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

(OLYMPUS Collaboration)

Research output: Contribution to journalArticle

34 Citations (Scopus)

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<ϵ<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.

Original languageEnglish (US)
Article number092501
JournalPhysical Review Letters
Volume118
Issue number9
DOIs
StatePublished - Mar 3 2017

Fingerprint

proton scattering
leptons
protons
positrons
photons
chambers
luminosity
recoilings
detectors
cross sections
photomultiplier tubes
gases
convolution integrals
acceptability
bremsstrahlung
scintillation counters
calorimeters
form factors
generators
telescopes

ASJC Scopus subject areas

  • Physics and Astronomy(all)

Cite this

Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering Determined by the OLYMPUS Experiment. / (OLYMPUS Collaboration).

In: Physical Review Letters, Vol. 118, No. 9, 092501, 03.03.2017.

Research output: Contribution to journalArticle

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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 M{\o}ller 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<ϵ<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.",
author = "{(OLYMPUS Collaboration)} and Henderson, {B. S.} and Ice, {L. D.} and D. Khaneft and C. O'Connor and R. Russell and A. Schmidt and Bernauer, {J. C.} and M. Kohl and N. Akopov and Ricardo Alarcon and O. Ates and A. Avetisyan and R. Beck and S. Belostotski and J. Bessuille and F. Brinker and Calarco, {J. R.} and V. Carassiti and E. Cisbani and G. Ciullo and M. Contalbrigo and {De Leo}, R. and J. Diefenbach and Donnelly, {T. W.} and K. Dow and G. Elbakian and Eversheim, {P. D.} and S. Frullani and Ch Funke and G. Gavrilov and B. Gl{\"a}ser and N. G{\"o}rrissen and Hasell, {D. K.} and J. Hauschildt and Ph Hoffmeister and Y. Holler and E. Ihloff and A. Izotov and R. Kaiser and G. Karyan and J. Kelsey and A. Kiselev and P. Klassen and A. Krivshich and I. Lehmann and P. Lenisa and D. Lenz and S. Lumsden and Y. Ma and F. Maas",
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AB - 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<ϵ<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.

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