The OLYMPUS Collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, R2γ, 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 ≈20° to 80°. The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved gas electron multiplier and multiwire proportional chamber detectors at 12°, as well as symmetric Møller or Bhabha calorimeters at 1.29°. A total integrated luminosity of 4.5 fb-1 was collected. In the extraction of R2γ, 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 R2γ, presented here for a wide range of virtual photon polarization 0.456<0.978, are smaller than some hadronic two-photon exchange calculations predict, but are in reasonable agreement with a subtracted dispersion model and a phenomenological fit to the form factor data.

Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering Determined by the OLYMPUS Experiment

CARASSITI, Vittore;CIULLO, Giuseppe;CONTALBRIGO, Marco;LENISA, Paolo;STATERA, Marco;
2017

Abstract

The OLYMPUS Collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, R2γ, 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 ≈20° to 80°. The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved gas electron multiplier and multiwire proportional chamber detectors at 12°, as well as symmetric Møller or Bhabha calorimeters at 1.29°. A total integrated luminosity of 4.5 fb-1 was collected. In the extraction of R2γ, 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 R2γ, presented here for a wide range of virtual photon polarization 0.456<0.978, are smaller than some hadronic two-photon exchange calculations predict, but are in reasonable agreement with a subtracted dispersion model and a phenomenological fit to the form factor data.
2017
Henderson, B. S.; Ice, L. D.; Khaneft, D.; O'Connor, C.; Russell, R.; Schmidt, A.; Bernauer, J. C; Kohl, M.; Akopov, N.; Alarcon, R.; Ates, O.; Avetisyan, A.; Beck, R.; Belostotski, S.; Bessuille, J.; Brinker, F.; Calarco, J. R.; Carassiti, Vittore; Cisbani, E.; Ciullo, Giuseppe; Contalbrigo, Marco; De Leo, R.; Diefenbach, J.; Donnelly, T. W.; Dow, K.; Elbakian, G.; Eversheim, P. D.; Frullani, S.; Funke, C. h.; Gavrilov, G.; Gläser, B.; Görrissen, N.; Hasell, D. K.; Hauschildt, J.; Hoffmeister, P. h.; Holler, Y.; Ihloff, E.; Izotov, A.; Kaiser, R.; Karyan, G.; Kelsey, J.; Kiselev, A.; Klassen, P.; Krivshich, A.; Lehmann, I.; Lenisa, Paolo; Lenz, D.; Lumsden, S.; Ma, Y.; Maas, F.; Marukyan, H.; Miklukho, O.; Milner, R. G.; Movsisyan, A.; Murray, M.; Naryshkin, Y.; Perez Benito, R.; Perrino, R.; Redwine, R. P.; Rodríguez Piñeiro, D.; Rosner, G.; Schneekloth, U.; Seitz, B.; Statera, Marco; Thiel, A.; Vardanyan, H.; Veretennikov, D.; Vidal, C.; Winnebeck, A.; Yeganov, V.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2368472
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