Measurement of the cross section for hard exclusive $\pi^0$ leptoproduction

M.G. Alexeev G.D. Alexeev A. Amoroso V. Andrieux N.V. Anfimov V. Anosov A. Antoshkin K. Augsten W. Augustyniak C.D.R. Azevedo B. Badełek F. Balestra M. Ball J. Barth R. Beck Y. Bedfer 1 J. Bernhard M. Bodlak P. Bordalo F. Bradamante A. Bressan M. Büchele V.E. Burtsev W.-C. Chang C. Chatterjee M. Chiosso A.G. Chumakov S.-U. Chung A. Cicuttin M.L. Crespo S. Dalla Torre S.S. Dasgupta O.Yu. Denisov L. Dhara S.V. Donskov N. Doshita Ch. Dreisbach W. Dünnweber R.R. Dusaev A. Efremov P.D. Eversheim M. Faessler A. Ferrero 1 M. Finger M.Finger Jr. H. Fischer C. Franco N. Du Fresne von Hohenesche J.M. Friedrich V. Frolov E. Fuchey 1 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. Hahne G. Hamar D. von Harrach R. Heitz F. Herrmann N. Horikawa N. d'Hose 1 C.-Y. Hsieh S. Huber S. Ishimoto A. Ivanov T. Iwata M. Jandek V. Jary R. Joosten P. Jörg K. Juraskova 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 Z. Kral M. Krämer F. Krinner Z.V. Kroumchtein Y. Kulinich F. Kunne 1 K. Kurek R.P. Kurjata A. Kveton S. Levorato Y.-S. Lian J. Lichtenstadt P.-J. Lin 1 R. Longo V.E. Lyubovitskij A. Maggiora A. Magnon N. Makins N. Makke G.K. Mallot S.A. Mamon B. Marianski A. Martin J. Marzec J. Matoušek T. Matsuda G.V. Meshcheryakov M. Meyer 1 W. Meyer Yu.V. Mikhailov M. Mikhasenko E. Mitrofanov N. Mitrofanov Y. Miyachi A. Moretti C. Naim 1 A. Nagaytsev D. Neyret 1 J. Nový W.-D. Nowak G. Nukazuka A.S. Nunes A.G. Olshevsky M. Ostrick D. Panzieri B. Parsamyan S. Paul J.-C. Peng F. Pereira M. Pešek D.V. Peshekhonov M. Pešková N. Pierre 1 S. Platchkov 1 J. Pochodzalla V.A. Polyakov J. Pretz M. Quaresma C. Quintans S. Ramos C. Regali G. Reicherz C. Riedl D.I. Ryabchikov A. Rybnikov A. Rychter V.D. Samoylenko A. Sandacz S. Sarkar I.A. Savin G. Sbrizzai H. Schmieden A. Selyunin L. Silva L. Sinha M. Slunecka J. Smolik A. Srnka D. Steffen M. Stolarski O. Subrt M. Sulc H. Suzuki A. Szabelski T. Szameitat P. Sznajder S. Tessaro F. Tessarotto A. Thiel J. Tomsa F. Tosello V. Tskhay S. Uhl B.I. Vasilishin A. Vauth B.M. Veit J. Veloso A. Vidon 1 M. Virius M. Wagner S. Wallner M. Wilfert K. Zaremba P. Zavada M. Zavertyaev E. Zemlyanichkina Y. Zhao M. Ziembicki
Abstract : We report on a measurement of hard exclusive $\pi^0$ muoproduction on the proton by COMPASS using 160 GeV/$c$ polarised $\mu^+$ and $\mu^-$ beams of the CERN SPS impinging on a liquid hydrogen target. From the average of the measured $\mu^+$ and $\mu^-$ cross sections, the virtual-photon proton cross section is determined as a function of the squared four-momentum transfer between initial and final proton in the range $0.08\,(\text{GeV/}c)^2 < |t| < 0.64\,(\text{GeV/}c)^2$. The average kinematics of the measurement are $\langle Q^2 \rangle =2.0\; {(\text{GeV}/c)^2}$, $\langle \nu \rangle = 12.8\; {\text{GeV}}$, $\langle x_{Bj} \rangle = 0.093 $ and $\langle -t \rangle = 0.256\; {(\text{GeV}/c)^2} $. Fitting the azimuthal dependence reveals a combined contribution by transversely and longitudinally polarised photons of $(8.1 \ \pm \ 0.9_{\text{stat}}{}_{- \ 1.0}^{+ \ 1.1}\big\rvert_{\text{sys}})\,{\text{nb}}/{(\text{GeV}/c)^{2}}$, as well as transverse-transverse and longitudinal-transverse interference contributions of $(-6.0 \pm 1.3_{\text{stat}}{}_{- \ 0.7}^{+ \ 0.7}\big\rvert_{\text{sys}})\,{\text{nb}}/{(\text{GeV}/c)^{2}}$ and $(1.4 \pm 0.5_{\text{stat}}{}_{- \ 0.2}^{+ \ 0.3}\big\rvert_{\text{sys}})\,{\text{nb}}/{(\text{GeV}/c)^{2}}$, respectively. Our results provide important input for modelling Generalised Parton Distributions. In the context of the phenomenological Goloskokov-Kroll model, the statistically significant transverse-transverse interference contribution constitutes clear experimental evidence for the chiral-odd GPD $\overline{E}_T$.
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M.G. Alexeev, G.D. Alexeev, A. Amoroso, V. Andrieux, N.V. Anfimov, et al.. Measurement of the cross section for hard exclusive $\pi^0$ leptoproduction. 2019. ⟨hal-02129556⟩

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