Search

A comprehensive set of azimuthal single-spin and double-spin asymmetries in semi-inclusive leptoproduction of pions, charged kaons, protons, and antiprotons from transversely polarized protons is presented. These asymmetries include the previously published HERMES results on Collins and Sivers asymmetries, the analysis of which has been extended to include protons and antiprotons and also to an extraction in a three-dimensional kinematic binning and enlarged phase space. They are complemented by corresponding results for the remaining four single-spin and four double-spin asymmetries allowed in the one-photon-exchange approximation of the semi-inclusive deep-inelastic scattering process for target-polarization orientation perpendicular to the direction of the incoming lepton beam. Among those results, significant non-vanishing cos (ϕ−ϕS) modulations provide evidence for a sizable worm-gear (II) distribution, g 1 T q x p T 2 . Most of the other modulations are found to be consistent with zero with the notable exception of large sin (ϕS) modulations for charged pions and K+. [ABSTRACT FROM AUTHOR]

Exclusive ρ-meson electroproduction is studied in the HERMES experiment, using a 27.6 GeV longitudinally polarized electron/positron beam and unpolarized hydrogen and deuterium targets in the kinematic region 0.5 GeV< Q<7.0 GeV, 3.0 GeV < W<6.3 GeV, and − t′<0.4 GeV. Real and imaginary parts of the ratios of the natural-parity-exchange helicity amplitudes T ( $\gamma^{*}_{T} \rightarrow\rho_{T}$), T ( $\gamma^{*}_{T} \rightarrow \rho_{L}$), T ( $\gamma^{*}_{L} \rightarrow\rho_{T}$), and T ( $\gamma^{*}_{-T} \rightarrow\rho_{T}$) to T ( $\gamma^{*}_{L}\rightarrow\rho_{L}$) are extracted from the data. For the unnatural-parity-exchange amplitude U, the ratio | U/ T| is obtained. The Q and t′ dependences of these ratios are presented and compared with perturbative QCD predictions. [ABSTRACT FROM AUTHOR]

Spin Density Matrix Elements (SDMEs) describing the angular distribution of exclusive ρ0 electroproduction and decay are determined in the HERMES experiment with 27.6 GeV beam energy and unpolarized hydrogen and deuterium targets. Eight (fifteen) SDMEs that are related (unrelated) to the longitudinal polarization of the beam are extracted in the kinematic region 1< Q2<7 GeV2, 3.0< W<6.3 GeV, and − t<0.4 GeV2. Within the given experimental uncertainties, a hierarchy of relative sizes of helicity amplitudes is observed. Kinematic dependences of all SDMEs on Q2 and t are presented, as well as the longitudinal-to-transverse ρ0 electroproduction cross-section ratio as a function of Q2. A small but statistically significant deviation from the hypothesis of s-channel helicity conservation is observed. An indication is seen of a contribution of unnatural-parity-exchange amplitudes; these amplitudes are naturally generated with a quark-exchange mechanism. [ABSTRACT FROM AUTHOR]

Double-spin asymmetries in the cross section of electroproduction of p0 and φ mesons on the proton and deuteron are measured at the HERMES experiment. The photoabsorption asymmetry in exclusive p0 electroproduction on the proton exhibits a positive tendency. This is consistent with theoretical predictions that the exchange of an object with unnatural parity contributes to exclusive p0 electroproduction by transverse photons. The photoabsorption asymmetry on the deuteron is found to be consistent with zero. Double-spin asymmetries in p0 and φ meson electroproduction by quasi-real photons were also found to be consistent with zero; the asymmetry in the case of the φ meson is compatible with a theoretical prediction which involves ss knockout from the nucleon. [ABSTRACT FROM AUTHOR]

The Gerasimov-Drell-Hearn (GDH) sum rule connects the anomalous contribution to the magnetic moment of the target nucleus with an energy-weighted integral of the difference of the helicity-dependent photoabsorption cross sections. Originally conceived for real photons, the GDH integral can be generalised to the case of photons with virtuality Q². For spin-1 /2 targets such as the nucleon, it then represents the non-perturbative limit of the first moment Γ1 of the spin structure function g1 (x, Q²) in deep inelastic scattering (DIS). The data collected by HERMES with a deuterium target are presented together with a re-analysis of previous measurements on the proton. This provides an unprecedented and complete measurement of the generalised GDH integral for photon-virtuality ranging over 1.2 1. From data on the deuteron and proton, the GDH integral for the neutron has been derived and the proton-neutron difference evaluated. This difference is found to satisfy the fundamental Bjorken sum rule at Q² = 5 GeV². [ABSTRACT FROM AUTHOR]

A beamline for 20 MeV electrons has been developed and mounted at Yerevan Physics Institute (YerPhI). The beamline was obtained by parallel transfer of the beam from the operating 75 MeV injector-linac of the 6 GeV Yerevan synchrotron. Parameters of the beam allow performing experimental studies of radiation of electrons in single crystals. [ABSTRACT FROM AUTHOR]