18-22 October 2021
Matsue, Shimane Prefecture, Japan
Asia/Tokyo timezone

Spin Density Matrix Elements in Exclusive Muoproduction of $\rho ^0$ and $\omega $ Mesons at COMPASS

18 Oct 2021, 21:40
Room 501 (Kunibiki Messe)

Room 501

Kunibiki Messe

Parallel Session Presentation Form factors and GPDs Form Factors and GPDs


Kamil Augsten (Czech Technical University in Prague)


We will present results on Spin Density Matrix Elements (SDMEs) measured in hard exclusivsive muoproduction of $\rho ^0$ and $\omega $ 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 considerable violation of $s$-channel helicity conservation. Additionally, for $\rho ^0$ production we observe a dominant contribution of natural-parity-exchange transitions and a small contribution of
unnatural-parity-exchange transitions observed only at small values of $W$. On the contrary,the contribution of unnatural-parity-exchange for $\omega $ production is significant. 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 $\rho ^0$ and $\omega $ production.

Primary author

Prof. Andrzej Sandacz (National Centre for Nuclear Research, Poland)

Presentation Materials