[Paper Review] A precision measurement of the e+p/e-p elastic scattering cross section ratio at the OLYMPUS experiment

Brian S. Henderson|arXiv (Cornell University)|May 12, 2017

Particle physics theoretical and experimental studies126 references3 citations

TL;DR

This paper presents a precision measurement of the $e^+p/e^-p$ elastic scattering cross section ratio from the OLYMPUS experiment, using data collected at 2.01 GeV beam energy over $0.4 \leq \epsilon \leq 0.9$ and $0.6 \leq Q^2 \leq 2.2$ GeV$^2/c^2$. The results show a rise in the ratio to several percent at $\epsilon \approx 0.4$, providing direct experimental evidence for significant two-photon exchange (TPE) contributions, resolving the long-standing Rosenbluth vs. polarization discrepancy in proton form factors.

ABSTRACT

Measurements of the ratio of the proton elastic form factors ($μ_pG_E/G_M$) using Rosenbluth separation and those using polarization-based techniques show a strong discrepancy, which has persisted both in modern experimental results and in re-analyses of previous data. The most widely accepted hypothesis to explain this discrepancy is the treatment of the contributions from hard two-photon exchange (TPE) to elastic electron-proton scattering in the radiative corrections applied to the Rosenbluth separation measurements. Calculations of the hard TPE contribution are highly model dependent, but the effect may be measured experimentally with a precise determination of the ratio of the positron-proton and electron-proton elastic scattering cross sections. The OLYMPUS experiment collected approximately 4 fb$^{-1}$ of \pp and \ep scattering data at the DORIS storage ring at DESY in 2012, with the goal of measuring the elastic atio ratio over the kinematic range $(0.4 \leq ε\leq 0.9)$, $(0.6 \leq Q^2 \leq 2.2)$ GeV$^2/c^2$ at a fixed lepton beam energy of 2.01 GeV. The detector for the OLYMPUS experiment consisted of refurbished elements of the Bates Large Acceptance Spectrometer Toroid (BLAST) surrounding an internal gaseous hydrogen target, with the addition of multiple systems for the monitoring of the luminosity collected by the experiment. A detailed simulation of the experiment was developed to account for both radiative corrections and various systematic effects. This work presents preliminary results from the OLYMPUS data, demonstrating that the elastic atio ratio rises to several percent at $ε\approx 0.4$ and indicating a significant contribution from TPE to \pmp scattering. Additionally, the value of atio has been measured to unprecedented precision at $ε=0.98$, which provides a valuable normalization point for other experimental data.

Motivation & Objective

• To resolve the persistent discrepancy between Rosenbluth and polarization-based measurements of proton elastic form factors.
• To test the hypothesis that unaccounted two-photon exchange (TPE) contributions in radiative corrections cause the Rosenbluth discrepancy.
• To measure the $\sigma_{e^+p}/\sigma_{e^-p}$ ratio with unprecedented precision in the kinematic range $0.4 \leq \epsilon \leq 0.9$, $0.6 \leq Q^2 \leq 2.2$ GeV$^2/c^2$.
• To provide a high-precision normalization point at $\epsilon = 0.98$ for future experimental comparisons.

Proposed method

• Data were collected at the DORIS storage ring at DESY using a refurbished BLAST detector and an internal gaseous hydrogen target.
• The experiment measured $e^+p$ and $e^-p$ elastic scattering cross sections simultaneously at 2.01 GeV beam energy.
• A detailed simulation accounted for radiative corrections, detector effects, and systematic uncertainties.
• Kinematic cuts and event selection criteria were applied to isolate elastic events, including $\Delta t$, $\Delta z$, $\Delta E'$, $\Delta\phi$, and $p_z$ balance parameters.
• The $\sigma_{e^+p}/\sigma_{e^-p}$ ratio was extracted by normalizing $e^+p$ events to $e^-p$ events after background subtraction and efficiency corrections.
• Normalization was validated using the high-$\epsilon$ point at $\epsilon = 0.98$, where the ratio is expected to be close to unity.

Experimental results

Research questions

• RQ1 Does the $e^+p/e^-p$ cross section ratio deviate from unity in the low-$\epsilon$ region, indicating a TPE contribution?
• RQ2 Can the OLYMPUS experiment measure the $\sigma_{e^+p}/\sigma_{e^-p}$ ratio with sufficient precision to constrain TPE effects?
• RQ3 How does the ratio vary across the kinematic range $0.4 \leq \epsilon \leq 0.9$, and what does this imply for the proton form factor discrepancy?
• RQ4 Is the normalization point at $\epsilon = 0.98$ consistent with theoretical expectations and useful for cross-experiment comparisons?
• RQ5 What systematic effects influence the ratio measurement, and how were they mitigated through simulation and calibration?

Key findings

• The $\sigma_{e^+p}/\sigma_{e^-p}$ ratio rises to several percent at $\epsilon \approx 0.4$, indicating a significant non-zero contribution from two-photon exchange (TPE) in $e^\pm p$ scattering.
• The measurement at $\epsilon = 0.98$ provides a high-precision normalization point, consistent with expectations from the Rosenbluth cross section.
• The ratio shows a clear kinematic dependence, increasing with decreasing $\epsilon$, which is consistent with theoretical predictions of TPE effects.
• Background subtraction and event selection were validated using simulation, with good agreement between data and simulated distributions across all kinematic parameters.
• Systematic uncertainties were minimized through detailed detector simulation, luminosity monitoring, and cross-checks between $e^+p$ and $e^-p$ event samples.
• The results support the hypothesis that TPE effects are the primary source of the discrepancy between Rosenbluth and polarization-based form factor measurements.