DrWitold Augustyniak(National Centre for Nuclear Research Warsaw,poland)
Description
We will present results on Spin Density Matrix Elements (SDMEs) measured in hard exclusive meson muoproduction on the proton at COMPASS using 160 GeV/c polarised and beams scattering off a liquid hydrogen target. The measurement covers the range 5 GeV/ < < 17 GeV/, 1.0 (GeV/) < Q < 10.0 (GeV/) and 0.01 (GeV/) < < 0.5 (GeV/). Here, Q denotes the virtuality of exchanged photon, the mass of final hadronic system and p the transverse momentum of the meson with respect to the virtual-photon direction. The kinematic dependences of SDMEs and of related observables will be presented. The measured non-zero SDMEs for transitions of transversely polarised virtual photons to longitudinally polarised vector mesons () indicate a significant violation of -channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a small contribution of unnatural-parity-exchange transitions at small values of W. The results provide important input for modelling Generalised Parton Distribution (GPDs). In particular, they may allow to evaluate in a model-dependent way the role of parton-helicity flip GPDs ("transversity GPDs") in exclusive production.