, measuring the charge and magnetic form factors of nucleons with much greater accuracy than is currently available. Virtual Compton scattering [the p(e,e'g) reaction] shows the promise of providing a powerful new tool for the study of the nucleon that is both sensitive to the resonance structure and essentially free of final state interaction complications. A detailed study of spin observables in the N->D transition will be performed. Precise, parity-violating electron scattering experiments will be used to isolate and study the weak neutral current structure of the nucleon. As can be seen in Figure 3, the asymmetry measured in these experiments will be sensitive to the details of the strange-quark contributions to the charge and magnetization distributions of the nucleons. Taken together, these and related data will provide stringent tests for microscopic models of the nucleon.
One reason it is doubtful that the simple quark model will continue to be successful is that it ignores the gluonic degree of freedom. While there is no evidence yet for states involving gluon excitation, model calculations indicate that most of the predicted "gluonic" states will decay in complicated many-particle modes which would not have been observed with the previous generation of detectors. The broad acceptance in both momentum and solid angle of the CLAS spectrometer should greatly facilitate the search for such states. This is one of the areas in which an eventual upgrade of the energy of the accelerator will be extremely important.
The accelerator's 4 GeV, cw electron beams will allow both (e, e' p) and (e->, e' p->) measurements to be carried out over a range of momentum transfers and internal nucleon momenta that will extend well beyond the region that has been studied to date. In heavy nuclei, these experiments will expand understanding of nuclear structure. An example is shown in Figure 4 which demonstrates the sensitivity of a planned 208Pb(e,e'p) experiment to different calculations of correlations in the nuclear wavefunction. Broad survey experiments carried out with CLAS and high-precision detailed studies planned for Hall A should provide us with important new information on how the nucleus absorbs energy and momentum. These and related measurements are expected to reveal the limitations of the conventional picture of nuclear structure based on nucleons interacting via meson exchange, and also to provide information on how the nucleonÕs properties change when it is embedded in the nuclear medium.
In few-body systems, where exact calculations can be performed for interacting nucleons, (e, e'p) and (e->, e' p->) experiments may reveal the complete breakdown of the meson exchange picture. Alternately, and perhaps more likely, we may discover that quark models at some point simply offer a much more economical description of the experimental data.
To support and guide the nuclear physics experimental program, Jefferson Lab maintains a strong theory group in partnership with local universities. The group includes expertise ranging from the nuclear many-body problem to perturbative QCD, as required at a laboratory working at the QCD transition region. In addition to supporting the experimental program directly, the group collaborates closely with theorists around the world working on related problems.