Spin density matrix elements in exclusive ρ ^0 meson muoproduction
G. D. Alexeev,M. G. Alexeev,C. Alice,A. Amoroso,V. Andrieux,V. Anosov,K. Augsten,W. Augustyniak,C. D. R. Azevedo,B. Badelek,J. Barth,R. Beck,Y. Bedfer,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,F. Del Carro,I. Denisenko,O. Yu. Denisov,S. V. Donskov,N. Doshita,Ch. Dreisbach,W. Dünnweber,R. R. Dusaev,D. Ecker,A. Efremov,D. Eremeev,P. Faccioli,M. Faessler,M. Finger,M. Finger jr.,H. Fischer,K. J. Flöthner,W. Florian,J. M. Friedrich,V. Frolov,L.G. Garcia Ordòñez,F. Gautheron,O. P. Gavrichtchouk,S. Gerassimov,J. Giarra,D. Giordano,M. Gorzellik,A. Grasso,A. Gridin,M. Grosse Perdekamp,B. Grube,M. Grüner,A. Guskov,D. von Harrach,M. Hoffmann,N. Horikawa,N. d’Hose,C.-Y. Hsieh,S. Huber,S. Ishimoto,A. Ivanov,T. Iwata,M. Jandek,V. Jary,R. Joosten,E. Kabuß,F. Kaspar,A. Kerbizi,B. Ketzer,A. Khatun,G. V. Khaustov,F. Klein,J. H. Koivuniemi,V. N. Kolosov,K. Kondo Horikawa,I. Konorov,V. F. Konstantinov,A. M. Korzenev,A. M. Kotzinian,O. M. Kouznetsov,A. Koval,Z. Kral,F. Krinner,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,A. Martin,J. Marzec,J. Matoušek,T. Matsuda,G. Mattson,C. Menezes Pires,F. Metzger,M. Meyer,W. Meyer,Yu. V. Mikhailov,M. Mikhasenko,E. Mitrofanov,D. Miura,Y. Miyachi,R. Molina,A. Moretti,A. Nagaytsev,C. Naim,D. Neyret,M. Niemiec,J. Nový,W.-D. Nowak,G. Nukazuka,A. G. Olshevsky,M. Ostrick,D. Panzieri,B. Parsamyan,S. Paul,H. Pekeler,J.-C. Peng,M. Pešek,D. V. Peshekhonov,M. Pešková,S. Platchkov,J. Pochodzalla,V. A. Polyakov,M. Quaresma,C. Quintans,G. Reicherz,C. Riedl,D. I. Ryabchikov,A. Rychter,A. Ryva,V. D. Samoylenko,A. Sandacz,S. Sarkar,I. A. Savin,G. Sbrizzai,H. Schmieden,A. Selyunin,K. Sharko,L. Sinha,M. Slunecka,D. Spülbeck,A. Srnka,M. Stolarski,O. Subrt,M. Sulc,H. Suzuki,S. Tessaro,F. Tessarotto,A. Thiel,J. Tomsa,F. Tosello,A. Townsend,T. Triloki,V. Tskhay,B. Valinoti,B. M. Veit,J. F. C. A. Veloso,B. Ventura,M. Virius,M. Wagner,S. Wallner,K. Zaremba,M. Zavertyaev,M. Zemko,E. Zemlyanichkina,M. Ziembicki
The European Physical Journal C（2023）
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摘要
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive ρ ^0 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/ 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 ρ ^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 ( 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 ρ ^0 production.
