Spin density matrix elements in exclusive ρ0 meson muoproduction

Abstract

We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive rho(0) meson muo-production at COMPASS using 160 GeV/c polarised mu(+) and mu-beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/c(2) < W < 17.0 GeV/c(2), 1.0 (GeV/c)(2) < Q(2) < 10.0 (GeV/c)(2) and 0.01 (GeV/c)(2) < p(T)(2) < 0.5 (GeV/c)(2). Here, W denotes the mass of the final hadronic system,Q(2) the virtuality of the exchanged photon, and p(T) the transverse momentum of the rho(0) meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons (gamma(& lowast; )(T)-> V-L) indicate a violation of s-channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they mayallow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive rho(0) production.