Two experiments at Jefferson Laboratory (JLab) in 1998 and 2000 have shown that the ratio of proton electric and magnetic form factors decreases sharply with momentum transfer, in definite disagreement with the results obtained by the standard "Rosenbluth separation" technique. The JLab experiments measured the polarization transfer from projectile to target in elastic ep scattering, rather than cross section. The current project will continue these investigations to the highest momentum transfer possible with the 6 GeV JLab accelerator. The findings from the first two experiments have definitively changed the way physicists understand the structure of the proton, and have directly challenged theoretical models of the proton. Among numerous theoretical work generated by our JLab results, we note that efforts to solve the theory of quantum chromodynamics (QCD) in the non-perturbative domain are beginning to have predictive power in the domain investigated by these experiments.
The experiments described in this proposal involve a large collaboration from faculty members at universities, with their postdoctoral associates, and graduate and undergraduate students. These students are diverse in origin and culture, and will receive exceptional training at a technologically advanced facility, JLab. Each summer 6-8 physics undergraduates join the effort and get hands-on practical training. This project support one WM graduate student and one of the two postdoctoral fellows associated with the core group.
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NSF grant 075377 supported the continuing investigation of the structure of the proton, using as a probe the elastic scattering of up to 6 GeV electrons at the Thomas Jefferson Laboratory in Newport News, Virginia. The grant recipient has been the initiator and a major player in a program to systematically increase the momentum transferred to the target proton, and in the process shrink the spatial region investigated to ever smaller size, very much like increasing the power of the lens of a microscope reveals smaller details of the sample. The consequence of this shrinking of the size of the region probed in the proton is a corresponding sensibility to the properties of the proton's constituents, which are the quarks, anti-quarks and gluons. An important revelation of this research is that the orbital motion of the quarks inside the proton manifests itself in elastic electron scattering. The orbital momentum of the constituents of the proton is responsible for a fraction of the spin of the proton, which is1/2.The question of the origin of the spin of the nucleon is presently being intensively investigated in a number of laboratories worldwide, using different probes, with the goal of attempting to resolve the so-called "spin crisis". The spin of the proton has its origin in the spin of its constituent quarks, anti-quarks and gluons, and in the angular moment due to the orbital rotation of these constituents. The crisis arises from the fact that so far experiments have revealed that the sum of these different contributions does not add to the spin 1/2 of the proton, a very fundamental property of both proton and neutron, the two constituents of all the visible matter in the universe; it is an important test of the current theory of strongly interacting matter. Elastic electron scattering is one of several reactions used to investigate the structure of the proton. What has contributed to the recent "fame" of elastic electron scattering however, is our discovery, 12 years ago, that earlier experiments, spread in time over the previous 50 years, had in fact lost their sensitivity to the structure of the proton at relatively small momentum transfer, of order 1 GeV/c. The technique available at JLab is different from that used in the past; the projectiles are polarized electrons, and it is the polarization transferred to the proton which is recorded, instead of just the probability of interaction, called the cross section. Such polarization transfer experiments were barely possible until JLab became operational 14 years ago. JLab is the laboratory in which these experiments can be carried out to the largest momentum transfers. Intensive theoretical analysis of the results of the two methods has lead to the conclusion that only the polarization data were correct, being affected to almost insignificant levels by more complex processes; in contrast, these more complex processes affect cross section measurements very significantly. This grant covered a period of time starting shortly after the conclusion of the third of the series of elastic scattering experiments, in the summer of 2008. During this time the main activity was the analysis of the data, which was fully completed in the Spring of 2011, and publication of the results in three papers, each one dedicated to a different aspect of these investigations. Another important activity was the presentation of the results in a number of Conferences and Seminars. The one year extension of this grant has further allowed us to reanalyze an earlier experiment, taking advantage of the technical knowledge acquired during the third experiment; these results have been now published, resulting in an set of data showing great internal consistency. It is clear from the interest these data have generated in the physics community, including both experimentalist and theorists, that a continuation of this program to as high a momentum transfer as will become available at the conclusion of the currently ongoing JLab upgrade to twice the present energy, is of very important. Proposals to do so have been submitted and approved at JLab during the period covered by this grant. They will increase the momentum imparted to the proton by 50%, correspondingly sharpening the acuteness, or ability to reveal structure details on a smaller scale of distances, of the probing electrons. Charles F. Perdrisat, September 11, 2012.
