Double-spin asymmetry in the cross section for exclusive rho(0) production in lepton-proton scattering

Abstract Evidence for a positive longitudinal double-spin asymmetry 〈 A 1 ρ 〉=0.24±0.11 stat ±0.02 syst in the cross section for exclusive diffractive ρ 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 2 〉=1.7 GeV 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.