Abstract
The transverse target spin azimuthal asymmetry AUTsin(ϕ−ϕS) in hard exclusive production of ρ0 mesons was measured at COMPASS by scattering 160 GeV/c muons off transversely polarised protons and deuterons. The measured asymmetry is sensitive to the nucleon helicity-flip generalised parton distributions Eq, which are related to the orbital angular momentum of quarks in the nucleon. The Q2, xBj and pT2 dependence of AUTsin(ϕ−ϕS) is presented in a wide kinematic range: 1(GeV/c)2<Q2<10(GeV/c)2, 0.003<xBj<0.3 and 0.05(GeV/c)2<pT2<0.5(GeV/c)2 for protons or 0.10(GeV/c)2<pT2<0.5(GeV/c)2 for deuterons. Results for deuterons are obtained for the first time. The measured asymmetry is small in the whole kinematic range for both protons and deuterons, which is consistent with the theoretical interpretation that contributions from GPDs Eu and Ed approximately cancel.
Highlights
1 IntroductionHard exclusive electro- and muoproduction of mesons on nucleons has played an important role in studies of strong interactions and recently gained renewed interest as it allows access to generalised parton distributions (GPDs) [1,2,3,4,5]
The spin-independent cross sections for deeply virtual Compton scattering (DVCS) and for vector meson production by longitudinal photons on a proton target are mostly sensitive to the nucleon-helicity-conserving GPDs Hf, with GPDs Ef being suppressed in the COMPASS kinematic domain
Diffractive dissociation background is examined using a Monte Carlo (MC) event generator called HEPGEN [26]. This generator is dedicated to studies of hard exclusive single photon or meson production processes in the COMPASS kinematic domain
Summary
Hard exclusive electro- and muoproduction of mesons on nucleons has played an important role in studies of strong interactions and recently gained renewed interest as it allows access to generalised parton distributions (GPDs) [1,2,3,4,5]. In perturbative QCD (pQCD), there exists a general proof of factorisation [9] for exclusive meson production by longitudinal virtual photons. The spin-independent cross sections for DVCS and for vector meson production by longitudinal photons on a proton target are mostly sensitive to the nucleon-helicity-conserving GPDs Hf , with GPDs Ef being suppressed in the COMPASS kinematic domain. It was pointed out that the spin dependent cross sections for DVCS on transversely polarised protons [13, 14] and for exclusive vector meson production by longitudinal photons on transversely polarised nucleons [15] are sensitive to the ‘elusive’ nucleon helicity-flip GPDs Ef. Access to GPDs Ef is possible by measurements of the cross section for DVCS on an unpolarised neutron target [14]. The experiment was carried out at CERN by the COMPASS collaboration using the 160 GeV/c muon beam and a polarised target filled either with lithium deuteride (6LiD) or ammonia (NH3) to provide polarised deuterons or protons, respectively
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