Spin density matrix elements in exclusive $$\omega $$ ω meson muoproduction

• Published in: The European physical journal. C, Particles and fields Vol. 81; no. 2; pp. 1 - 22
• Main Authors: M. G. Alexeev, V. Andrieux, A. Antoshkin, C. D. R. Azevedo, B. Badełek, F. Balestra, J. Barth, R. Beck, Y. Bedfer, J. Bernhard, C. Chatterjee, M. Chiosso, S.-U. Chung, M. L. Crespo, S. Dasgupta, I. Denisenko, O. Yu. Denisov, A. Efremov, P. D. Eversheim, P. Faccioli, M. Faessler, M. Finger, J. M. Friedrich, V. Frolov, J. Giarra, M. Gorzellik, A. Gridin, B. Grube, D. von Harrach, R. Heitz, F. Herrmann, C.-Y. Hsieh, S. Huber, V. Jary, R. Joosten, E. Kabuß, F. Kaspar, Yu. A. Khokhlov, Yu. Kisselev, F. Klein, V. N. Kolosov, K. Kondo Horikawa, I. Konorov, A. Koval, Z. Kral, Y. Kulinich, F. Kunne, R. P. Kurjata, A. Kveton, S. Levorato, Y.-S. Lian, J. Lichtenstadt, V. E. Lyubovitskij, A. Maggiora, A. Magnon, N. Makins, A. Maltsev, B. Marianski, W. Meyer, M. Mikhasenko, E. Mitrofanov, N. Mitrofanov, A. Nagaytsev, D. Neyret, W.-D. Nowak, G. Nukazuka, A. S. Nunes, D. Panzieri, S. Paul, J.-C. Peng, M. Pešek, D. V. Peshekhonov, M. Pešková, N. Pierre, S. Platchkov, J. Pochodzalla, V. A. Polyakov, M. Quaresma, C. Quintans, C. Regali, T. Rudnicki, A. Rybnikov, A. Rychter, A. Sandacz, G. Sbrizzai, A. Selyunin, L. Sinha, J. Smolik, D. Steffen, M. Sulc, F. Tessarotto, A. Thiel, A. Vauth, B. M. Veit, J. Veloso, S. Wallner, K. Zaremba, P. Zavada, M. Zemko, Y. Zhao
• Format: Journal Article
• Language: English
• Published: SpringerOpen 01.02.2021
• ISSN: 1434-6044; 1434-6052
• DOI: 10.1140/epjc/s10052-020-08740-y

Abstract We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive $$\omega $$ ω meson muoproduction on the proton at COMPASS using 160 GeV/c polarised $$ \mu ^{+}$$ μ + and $$ \mu ^{-}$$ μ - beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/ $$c^2$$ c 2 $$< W<$$ < W < 17.0 GeV/ $$c^2$$ c 2 , with the average kinematics $$\langle Q^{2} \rangle =$$ ⟨ Q 2 ⟩ = 2.1 (GeV/c) $$^2$$ 2 , $$\langle W \rangle = 7.6$$ ⟨ W ⟩ = 7.6  GeV/ $$c^2$$ c 2 , and $$\langle p^{2}_{\mathrm{T}} \rangle = 0.16$$ ⟨ p T 2 ⟩ = 0.16  (GeV/c) $$^2$$ 2 . Here, $$Q^2$$ Q 2 denotes the virtuality of the exchanged photon, W the mass of the final hadronic system and $$p_T$$ p T the transverse momentum of the $$\omega $$ ω 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 ( $$\gamma ^*_T \rightarrow V_L$$ γ T ∗ → V L ) indicate a violation of s-channel helicity conservation. Additionally, we observe a sizeable contribution of unnatural-parity-exchange (UPE) transitions that decreases with increasing W. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow to evaluate in a model-dependent way the contribution of UPE transitions and assess the role of parton helicity-flip GPDs in exclusive $$\omega $$ ω production.