Paper

Measurement of the hard exclusive \\(^0\\) muoproduction cross section at COMPASS

Pešková, M, Riedl, C, Florian, W, Sinha, L, Chatterjee, C, Tessarotto, F, Mitrofanov, E, Kolosov, V N, Ishimoto, S, Faessler, M, Pochodzalla, J, Vijayakumar, A, Moretti, A, W -C Chang, Reicherz, G, Ecker, D, Schmieden, H, M Grosse Perdekamp, Marzec, J, Zemko, M, Ketzer, B, Barth, J, Beck, R, Sulc, M, Y -S Lian, Stolarski, M, Gridin, A, D von Harrach, Wallner, S, Grüner, M, C Menezes Pires, Nagaytsev, A, Azevedo, C D R, Gorzellik, M, Triloki, T, W -D Nowak, Virius, M, J -C Peng, Tosello, F, D'Ago, D, Khaustov, G V, Polyakov, V A, Quintans, C, Thiel, A, P -J Lin, Levorato, S, Anosov, V, Maggiora, A, Savin, I A, Alexeev, M G, Matoušek, J, Jörg, P, K Kondo Horikawa, Kouznetsov, O M, Bodlak, M, Jary, V, Grasso, A, Andrieux, V, Donskov, S V, Friedrich, J M, Nukazuka, G, Bernhard, J, Denisenko, I, Metzger, F, Delcarro, F, Mikhasenko, M, Lichtenstadt, J, Longo, R, Miura, D, Zavertyaev, M, Dusaev, R R, Eremeev, D, Flöthner, K J, Wagner, M, Makke, N, Bressan, A, J F C A Veloso, Giarra, J, Kaspar, F, Dasgupta, S S, Dünnweber, W, Spülbeck, D, Pekeler, H, Srnka, A, C -Y Hsieh, Martin, A, Cicuttin, A, Hoffmann, M, Rymbekova, A, Niemiec, M, Dreisbach, Ch, Grube, B, Suzuki, H, O Yu D

2024

Overview

A new and detailed measurement of the cross section for hard exclusive neutral-pion muoproduction on the proton was performed in a wide kinematic region, with the photon virtuality \\(Q^2\\) ranging from 1 to 8 (GeV/\\(c\\))\\(^\\, 2\\) and the Bjorken variable \\(x_ Bj\\) ranging from 0.02 to 0.45. The data were collected at COMPASS at CERN using 160 GeV/\\(c\\) longitudinally polarised \\(^+\\) and \\(^-\\) beams scattering off a 2.5 m long liquid hydrogen target. From the average of the measured \\(^+\\) and \\(^-\\) cross sections, the virtual-photon--proton cross section is determined as a function of the squared four-momentum transfer between the initial and final state proton in the range 0.08 (GeV/\\(c\\))\\(^\\, 2\\) \\(< |t| <\\) 0.64 (GeV/\\(c\\))\\(^\\, 2\\). From its angular distribution, the combined contribution of transversely and longitudinally polarised photons are determined, as well as transverse--transverse and longitudinal--transverse interference contributions. They are studied as functions of four-momentum transfer \\(|t|\\), photon virtuality \\(Q^2\\) and virtual-photon energy \\(\\). The longitudinal--transverse interference contribution is found to be compatible with zero. The significant transverse--transverse interference contribution reveals the existence of a dominant contribution by transversely polarized photons. This provides clear experimental evidence for the chiral-odd GPD \\(E_T\\). In addition, the existence of a non-negligible contribution of longitudinally polarized photons is suggested by the \\(|t|\\)-dependence of the cross section at \\(x_ Bj < \\) 0.1 . Altogether, these results provide valuable input for future modelling of GPDs and thus of cross sections for exclusive pseudo-scalar meson production. Furthermore, they can be expected to facilitate the study of next-to-leading order corrections and higher-twist contributions.

• Publisher: Cornell University Library, arXiv.org
• Subject: Angular distribution / CERN / Cross-sections / Kinematics / Liquid hydrogen / Momentum transfer / Particle production / Photons / Pions / Protons