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

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 tensor structure function b(1) of the deuteron

The Hermes experiment has investigated the tensor spin structure of the deuteron using the 27.6 GeV/c positron beam of DESY HERA. The use of a tensor-polarized deuteron gas target with only a negligible residual vector polarization enabled the first measurement of the tensor asymmetry A(zz)(d) and the tensor structure function b(1)(d) for average values of the Bjorken variable 0.01 < 5 GeV2. The quantities A(zz)(d) and b(1)(d) are found to be nonzero. The rise of b(1)(d) for decreasing values of x can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering

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)