We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive ρ meson muoproduction at COMPASS using 160 GeV/c polarised μ+ and μ- beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/ c2< W< 17.0 GeV/ c2 , 1.0 (GeV/c) 2< Q2< 10.0 (GeV/c) 2 and 0.01 (GeV/c) 2<pT2< 0.5 (GeV/c) 2 . Here, W denotes the mass of the final hadronic system, Q2 the virtuality of the exchanged photon, and pT the transverse momentum of the ρ meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons (γT∗→VL) indicate a violation of s-channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive ρ production.

Original languageEnglish
Article number924
JournalEuropean Physical Journal C
Issue number10
StatePublished - Oct 2023
Externally publishedYes


Dive into the research topics of 'Spin density matrix elements in exclusive ρ meson muoproduction'. Together they form a unique fingerprint.

Cite this