Exclusive Leptoproduction of Real Photons on a Longitudinally Polarised   Hydrogen Target

The HERMES Collaboration: A. Airapetian; N. Akopov; Z. Akopov; E.C.; Aschenauer; W. Augustyniak; R. Avakian; A. Avetissian; E. Avetisyan; B. Ball,; S. Belostotski; N. Bianchi; H.P. Blok; H. Boettcher; A. Borissov; J. Bowles,; I. Brodski; V. Bryzgalov; J. Burns; M. Capiluppi; G.P. Capitani; E. Cisbani,; G. Ciullo; M. Contalbrigo; P.F. Dalpiaz; W. Deconinck; R. De Leo; L. De; Nardo; E. De Sanctis; M. Diefenthaler; P. Di Nezza; M. Dueren; M. Ehrenfried,; G. Elbakian; F. Ellinghaus; R. Fabbri; A. Fantoni; L. Felawka; S. Frullani,; D. Gabbert; G. Gapienko; V. Gapienko; F. Garibaldi; G. Gavrilov; V.; Gharibyan; F. Giordano; S. Gliske; M. Golembiovskaya; C. Hadjidakis; M.; Hartig; D. Hasch; G. Hill; A. Hillenbrand; M. Hoek; Y. Holler; I. Hristova,; Y. Imazu; A. Ivanilov; A. Izotov; H.E. Jackson; H.S. Jo; S. Joosten; R.; Kaiser; G. Karyan; T. Keri; E. Kinney; A. Kisselev; N. Kobayashi; V.; Korotkov; V. Kozlov; P. Kravchenko; V.G. Krivokhijne; L. Lagamba; R. Lamb; L.; Lapikas; I. Lehmann; P. Lenisa; L.A. Linden-Levy; A. Lopez~Ruiz; W. Lorenzon,; X.-G. Lu; X.-R. Lu; B.-Q. Ma; D. Mahon; N.C.R. Makins; S.I. Manaenkov; L.; Manfre; Y. Mao; B. Marianski; A. Martinez de la Ossa; H. Marukyan; C.A.; Miller; Y. Miyachi; A. Movsisyan; V. Muccifora; M. Murray; A. Mussgiller; E.; Nappi; Y. Naryshkin; A. Nass; M. Negodaev; W.-D. Nowak; L.L. Pappalardo; R.; Perez-Benito; N. Pickert; M. Raithel; P.E. Reimer; A.R. Reolon; C. Riedl; K.; Rith; G. Rosner; A. Rostomyan; J. Rubin; D. Ryckbosch; Y. Salomatin; F.; Sanftl; A. Schaefer; G. Schnell; K.P. Schueler; B. Seitz; T.-A. Shibata; V.; Shutov; M. Stancari; M. Statera; E. Steffens; J.J.M. Steijger; H. Stenzel; J.; Stewart; F. Stinzing; S. Taroian; A. Terkulov; A. Trzcinski; M. Tytgat; A.; Vandenbroucke; P.B. van der Nat; Y. Van Haarlem; C. Van Hulse; D.; Veretennikov; V. Vikhrov; I. Vilardi; C. Vogel; S. Wang; S. Yaschenko; H. Ye,; Z. Ye; S. Yen; W. Yu; D. Zeiler; B. Zihlmann; P. Zupranski

arXiv:1004.0177·hep-ex·November 20, 2014

Exclusive Leptoproduction of Real Photons on a Longitudinally Polarised Hydrogen Target

The HERMES Collaboration: A. Airapetian, N. Akopov, Z. Akopov, E.C., Aschenauer, W. Augustyniak, R. Avakian, A. Avetissian, E. Avetisyan, B. Ball,, S. Belostotski, N. Bianchi, H.P. Blok, H. Boettcher, A. Borissov, J. Bowles,, I. Brodski, V. Bryzgalov, J. Burns, M. Capiluppi

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TL;DR

This paper measures polarization asymmetries in exclusive real photon leptoproduction on a polarized hydrogen target, providing insights into the underlying parton distributions and testing theoretical models.

Contribution

It presents the first measurement of azimuthal asymmetries in this process with a longitudinally polarized hydrogen target, including a novel observation of a sin(2*phi) modulation.

Findings

01

A_UL and A_LL asymmetries are compatible with model predictions.

02

A significant sin(2*phi) modulation in A_UL asymmetry is observed.

03

Results support the validity of generalized parton distribution models.

Abstract

Polarisation asymmetries are measured for the hard exclusive leptoproduction of real photons from a longitudinally polarised hydrogen target. These asymmetries arise from the deeply virtual Compton scattering and Bethe-Heitler processes. From the data are extracted two asymmetries in the azimuthal distribution of produced real photons about the direction of the exchanged virtual photon: A_UL with respect to the target polarisation and A_LL with respect to the product of the beam and target polarisations. Results for both asymmetries are compared to the predictions from a generalised parton distribution model. The sin(phi) and cos(0*phi) amplitudes observed respectively for the A_UL and A_LL asymmetries are compatible with the sizeable predictions from the model. Unexpectedly, a sin(2*phi) modulation in the A_UL asymmetry with a magnitude similar to that of the sin(phi) modulation is…

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