2022/10/30 by Alexeev, G. D., Alexeev, M. G., Alice, C. +186
Physics and Astronomy · #FOS: Physical sciences #High Energy Physics - Experiment (hep-ex) #High Energy Physics - Phenomenology (hep-ph) #High-Energy Particle Collisions Research #Particle physics theoretical and experimental studies #Quantum Chromodynamics and Particle Interactions
paper · pdf · doi:10.48550/arxiv.2210.16932
openalex publication_date 2022/10/30 · openalex created_date 2022/11/06 · openalex updated_date 2026/07/28
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive ρ0 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 < p_\rmT2 < 0.5 (GeV/c)2. Here, W denotes the mass of the final hadronic system, Q2 the virtuality of the exchanged photon, and p_\rmT the transverse momentum of the ρ0 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 (γ^*T → V L) 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 ρ0 production.