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26 April 2024

» arxiv » 1611.4685

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Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering: Determined by the OLYMPUS Experiment
B.S. Henderson ; L.D. Ice ; D. Khaneft ; C. O'Connor ; R. Russell ; A. Schmidt ; J.C. Bernauer ; M. Kohl ; N. Akopov ; R. Alarcon ; O. Ates ; A. Avetisyan ; R. Beck ; S. Belostotski ; J. Bessuille ; F. Brinker ; J.R. Calarco ; V. Carassiti ; E. Cisbani ; G. Ciullo ; M. Contalbrigo ; R. De Leo ; J. Diefenbach ; T.W. Donnelly ; K. Dow ; G. Elbakian ; P.D. Eversheim ; S. Frullani ; Ch. Funke ; G. Gavrilov ; B. Gläser ; N. Görrissen ; D.K. Hasell ; J. Hauschildt ; Ph. Hoffmeister ; Y. Holler ; E. Ihloff ; A. Izotov ; R. Kaiser ; G. Karyan ; J. Kelsey ; A. Kiselev ; P. Klassen ; A. Krivshich ; I. Lehmann ; P. Lenisa ; D. Lenz ; S. Lumsden ; Y. Ma ; F. Maas ; H. Marukyan ; O. Miklukho ; R.G. Milner ; A. Movsisyan ; M. Murray ; Y. Naryshkin ; R. Perez Benito ; R. Perrino ; R.P. Redwine ; D. Rodríguez Pi neiro ; G. Rosner ; U. Schneekloth ; B. Seitz ; M. Statera ; A. Thiel ; H. Vardanyan ; D. Veretennikov ; C. Vidal ; A. Winnebeck ; V. Yeganov ;
Date:	15 Nov 2016
Abstract:	The OLYMPUS collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio $R_{2gamma}$, a direct measure of the contribution of hard two-photon exchange to the elastic cross section. In the OLYMPUS measurement, 2.01 GeV electron and positron beams were directed through a hydrogen gas target internal to the DORIS storage ring at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight scintillators detected elastically scattered leptons in coincidence with recoiling protons over a scattering angle range of $approx 20^circ$ to $80^circ$. The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved GEM and MWPC detectors at $12^circ$, as well as symmetric M{o}ller/Bhabha calorimeters at $1.29^circ$. A total integrated luminosity of 4.5 fb$^{-1}$ was collected. In the extraction of $R_{2gamma}$, radiative effects were taken into account using a Monte Carlo generator to simulate the convolutions of internal bremsstrahlung with experiment-specific conditions such as detector acceptance and reconstruction efficiency. The resulting values of $R_{2gamma}$, presented here for a wide range of virtual photon polarization $0.456<epsilon<0.978$, are smaller than hadronic two-photon exchange calculations predict, but are consistent with phenomenological models.
Source:	arXiv, 1611.4685
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