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

TL;DR

This paper introduces a new experimental technique using a mixed electron-positron beam to directly measure two-photon exchange effects in elastic electron and positron proton scattering, addressing a key source of discrepancy in proton form factor measurements.

Contribution

The paper presents a novel method for generating and using a combined electron-positron beam to directly compare scattering, providing more precise data on two-photon exchange effects over a broad kinematic range.

Findings

Measured the positron to electron scattering ratio as 1.027±0.005±0.05.

Demonstrated the ability to identify elastic scattering events and correct for acceptance differences.

Collected data with statistics comparable to previous best measurements at low Q^2.

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: $R=1.027\pm0.005\pm0.05$ for $<Q^2>=0.206$ GeV$^2$ and $0.830\leq \epsilon\leq 0.943$.