Speaker
Description
We will present results on Spin Density Matrix Elements (SDMEs) measured in hard exclusive $\rho^{0}$ meson muoproduction on the proton at COMPASS using 160 GeV/c polarised $\mu^{+}$ and $\mu^{-}$ beams scattering off a liquid hydrogen target. The measurement covers the range 5 GeV/$c^{2}$ < $W$ < 17 GeV/$c^{2}$, 1.0 (GeV/$c$)$^{2}$ < Q$^{2}$ < 10.0 (GeV/$c$)$^{2}$ and 0.01 (GeV/$c$)$^{2}$ < $p_{T}^{2}$ < 0.5 (GeV/$c$)$^{2}$. Here, Q$^{2}$ denotes the virtuality of exchanged photon, $W$ the mass of final hadronic system and p$_{T}$ the transverse momentum of the $\rho^{0}$ 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 ($\gamma^{*}_{T} \rightarrow V_{L}$) indicate a significant 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 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 $\rho^{0}$ production.
