Orateur
Description
We will present preliminary results on Spin Density Matrix Elements (SDMEs) measured in hard exclusive $\phi$ meson muoproduction on protons at COMPASS using 160 GeV/$c$ polarised $\mu ^{+}$ and $\mu ^{-}$ beams scattering off a liquid hydrogen target. The measurements cover the range 6 GeV/$c^2$ < W < 17 GeV/$c^2$, 1.0 (GeV/$c$)$^2 <$ Q$^2 <$ 10.0 (GeV/$c$)$^2$ and 0.01 (GeV/$c$)$^2$ < p$_\rm{T}$ < 0.5 (GeV/c)$^2$. Here, Q$^2$ denotes the virtuality of exchanged photon, W the mass of final hadronic system and p$_\rm{T}$ the transverse momentum of the vector meson with respect to the virtual-photon direction. The measured non-zero SDMEs for transitions of transversely polarised virtual photons to longitudinally polarised vector mesons ($\gamma _{\rm{T}}$ $\rightarrow$ V{$_\rm{L}$) indicate a violation of s-channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a small contribution of unnatural-parity-exchange transitions. These results, combined with the published COMPASS results on SDMEs in exclusive $\rho ^0$ and $\omega$ productions, may provide an important input for modelling Generalised Parton Distribution (GPDs). In particular, they may allow to estimate the role of parton-helicity flip GPDs ("transversity GPDs") for the quarks of dfferent flavours and for gluons.
