T. H. Chang

Parton Energy Loss Limits and Shadowing in Drell-Yan Dimuon Production

A precise measurement of the ratios of the Drell-Yan cross section per nucleon for an 800 GeV/{ital c} proton beam incident on Be, Fe, and W targets is reported. The behavior of the Drell-Yan ratios at small target-parton momentum fraction is well described by an existing fit to the shadowing observed in deep-inelastic scattering. The cross-section ratios as a function of the incident-parton momentum fraction set tight limits on the energy loss of quarks passing through a cold nucleus. {copyright} {ital 1999} {ital The American Physical Society}

Final analysis of proton form factor ratio data at Q2 = 4.0, 4.8, and 5.6 GeV2

Precise measurements of the proton electromagnetic form factor ratio R = mu(p)G(E)(p)/G(M)(p) using the polarization transfer method at Jefferson Lab have revolutionized the understanding of nucleon structure by revealing the strong decrease of R with momentum transfer Q(2) for Q(2) greater than or similar to 1 GeV2, in strong disagreement with previous extractions of R from cross-section measurements. In particular, the polarization transfer results have exposed the limits of applicability of the one-photon-exchange approximation and highlighted the role of quark orbital angular momentum in the nucleon structure. The GEp-II experiment in Jefferson Labs Hall A measured R at four Q(2) values in the range 3.5 GeV2 <= Q(2) <= 5.6 GeV2. A possible discrepancy between the originally published GEp-II results and more recent measurements at higher Q(2) motivated a new analysis of the GEp-II data. This article presents the final results of the GEp-II experiment, including details of the new analysis, an expanded description of the apparatus, and an overview of theoretical progress since the original publication. The key result of the final analysis is a systematic increase in the results for R, improving the consistency of the polarization transfer data in the high-Q(2) region. This increase is the result of an improved selection of elastic events which largely removes the systematic effect of the inelastic contamination, underestimated by the original analysis. (Less)

Improved measurement of the anti-d / anti-u asymmetry in the nucleon sea

Measurements of the ratio of Drell-Yan yields from an 800 \rm{GeV/c} proton beam incident on liquid hydrogen and deuterium targets are reported. Approximately 360,000 Drell-Yan muon pairs remained after all cuts on the data. From these data, the ratio of anti-down (

Cross section measurements of charged pion photoproduction in hydrogen and deuterium from 1.1 to 5.5 GeV

\bar{d}

Observation of polarization in bottomonium production at

) to anti-up (

\sqrt{s}

\bar{u}

= 38.8-GeV

) quark distributions in the proton sea is determined over a wide range in Bjorken-

Measurement of G(Ep) / G(Mp) in polarized-e p ---> e polarized-p to Q**2 = 5.6-GeV**2

x

Anti-d / anti-u asymmetry and the origin of the nucleon sea

. These results confirm previous measurements by E866 and extend them to lower

{bar d}/{bar u} asymmetry and the origin of the nucleon sea

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