Measurement of two-photon exchange effect with CLAS

Journal Article · Thu Aug 05 00:00:00 EDT 2010 · AIP Conference Proceedings

DOI:https://doi.org/10.1063/1.3483406· OSTI ID:21426469

Raue, Brian A ^[1]

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1. Florida International University (United States)

The structure of the proton is one the most important and most studied topics in nuclear physics. However, discrepant measurements of the proton's electromagnetic form factor ratio, G{sub E}/G{sub M}, seriously jeopardize a definitive understanding of the proton's structure. Measurements of G{sub E}G{sub M} using the Rosenbluth separation technique disagree with those using polarization transfer methods by about a factor of three at Q{sup 2{approx}}5.6 GeV{sup 2}. It has been hypothesized that this discrepancy is due to two-photon exchange (TPE) effects that are not part of the usual radiative corrections. Theoretical corrections for the TPE effect are difficult due to the fact that a large number of excited nucleon states can contribute to the process. However, the TPE effect can be directly determined by measuring the ratio of the positron-proton to electron-proton elastic scattering cross sections, R = {sigma}(e{sup +})/{sigma}(e{sup -}), as the TPE effect changes sign with respect to the charge of the incident particle. A brief test run of a modified beamline and the CLAS detector at Jefferson Lab has resulted in the most precise measurements of the R to date. We will present results from the test run covering Q{sup 2}<0.8 GeV{sup 2} and 0.78{<=}{epsilon}{<=}0.97. In addition, the test run demonstrated the feasibility of producing a mixed electron/positron beam of good quality so that the measurements can be extended up to Q{sup 2{approx}}3.0 GeV{sup 2}.

• OSTI ID: 21426469
• Journal Information: AIP Conference Proceedings, Vol. 1257, Issue 1; Conference: HADRON 2009: 13. international conference on hadron spectroscopy, Tallahassee, FL (United States), 29 Nov - 4 Dec 2009; Other Information: DOI: 10.1063/1.3483406; (c) 2010 American Institute of Physics; ISSN 0094-243X
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
CEBAF ACCELERATOR
CROSS SECTIONS
ELASTIC SCATTERING
ELECTROMAGNETIC FORM FACTORS
ELECTRON-PROTON INTERACTIONS
ELECTRONS
FOUR MOMENTUM TRANSFER
MULTIPARTICLE SPECTROMETERS
MULTI-PHOTON PROCESSES
PARTICLE IDENTIFICATION
PARTICLE STRUCTURE
PHOTONS
POLARIZATION
POLARIZED BEAMS
POLARIZED TARGETS
POSITRON-POSITRON INTERACTIONS
POSITRONS
PROTONS
RADIATIVE CORRECTIONS
SHOWER COUNTERS
ACCELERATORS
ANTILEPTONS
ANTIMATTER
ANTIPARTICLES
BARYONS
BEAMS
BOSONS
CORRECTIONS
DIMENSIONLESS NUMBERS
ELECTRON-NUCLEON INTERACTIONS
ELEMENTARY PARTICLES
FERMIONS
FORM FACTORS
HADRONS
INTERACTIONS
LEPTON-BARYON INTERACTIONS
LEPTON-HADRON INTERACTIONS
LEPTON-LEPTON INTERACTIONS
LEPTON-NUCLEON INTERACTIONS
LEPTONS
LINEAR ACCELERATORS
MASSLESS PARTICLES
MATTER
MEASURING INSTRUMENTS
MOMENTUM TRANSFER
NUCLEONS
PARTICLE INTERACTIONS
PARTICLE PROPERTIES
RADIATION DETECTORS
SCATTERING
SPECTROMETERS
TARGETS