Double hadron leptoproduction in the nuclear medium

First measurement of double-hadron production in deep-inelastic scattering has been measured with the HERMES spectrometer at HERA using a 27.6 GeV positron beam with deuterium, nitrogen, krypton and xenon targets. The influence of the nuclear medium on the ratio of double-hadron to single-hadron yields has been investigated. Nuclear effects are clearly observed but with substantially smaller magnitude and reduced $A$-dependence compared to previously measured single-hadron multiplicity ratios. The data are in fair agreement with models based on partonic or pre-hadronic energy loss, while they seem to rule out a pure absorptive treatment of the final state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter

Quark helicity distributions in the nucleon for up, down, and strange quarks from semi--inclusive deep--inelastic scattering

Polarized deep--inelastic scattering data on longitudinally polarized hydrogen and deuterium targets have been used to determine double spin asymmetries of cross sections. Inclusive and semi--inclusive asymmetries for the production of positive and negative pions from hydrogen were obtained in a re--analysis of previously published data. Inclusive and semi--inclusive asymmetries for the production of negative and positive pions and kaons were measured on a polarized deuterium target. The separate helicity densities for the up and down quarks and the anti--up, anti--down, and strange sea quarks were computed from these asymmetries in a ``leading order'' QCD analysis. The polarization of the up--quark is positive and that of the down--quark is negative. All extracted sea quark polarizations are consistent with zero, and the light quark sea helicity densities are flavor symmetric within the experimental uncertainties. First and second moments of the extracted quark helicity densities in the measured range are consistent with fits of inclusive data

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

Evidence for a narrow |S|=1 baryon state at a mass of 1528 MeV in quasi-real photoproduction

Evidence for a narrow baryon state is found in quasi-real photoproduction on a deuterium target through the decay channel p K^0_S --> p pi^+ pi^-. A peak is observed in the p K^0_S invariant mass spectrum at 1528 +/- 2.6 (stat) +/-2.1 (syst) MeV. Depending on the background model,the naive statistical significance of the peak is 4--6 standard deviations and its width may be somewhat larger than the experimental resolution of sigma=4.3 -- 6.2 MeV. This state may be interpreted as the predicted S=+1 exotic Theta^{+}(uuddbar(s)) pentaquark baryon. No signal for an hypothetical Theta^{++} baryon was observed in the pK^+ invariant mass distribution. The absence of such a signal indicates that an isotensor Theta is excluded and an isovector Theta is unlikely.Comment: 8 pages, 4 figure

Relativistic effects in the electrodisintegration of deuterium

The structure function R(LT) and the cross-section asymmetry A(phi) with respect to the direction of the momentum transfer in the reaction H-2(e, e'p) have been measured at a four-momentum transfer squared of 0.2 (GeV/c)(2), for missing momenta between 160 and 220 MeV/c at an invariant mass of 1050 MeV. For a proper description of these data calculations that include a relativistic form of the nucleon current operator are favoured. The absolute 2H(e, e'p) cross-section data favour a covariant calculation over non-relativistic calculations with relativistic corrections

Deuteron electrodisintegration at high missing momenta

The reaction (2)H(e, e'p) has been studied at an invariant mass W of 1050 MeV, i.e. well below the Delta(1232) resonance, Cross sections have been obtained at values of Q(2), the four-momentum transfer squared, of 0.10, 0.20, and 0.28 (GeV/c)(2), covering a missing-momentum range from 150 to 700 MeV/c. The data are compared to the results of covariant calculations of Tjon, and the results of calculations based on a Schrodinger formalism due to Laget and the Mainz group, respectively. The data are well described by the calculations of the Mainz group, whereas they are underestimated by Tjon's calculations at high missing momenta. The calculations of Laget, on the other hand, overestimate the data at low missing momenta, but give a good account of the data at high missing momenta. More detailed considerations reveal that the Delta(1232) contributions are dominant at high missing momenta. However, the lacking Delta(1232) contribution in Tjon's calculations is not enough to explain the large discrepancy between his calculation and the present (2)H(e, e'p) data at high missing momentum. Probably the deuteron wave function employed in the covariant calculations has a D-state contribution that is too small

Deuteron electrodisintegration in the Delta-resonance region

The differential cross section and the transverse-transverse interference structure function for the reaction H-2(e,e'p)n have been determined at an np invariant mass of 2.16 GeV. The data, covering a 40 degrees range in the proton emission angle, indicate that Delta excitation and subsequent N Delta interaction is the dominant reaction mechanism. Calculations performed within an N Delta coupled-channel approach reproduce the cross section data, but underestimate the f(TT) results by 30 to 40 percent

Proton detection with large-acceptance scintillator detection systems in electron-scattering environments

Two highly segmented plastic-scintillator arrays have been developed for proton detection in electron scattering experiments. The detectors subtend solid angles of 225 and 550 msr and cover energy ranges of 50-225 and 25-165 MeV, respectively. The charge and arrival time of each photomultiplier signal are digitized by flash ADCs and temporarily stored in a dual-port memory. The readout parameters are computer controlled, tuned, and monitored. These detectors have been employed in (e, e'p) and (e, e'pp) experiments for proton emission angles greater than 30 degrees and for luminosities up to 10(36) nucleons cm(-2) s(-1). The singles counting rates in the scintillator elements of the first layers were about 0.5 x 10(6) particles s(-1) and the trigger rate 1 MHz. The measured resolution in the excitation energy and timing spectra are 2.7% and 0.7 ns, respectively. (C) 1999 Elsevier Science B.V. All rights reserved

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