Demonstration of a novel technique to measure two-photon exchange effects in elastic $e^{\pm }p$ scattering

Abstract

The discrepancy between proton electromagnetic form factors extracted using unpolarized and polarized scattering data is believed to be a consequence of two-photon exchange (TPE) effects. However, the calculations of TPE corrections have significant model dependence, and there is limited direct experimental evidence for such corrections. We present the results of a new experimental technique for making direct $e^{\pm }p$ comparisons, which has the potential to make precise measurements over a broad range in $Q^2$ and scattering angles. We use the Jefferson Lab electron beam and the Hall B photon tagger to generate a clean but untagged photon beam. The photon beam impinges on a converter foil to generate a mixed beam of electrons, positrons, and photons. A chicane is used to separate and recombine the electron and positron beams while the photon beam is stopped by a photon blocker. This provides a combined electron and positron beam, with energies from 0.5 to 3.2 GeV, which impinges on a liquid hydrogen target. The large acceptance CLAS detector is used to identify and reconstruct elastic scattering events, determining both the initial lepton energy and the sign of the scattered lepton. The data were collected in two days with a primary electron beam energy of only 3.3 GeV, limiting the data from this run to smaller values of $Q^2$ and scattering angle. Nonetheless, this measurement yields a data sample for $e^{\pm }p$ with statistics comparable to those of the best previous measurements. We have shown that we can cleanly identify elastic scattering events and correct for the difference in acceptance for electron and positron scattering. The final ratio of positron to electron scattering: $R=1.027\pm 0.005\pm 0.05$ for $<Q^2>=0.206$ GeV${}^2$ and $0.830\le \epsilon \le 0.943$.

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Cited by in corpus (12)

• Measurement of the two-photon exchange contribution to the elastic $e^{\pm }p$ scattering cross sections at the VEPP-3 storage ring
• Two-photon exchange in elastic electron-proton scattering
• Towards a resolution of the proton form factor problem: new electron and positron scattering data
• Nucleon electromagnetic form factors from lattice QCD using a nearly physical pion mass
• Subtracted dispersion relation formalism for the two-photon exchange correction to elastic electron-proton scattering: comparison with data
• Measurement of two-photon exchange effect by comparing elastic $e^{\pm }p$ cross sections
• Two-photon exchange contribution to elastic $e^{-}$-proton scattering: Full dispersive treatment of $\pi N$ states and comparison with data
• A new event generator for the elastic scattering of charged leptons on protons
• $\Delta (1232)$ Resonance Contribution to Two-Photon Exchange in Electron-Proton Scattering Revisited
• Applications of Neural Networks in Hadron Physics
• Super-Rosenbluth Measurements with Electrons and Positrons
• Resonance Contributions to Radiative Corrections in Charged-Current Elastic (Anti)Neutrino-Nucleon Scattering at GeV Energies