Speaker
Description
We will present results on Spin Density Matrix Elements (SDMEs) measured in hard exclusive muoproduction of $\rho ^0$, $\omega $ and $\phi$ mesons on the proton at COMPASS using 160 GeV/$c$ polarised $\mu ^{+}$ and $\mu^{-}$ beams scattering off a liquid hydrogen target. The measurements cover 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 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 ($\gamma _{T} \rightarrow V_{L}$) indicate a violation of $s$-channel helicity conservation. Additionally, for $\rho ^0$ and $\phi$ production we observe a dominant contribution of natural-parity-exchange (NPE) transitions and a small contribution of unnatural-parity-exchange (UPE) transitions. On the contrary the UPE contribution for $\omega $ production is significant and it decreases with increasing $W$, being still non-negligible at the largest $W$ values accessible at COMPASS. The results provide an 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 vector meson production.
| Submitted on behalf of a Collaboration? | Yes |
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| Participate in poster competition? | No |
