Virtual Compton Scattering and Neutral Pion Electroproduction in the Resonance Region up to the Deep Inelastic Region at Backward Angles

TL;DR

This paper reports the first measurements of virtual Compton scattering in the nucleon resonance region at backward angles, revealing resonance structures and potential signs of a transition to quark-level scattering mechanisms.

Contribution

It provides novel experimental data on VCS in the resonance region, highlighting resonance structures and the Q^2-independence at high W, suggesting a transition to perturbative QCD.

Findings

Resonant structures observed in the first and second resonance regions.

Q^2-dependence is smooth across the measured range.

Possible indication of a transition to perturbative scattering at high W.

Abstract

We have made the first measurements of the virtual Compton scattering (VCS) process via the H$(e,e'p)\gamma$ exclusive reaction in the nucleon resonance region, at backward angles. Results are presented for the $W$-dependence at fixed $Q^2=1$ GeV$^2$, and for the $Q^2$-dependence at fixed $W$ near 1.5 GeV. The VCS data show resonant structures in the first and second resonance regions. The observed $Q^2$-dependence is smooth. The measured ratio of H$(e,e'p)\gamma$ to H$(e,e'p)\pi^0$ cross sections emphasizes the different sensitivity of these two reactions to the various nucleon resonances. Finally, when compared to Real Compton Scattering (RCS) at high energy and large angles, our VCS data at the highest $W$ (1.8-1.9 GeV) show a striking $Q^2$- independence, which may suggest a transition to a perturbative scattering mechanism at the quark level.