Speaker
Description
Preliminary results on Spin Density Matrix Elements in hard exclusive muoproduction of $\phi$ mesons on the proton are presented. The measurements were performed at COMPASS using 160 GeV/c polarised positive and negative muon beams incident impinging on a liquid hydrogen target. The covered kinematic range is $5 < W < 17~\mathrm{GeV}$, $1.0 < Q^2 < 10.0~\mathrm{GeV}^2$, and $0.01 < p_T^2 < 0.5~\mathrm{GeV}^2$. Here, $Q^2$ denotes the virtuality of the exchanged photon, $W$ the invariant mass of the final-state hadronic system, and $p_T$ the transverse momentum of the $\phi$ meson with respect to the virtual-photon direction.
The extracted Spin Density Matrix Elements allow the study of helicity transitions in exclusive $\phi$ production, in particular transitions from transversely polarised virtual photons to longitudinally polarised vector mesons, which provide a test of s-channel helicity conservation. Contributions from natural-parity-exchange and unnatural-parity-exchange amplitudes are investigated as functions of $W$, $Q^2$, $p_T$, and $x_B$.
These results provide constraints for phenomenological models based on Generalised Parton Distributions (GPDs) and may allow a model-dependent assessment of the role of parton-helicity-flip (transversity) GPDs in exclusive $\phi$ meson production.
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