Spin density matrix elements in exclusive ω meson muoproduction
G. D. Alexeev,M. G. Alexeev,A. Amoroso,V. Andrieux,V. Anosov,A. Antoshkin,K. Augsten,W. Augustyniak,C. D. R. Azevedo,B. Badełek,F. Balestra,M. Ball,J. Barth,R. Beck,Y. Bedfer,J. Berenguer Antequera,J. Bernhard,M. Bodlak,F. Bradamante,A. Bressan,V. E. Burtsev,W.-C. Chang,C. Chatterjee,M. Chiosso,A. G. Chumakov,S.-U. Chung,A. Cicuttin,P. M. M. Correia,M. L. Crespo,D. D’Ago,S. Dalla Torre,S. S. Dasgupta,S. Dasgupta,I. Denisenko,O. Yu. Denisov,S. V. Donskov,N. Doshita,Ch. Dreisbach,W. Dünnweber,R. R. Dusaev,A. Efremov,P. D. Eversheim,P. Faccioli,M. Faessler,A. Ferrero,M. Finger,M. Finger Jr.,H. Fischer,C. Franco,J. M. Friedrich,V. Frolov,F. Gautheron,O. P. Gavrichtchouk,S. Gerassimov,J. Giarra,I. Gnesi,M. Gorzellik,A. Grasso,A. Gridin,M. Grosse Perdekamp,B. Grube,A. Guskov,D. von Harrach,R. Heitz,F. Herrmann,N. Horikawa,N. d’Hose,C.-Y. Hsieh,S. Huber,S. Ishimoto,A. Ivanov,T. Iwata,M. Jandek,V. Jary,P. Jörg,R. Joosten,E. Kabuß,F. Kaspar,A. Kerbizi,B. Ketzer,G. V. Khaustov,Yu. A. Khokhlov,Yu. Kisselev,F. Klein,J. H. Koivuniemi,V. N. Kolosov,K. Kondo Horikawa,I. Konorov,V. F. Konstantinov,A. M. Kotzinian,O. M. Kouznetsov,A. Koval,Z. Kral,F. Krinner,Y. Kulinich,F. Kunne,K. Kurek,R. P. Kurjata,A. Kveton,K. Lavickova,S. Levorato,Y.-S. Lian,J. Lichtenstadt,P.-J. Lin,R. Longo,V. E. Lyubovitskij,A. Maggiora,A. Magnon,N. Makins,N. Makke,G. K. Mallot,A. Maltsev,S. A. Mamon,B. Marianski,A. Martin,J. Marzec,J. Matoušek,T. Matsuda,G. Mattson,G. V. Meshcheryakov,M. Meyer,W. Meyer,Yu. V. Mikhailov,M. Mikhasenko,E. Mitrofanov,N. Mitrofanov,Y. Miyachi,A. Moretti,A. Nagaytsev,C. Naim,D. Neyret,J. Nový,W.-D. Nowak,G. Nukazuka,A. S. Nunes,A. G. Olshevsky,M. Ostrick,D. Panzieri,B. Parsamyan,S. Paul,H. Pekeler,J.-C. Peng,M. Pešek,D. V. Peshekhonov,M. Pešková,N. Pierre,S. Platchkov,J. Pochodzalla,V. A. Polyakov,J. Pretz,M. Quaresma,C. Quintans,C. Regali,G. Reicherz,C. Riedl,T. Rudnicki,D. I. Ryabchikov,A. Rybnikov,A. Rychter,V. D. Samoylenko,A. Sandacz,S. Sarkar,I. A. Savin,G. Sbrizzai,H. Schmieden,A. Selyunin,L. Sinha,M. Slunecka,J. Smolik,A. Srnka,D. Steffen,M. Stolarski,O. Subrt,M. Sulc,H. Suzuki,T. Szameitat,P. Sznajder,S. Tessaro,F. Tessarotto,A. Thiel,J. Tomsa,F. Tosello,A. Townsend,V. Tskhay,S. Uhl,B. I. Vasilishin,A. Vauth,B. M. Veit,J. Veloso,B. Ventura,A. Vidon,M. Virius,M. Wagner,S. Wallner,K. Zaremba,P. Zavada,M. Zavertyaev,M. Zemko,E. Zemlyanichkina,Y. Zhao,M. Ziembicki
The European Physical Journal C（2021）
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摘要
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive ω meson muoproduction on the proton at COMPASS using 160 GeV/ c polarised μ ^+ and μ ^- beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/ c^2 < W< 17.0 GeV/ c^2 , with the average kinematics ⟨ Q^2⟩ = 2.1 (GeV/ c ) ^2 , ⟨ W ⟩ = 7.6 GeV/ c^2 , and ⟨ p^2_T⟩ = 0.16 (GeV/ c ) ^2 . Here, Q^2 denotes the virtuality of the exchanged photon, W the mass of the final hadronic system and p_T the transverse momentum of the 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 ( 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 ω production.
