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

Area of Science:

• Particle Physics
• Quantum Electrodynamics

Background:

• Understanding lepton-proton elastic scattering is crucial for probing fundamental interactions.
• The contribution of two-photon exchange (TPE) to elastic scattering requires precise measurement.
• Previous theoretical calculations for hadronic TPE varied, necessitating experimental verification.

Purpose of the Study:

• To precisely measure the ratio of positron-proton to electron-proton elastic cross sections (R_{2γ}).
• To directly quantify the contribution of hard two-photon exchange (TPE) in elastic scattering.
• To compare experimental results with theoretical predictions and phenomenological models.

Main Methods:

• Utilized 2.01 GeV electron and positron beams at the DORIS storage ring.
• Employed a toroidal magnetic spectrometer to detect scattered leptons and recoiling protons.
• Monitored relative luminosity using specialized detectors and calorimeters.
• Applied Monte Carlo simulations to account for radiative effects and detector efficiencies.

Main Results:

• Collected a total integrated luminosity of 4.5 fb⁻¹.
• Measured R_{2γ} over a wide range of virtual photon polarization (0.456 < ε < 0.978).
• Observed R_{2γ} values smaller than some hadronic TPE calculations.

Conclusions:

• The experimental R_{2γ} values are in good agreement with a subtracted dispersion model.
• Results align with a phenomenological fit to form factor data.
• Provides crucial experimental data for refining theoretical models of TPE in elastic lepton-proton scattering.