Compton-Scattering Cross Section on the Proton at High Momentum Transfer

Cross-section values for Compton scattering on the proton were measured at 25 kinematic settings over the range $s=5–11$ and $-t=2–7{\mathrm{GeV}}^2$ with a statistical accuracy of a few percent. The scaling power for the $s$ dependence of the cross section at fixed center-of-mass angle was found to be $8.0\pm 0.2$, strongly inconsistent with the prediction of perturbative QCD. The observed cross-section values are in fair agreement with the calculations using the handbag mechanism, in which the external photons couple to a single quark.

Figure 1

Schematic layout of the present experiment.

Figure 2

Distribution of photon-proton coincidence events in $\delta x\mathrm{\text{−}}\delta y$ space, as defined in the text, for the measurement at $s=6.79$, $t=-3.04{\mathrm{GeV}}^2$. The peaks near $\delta x=\delta y=0$ and $\delta x=-25\mathrm{cm}$, $\delta y=0$ originate from the RCS and $ep$ events, respectively. The continuum events are photons from ${\pi }^0$ photoproduction. The events to the right of the solid line are used to normalize the ${\pi }^0$ events yield in the Monte Carlo simulation.

Figure 3

The $\delta x$-distribution for a coplanarity cut $|\delta y|\le 15\mathrm{cm}$, with the RCS and $ep$ peaks indicated. The curve is a distribution of the continuum ${\pi }^0$ events. The vertical dashed lines show the cuts used to calculate the number of RCS events.

Figure 4

Present cross section for the RCS process (points) along with theoretical curves discussed in the text.

Figure 5

Scaling of the RCS cross section at fixed ${\theta }_{\mathrm{cm}}$. Open points are results from the Cornell experiment [1]. Closed points are results from the present experiment, with error bars determined from the combined statistical and point-to-point systematic uncertainties. The line at $n=6$ is the prediction of asymptotic perturbative QCD, while the shaded area shows the fit range obtained from the cross sections of GPDs-based handbag calculation [8].