31st International Workshop on Deep Inelastic Scattering

Spin Density Matrix Elements in hard exclusive ϕ meson muoproduction at COMPASS

9 avr. 2024, 10:10

20m

Maison MINATEC, Grenoble, FRANCE

Regular parallel talk WG5: Spin and 3D Structure WG5

Orateur

Dr Witold Augustyniak (National Centre for Nuclear Research)

Description

We will present preliminary results on Spin Density Matrix Elements (SDMEs) measured in hard exclusive $\varphi$ 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${}_{\mathrm{T}}$ < 0.5 (GeV/c)${}^2$. Here, Q${}^2$ denotes the virtuality of exchanged photon, W the mass of final hadronic system and p${}_{\mathrm{T}}$ 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 (${\gamma }_{\mathrm{T}}$ $\to$ V{${}_{\mathrm{L}}$) 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.