Spin density matrix elements in exclusive $$\rho ^0$$ meson muoproduction

Null, Null; Alexeev, G. D.; Alexeev, M. G.; Alice, C.; Amoroso, A.; Andrieux, V.; Anosov, V.; Augsten, K.; Augustyniak, W.; Azevedo, C. D. R.; Badelek, B.; Barth, J.; Beck, R.; Bedfer, Y.; Bernhard, J.; Bodlak, M.; Bradamante, F.; Bressan, A.; Burtsev, V. E.; Chang, W. -C.; Chatterjee, C.; Chiosso, M.; Chumakov, A. G.; Chung, S. -U.; Cicuttin, A.; Correia, P. M. M.; Crespo, M. L.; D'Ago, D.; Torre, S. Dalla; Dasgupta, S. S.; Dasgupta, S.; Carro, F. Del; Denisenko, I.; Denisov, O. Yu.; Donskov, S. V.; Doshita, N.; Dreisbach, Ch.; Dünnweber, W.; Dusaev, R. R.; Ecker, D.; Efremov, A.; Eremeev, D.; Faccioli, P.; Faessler, M.; Finger, M.; Finger, M.; Fischer, H.; Flöthner, K. J.; Florian, W.; Friedrich, J. M.; Frolov, V.; Garcia Ordòñez, L. G.; Gautheron, F.; Gavrichtchouk, O. P.; Gerassimov, S.; Giarra, J.; Giordano, D.; Gorzellik, M.; Grasso, A.; Gridin, A.; Perdekamp, M. Grosse; Grube, B.; Grüner, M.; Guskov, A.; Harrach, D. von; Hoffmann, M.; Horikawa, N.; D'Hose, N.; Hsieh, C. -Y.; Huber, S.; Ishimoto, S.; Ivanov, A.; Iwata, T.; Jandek, M.; Jary, V.; Joosten, R.; Kabuß, E.; Kaspar, F.; Kerbizi, A.; Ketzer, B.; Khatun, A.; Khaustov, G. V.; Klein, F.; Koivuniemi, J. H.; Kolosov, V. N.; Horikawa, K. Kondo; Konorov, I.; Konstantinov, V. F.; Korzenev, A. M.; Kotzinian, A. M.; Kouznetsov, O. M.; Koval, A.; Kral, Z.; Krinner, F.; Kunne, F.; Kurek, K.; Kurjata, R. P.; Kveton, A.; Lavickova, K.; Levorato, S.; Lian, Y. -S.; Lichtenstadt, J.; Lin, P. -J.; Longo, R.; Lyubovitskij, V. E.; Maggiora, A.; Magnon, A.; Makins, N.; Makke, N.; Mallot, G. K.; Maltsev, A.; Mamon, S. A.; Martin, A.; Marzec, J.; Matoušek, J.; Matsuda, T.; Mattson, G.; Pires, C. Menezes; Metzger, F.; Meyer, M.; Meyer, W.; Mikhailov, Yu. V.; Mikhasenko, M.; Mitrofanov, E.; Miura, D.; Miyachi, Y.; Molina, R.; Moretti, A.; Nagaytsev, A.; Naim, C.; Neyret, D.; Niemiec, M.; Nový, J.; Nowak, W. -D.; Nukazuka, G.; Olshevsky, A. G.; Ostrick, M.; Panzieri, D.; Parsamyan, B.; Paul, S.; Pekeler, H.; Peng, J. -C.; Pešek, M.; Peshekhonov, D. V.; Pešková, M.; Platchkov, S.; Pochodzalla, J.; Polyakov, V. A.; Quaresma, M.; Quintans, C.; Reicherz, G.; Riedl, C.; Ryabchikov, D. I.; Rychter, A.; Ryva, A.; Samoylenko, V. D.; Sandacz, A.; Sarkar, S.; Savin, I. A.; Sbrizzai, G.; Schmieden, H.; Selyunin, A.; Sharko, K.; Sinha, L.; Slunecka, M.; Spülbeck, D.; Srnka, A.; Stolarski, M.; Subrt, O.; Sulc, M.; Suzuki, H.; Tessaro, S.; Tessarotto, F.; Thiel, A.; Tomsa, J.; Tosello, F.; Townsend, A.; Triloki, T.; Tskhay, V.; Valinoti, B.; Veit, B. M.; Veloso, J. F. C. A.; Ventura, B.; Virius, M.; Wagner, M.; Wallner, S.; Zaremba, K.; Zavertyaev, M.; Zemko, M.; Zemlyanichkina, E.; Ziembicki, M.

doi:10.1140/epjc/s10052-023-11359-4

We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive rho(0) meson muo-production at COMPASS using 160 GeV/c polarised mu(+) and mu-beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/c(2) < W < 17.0 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, W denotes the mass of the final hadronic system,Q(2) the virtuality of the exchanged photon, and p(T) the transverse momentum of the rho(0) 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(& lowast; )(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 mayallow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive rho(0) production.