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Light Nuclei: Many-Body Forces, Medium Modifications, and More A central theme of our experimental program is the delineation of the properties and symmetries of the many-body nuclear force. In particular, we wish to explore and understand the nature of the short-range part of the strong interaction in the nuclear environment. Thus, one of the main thrusts of our effort continues to be the investigation of the nuclear three-body force. The investigation of this short-range phenomenon is hampered by the fact that the effects usually are small compared to those due to the ever-present two-body force. Furthermore, the extraction of these small effects requires not only the suppression of two-body effects but also the detection of multi-particle events in the final state, often including neutral particles. However, at Jefferson Lab, the exciting prospect of identifying and quantifying the nature of the many-body part of the nuclear force is made much more feasible when we have at our disposal the combined power of a well-understood short-wavelength electromagnetic probe and an efficient large-acceptance detector. Accordingly, we have completed a detailed study of the nuclear three-body force in its most elementary manifestation, the three-body photodisintegration of 3He. This has constituted the Ph.D. thesis experiment for Silvia Niccolai, summarized in Section 1 below. The same g3 data set that we are using for this analysis has provided us with an unparalleled opportunity to study other reaction channels in few-body nuclei. In particular, the modification of the elementary photoproduction amplitudes for mesons and baryons on the nucleon by the presence of other strongly interacting particles can be delineated in our data. We study the simple breakup channels in Sections 2-4 and coherent pion production in Sections 5 and 6; the latter shows somewhat surprising evidence for an h meson in the intermediate state. In Sections 7-10, we examine modifications of the r , the S , the D , and the helicity dependence of two-p production in 3He; and in all cases we also have the data to study the density dependence of these medium effects with the much more tightly bound nucleus 4He. Finally, Section 11 details our study of the modification of the nucleon form factors via their spin transfer in 4He.
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