54 research outputs found
The Q(2)-dependence of the generalised Gerasimov-Drell-Hearn integral for the deuteron, proton and neutron
No full textThe 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 Q(2). For spin-1/2 targets such as the nucleon, it then represents the non-perturbative limit of the first moment Gamma(1) of the spin structure function g(1)(x,Q(2)) 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 lt Q(2) lt 12.0 GeV2 and for photon-nucleon invariant mass squared W-2 ranging over I lt W2 lt 45 GeV2. thus covering simultaneously the nucleon-resonance and the deep inelastic scattering regions. These data allow the study of the Q(2)- dependence of the full GDH integral, which is sensitive to both the Q(2)-evolution of the resonance form factors and contributions of higher twist. The contribution of the nucleon- resonance region is seen to decrease rapidly with increasing Q(2). The DIS contribution is sizeable over the full measured range, even down to the lowest measured Q(2). As expected, at higher Q(2) the data are found to be in agreement with previous measurements of the first moment of g(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 Q(2) = 5 GeV2
Single-spin azimuthal asymmetries in electroproduction of neutral pions in semi-inclusive deep-inelastic scattering
No full textA single-spin asymmetry in the azimuthal distribution of neutral pions relative to the lepton scattering plane has been measured for the first time in deep-inelastic scattering of positrons off longitudinally polarized protons. The analyzing power in the sin phi moment of the cross section is 0.019+or-0.007(stat)+or-0.003(syst). This result is compared to single-spin asymmetries for charged pion production measured in the same kinematic range. The pi /sup 0/ asymmetry is of the same size as the pi /sup +/ asymmetry and shows a similar dependence on the relevant kinematic variables. The asymmetry is described by a phenomenological calculation based on a fragmentation function that represents sensitivity to the transverse polarization of the struck quark
Single-spin azimuthal asymmetry in exclusive electroproduction of pi /sup +/ mesons
No full textA single-spin asymmetry in the distribution of exclusively produced pi /sup +/ mesons azimuthally around the virtual photon direction relative to the lepton scattering plane has been measured for the first time in deep-inelastic scattering of positrons off longitudinally polarized protons. Integrated over the experimental acceptance, the sin phi moment of the polarization asymmetry of the cross section is measured to be -0.18+or-0.05(stat.)+or-0.02(syst.). The asymmetry is also studied as a function of the relevant kinematic variables, and its magnitude is found to grow with decreasing x and increasing -t and vanish at t to t/sub min/ |t|. Further, the data show that the longitudinal target polarization component with respect to the virtual photon direction provides the dominant contribution to the measured asymmetry at small x. A next-to-leading twist calculation would thus be required for a complete description of the measurement. The present data, in combination with future HERMES measurements on a transversely polarized target [18] for which a large asymmetry is expected, opens the way to experimentally disentangle the asymmetry arising from the two target polarization components. This information may provide a first glimpse of certain unknown Generalized Parton Distributions
Evidence for quark-hadron duality in the proton spin asymmetry A(1)
No full textSpin-dependent lepton-nucleon scattering data have been used to investigate the validity of the concept of quark-hadron duality for the spin asymmetry A(1). Longitudinally polarized positrons were scattered off a longitudinally polarized hydrogen target for values of Q(2) between 1.2 and 12 GeV2 and values of W-2 between 1 and 4 GeV2. The average double-spin asymmetry in the nucleon resonance region is found to agree with that measured in deep-inelastic scattering at the same values of the Bjorken scaling variable x. This finding implies that the description of A(1) in terms of quark degrees of freedom is valid also in the nucleon resonance region for values of Q(2) above 1.6 GeV2
Nuclear effects on R = sigma(L)/sigma(T) in deep-inelastic scattering
No full textCross 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
No full textThe 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
Evidence for a single-spin azimuthal asymmetry in semi-inclusive pion ekectroproduction.
No full textSingle-spin asymmetries for semi-inclusive pion production in deep-inelastic scattering have been measured for the first time. A significant target-spin asymmetry of the distribution in the azimuthal angle φ of the pion relative to the lepton scattering plane was formed for π+ electroproduction on a longitudinally polarized hydrogen target. The corresponding analyzing power in the sinφ moment of the cross section is 0.022±0.005±0.003. This result can be interpreted as the effect of terms in the cross section involving chiral-odd spin distribution functions in combination with a chiral-odd fragmentation function that is sensitive to the transverse polarization of the fragmenting quark
Exclusive leptoproduction of rho-0 mesons from Hydrogen at intermediate virtual photon energies
No full textMeasurements 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 GeV2. The present data together with roost of the previous data in the intermediate W-domain are well described by a model that infers the W-dependence of the cross section from the dependence on the Bjorken scaling variables of the unpolarized structure function for deep-inelastic scattering. In addition, a model calculation based on OR-Forward Parton Distributions gives a fairly good account of the longitudinal component of the rho (0) production cross section for Q(2) > 2 GeV2
Nuclear effects on R = sigma(L)/sigma(T) in deep-inelastic scattering (vol 475, pg 386, 2000)
No full texterratum to Nuclear effects on R = sigma(L)/sigma(T) in deep-inelastic scattering (vol 475, pg 386, 2000
Double-spin asymmetry in the cross section for exclusive ρ0 production in lepton–proton scattering
No full textEvidence for a positive longitudinal double-spin asymmetry (A/sub 1//sup rho /)=0.24+or-0.11/sub stat/+or-0.02/sub syst/ in the cross section for exclusive diffractive rho /sup 0/(770) vector-meson production in polarised lepton-proton scattering was observed by the HERMES experiment. The longitudinally polarised 27.56 GeV HERA positron beam was scattered off a longitudinally polarised pure hydrogen gas target. The average invariant mass of the photon-proton system has a value of (W)=4.9 GeV, while the average negative squared four-momentum of the virtual photon is (Q/sup 2/)=1.7 GeV/sup 2/. The ratio of the present result to the corresponding spin asymmetry in inclusive deep-inelastic scattering is in agreement with an early theoretical prediction based on the generalised vector-meson dominance model
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