We report the results of a new Rosenbluth measurement of the proton electromagnetic form factors at Q2 values of 2.64, 3.20, and 4.10 GeV2. Cross sections were determined by detecting the recoiling proton, in contrast to previous measurements which detected the scattered electron. Cross sections were determined to 3%, with relative uncertainties below 1%. The ratio mu(p)G(E)/G(M) was determined to 4%-8% and showed mu(p)G(E)/G(M) approximately 1. These results are consistent with, and much more precise than, previous Rosenbluth extractions. They are inconsistent with recent polarization transfer measurements of similar precision, implying a systematic difference between the techniques.

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR Collaboration presents a measurement of ρ 0 and direct π + π − photoproduction in ultraperipheral relativistic heavy ion collisions at √ s NN = 200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of σ (AuAu → Au * Au * ρ 0 ) = 530 ± 19(stat.) ± 57(syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The ρ 0 transverse momentum spectrum (p 2 T ) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find σ inc /σ coh = 0.29 ± 0.03 (stat.) ± 0.08 (syst.). The ratio of direct π + π − to ρ 0 production is comparable to that observed in γp collisions at HERA and appears to be independent of photon energy. Finally, the measured ρ 0 spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.

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The STAR collaboration at the BNL Relativistic Heavy-Ion Collider (RHIC) reports measurements of the inclusive yield of nonphotonic electrons, which arise dominantly from semileptonic decays of heavy flavor mesons, over a broad range of transverse momenta (1.2

Background: The spatial distribution of charge and magnetization in the proton and neutron are encoded in the nucleon electromagnetic form factors. The form factors are all approximated by a simple dipole function, normalized to the charge or magnetic moment of the nucleon. The differences between the proton and neutron form factors and the deviation of G n E from zero are sensitive to the difference between up-and down-quark contributions to the form factors.Purpose: Recent measurements of G n E up to 3.4 (GeV/c) 2 allow for a much more detailed examination of the form factors. The flavor-separated form factors provide information on the quark flavor dependence of the nucleon structure and test theoretical models of the form factors.Methods: We combine recent measurements of the neutron form factors with updated extractions of the proton form factors, accounting for two-photon exchange corrections and including an estimate of the uncertainties for all of the form factors to obtain a complete set of measurements up to Q 2 ≈ 4 (GeV/c) 2 . We use this to extract the up-and down-quark contributions which we compare to recent fits and calculations.Results: We find a large differences between the up-and down-quark contributions to GE and GM , implying significant flavor dependence in the charge and magnetization distributions. The rapid falloff of the ratio G p E /G p M does not appear in the individual quark form factors, but arises from a cancellation between the up-and down-quark contributions. We see indications that the down-quark contributions to the Dirac and Pauli form factors deviate from the suggested 1/Q 4 scaling behavior suggested by a previous analysis. While recent models provide a generally good qualitative description of the data, the down-quark contribution to GE/GM and F2/F1 are not reproduced by any of the models. Finally, we note that while the inclusion of recent G n M data from CLAS modifies the high-Q 2 slightly, the tension between these data and previous measurements at lower Q 2 has a more significant impact, suggesting the need for additional data in this region.