Double-spin asymmetry in the cross section for exclusive ρ0 production in lepton–proton scattering

Evidence for a positive longitudinal double-spin asymmetry 〈A1ρ〉=0.24±0.11stat±0.02syst in the cross section for exclusive diffractive ρ0(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 〈Q2〉=1.7 GeV2. 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.