Exclusive Leptoproduction of rho^0 Mesons from Hydrogen at Intermediate Virtual Photon Energies

Measurements of the cross section for exclusive virtual-photoproduction of rho^0 mesons from hydrogen are reported. The data were collected by the HERMES experiment using 27.5 GeV positrons incident on a hydrogen gas target in the HERA storage ring. The invariant mass W of the photon-nucleon system ranges from 4.0 to 6.0 GeV, while the negative squared four-momentum Q^2 of the virtual photon varies from 0.7 to 5.0 GeV^2. The present data together with most of the previous data at W > 4 GeV are well described by a model that infers the W-dependence of the cross section from the dependence on the Bjorken scaling variable x of the unpolarized structure function for deep-inelastic scattering. In addition, a model calculation based on Off-Forward Parton Distributions gives a fairly good account of the longitudinal component of the rho^0 production cross section for Q^2 > 2 GeV^2.Comment: 10 pages, 6 embedded figures, LaTeX for SVJour(epj) document class. Revisions: curves added to Fig. 1, several clarifications added to tex

Search for Nucleon-nucleon Correlations in the Proton Spectral-function of Pb-208

Cross sections for the reaction Pb-208 (e,e'p) have been measured with the continuous electron beam from the Amsterdam Pulse-Stretcher facility at NIKHEF-K. The spectral function has been extracted for protons with initial momenta of 300 to 500 MeV/c and binding energies up to 26 MeV. The data are compared to calculations with and without inclusion of nucleon-nucleon correlations. Mean-field predictions significantly underestimate the data and the discrepancy increases with binding energy. For transitions to the valence states the discrepancy is removed by introducing long-range correlations. Above the two-nucleon emission threshold long-range and short-range correlations reduce the discrepancy, but are insufficient to fully account for the measured strength

Nuclear effects on R = sigma(L)/sigma(T) in deep-inelastic scattering

Cross section ratios for deep-inelastic scattering from 14N and 3He with respect to 2H have been measured by the HERMES experiment at DESY using a 27.5 GeV positron beam. The data cover a range in the Bjorken scaling variable x between 0.013 and 0.65, while the negative squared four-momentum transfer Q2 varies from 0.5 to 15 GeV2. The data are compared to measurements performed by NMC, E665, and SLAC on 4He and 12C, and are found to be different for x<0.06 and Q2<1.5 GeV2. The observed difference is attributed to an A-dependence of the ratio R = σL/σT of longitudinal to transverse deep-inelastic scattering cross sections at low x and low Q

Measurement of the beam-spin azimuthal asymmetry associated with deeply-virtual Compton scattering

The beam-spin asymmetry in hard electroproduction of photons has been measured. The data have been accumulated by the HERMES experiment at DESY using the HERA 27.6 GeV longitudinally polarized positron beam and an unpolarized hydrogen-gas target. The asymmetry in the azimuthal distribution of the produced photons in the angle phi relative to the lepton scattering plane was determined with respect to the helicity state of the incoming positron beam. The beam-spin analyzing power in the sin phi moment was measured to be -0.23 +/- 0.04(stat) +/- 0.03(syst) in the missing-mass range below 1.7 GeV. The observed asymmetry is attributed to the interference of the Bethe-Heitler and deeply virtual Compton scattering processes

Multiplicity of charged and neutral pions in deep-inelastic scattering of 27.5 GeV positrons on hydrogen

Measurements of the individual multiplicities of pi (+), pi (-) and pi (0) produced in thc deep-inelastic scattering of 27.5 GeV positrons on hydrogen are presented. The average charged pion multiplicity is the same as for neutral pions, up to z approximate to 0.7, where z is the fraction of the energy transferred in the scattering process carried by the pion. This result (below z approximate to 0.7) is consistent with isospin invariance. The total energy fraction associated with charged and neutral pions is 0.51 +/- 0.01(stat.) +/- 0.08 (syst.) and 0.26 +/- 0.01 (stat.) +/- 0.04(syst.), respectively. For fixed z, the measured multiplicities depend oil both the negative squared four momentum transfer Q(2) and the Bjorken variable x. The observed dependence oil Q(2) agrees qualitatively with the expected behaviour based oil NLO-QCD evolution, while, the dependence on x is consistent with that of previous data after corrections have been made for the expected Q(2)-dependence

Hadron formation in deep-inelastic positron scattering in a nuclear environment

The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep-inelastic scattering has been studied by the HERMES experiment at DESY using a 27.5 GeV positron beam. The differential multiplicity of charged hadrons and identified charged pions from nitrogen relative to that from deuterium has been measured as a function of the virtual photon energy nu and the fraction z of this energy transferred to the hadron. There are observed substantial reductions of the multiplicity ratio R-M(h) at low nu and at high z, both of which are well described by a gluon-bremsstrahlung model of hadronization. A significant difference of the nu -dependence of R-M(h) is found between positive and negative A hadrons. This is interpreted in terms of a difference between the formation times of protons and pions, using a phenomenological model to describe the nu- and z-dependence of R-M(h)