We report results of an electron-beam-dump search for neutral particles with masses in the range 1 to 15 MeV and lifetimes r between 10 ~1 4 and 10 -10 s. No evidence was found for such an object. We rule out the existence of any 1.8-MeV pseudoscalar boson with r > 8.2 x 10 ~1 5 s and an absorption cross section in matter less than 1 mb per nucleon, and exclude r> 1 x 10 ~1 4 s were its cross section to equal 50 mb per nucleon. In conjunction with measurements of the electron's anomalous magnetic moment, this experiment shows that the narrow positron peaks observed in heavy-ion collisions at the Gessellschaft fur Schwerionenforschung are not due to an elementary pseudoscalar.
New Jefferson Lab data are presented on the nuclear dependence of the inclusive cross section from (2)H, (3)He, (4)He, (9)Be and (12)C for 0.3 < x < 0.9, Q(2) approximately 3-6 GeV(2). These data represent the first measurement of the EMC effect for (3)He at large x and a significant improvement for (4)He. The data do not support previous A-dependent or density-dependent fits to the EMC effect and suggest that the nuclear dependence of the quark distributions may depend on the local nuclear environment.
We present new measurements of electron scattering from high-momentum nucleons in nuclei. These data allow an improved determination of the strength of two-nucleon correlations for several nuclei, including light nuclei where clustering effects can, for the first time, be examined. The data also include the kinematic region where three-nucleon correlations are expected to dominate.
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