Prompt Photon Production and Observation of Deeply Virtual Compton Scattering

TL;DR

This paper reports ZEUS measurements of two real-photon processes in $e^+p$ collisions at HERA: prompt-$\\gamma$ production in photoproduction and the first observation of Deeply Virtual Compton Scattering in DIS. The prompt-$\\gamma$ analysis uses calorimeter shower shapes to isolate prompt photons and compares cross sections to NLO pQCD predictions with different $\\gamma$-parton densities, showing sensitivity to the photon PDF. The DVCS study selects events with two EM clusters and uses dedicated MC tools to quantify elastic DVCS, QEDC, and their interference, finding that DVCS is necessary to explain the observed rates and angular distributions, providing first evidence at high energy and supporting the SPD framework. The results demonstrate the feasibility of using prompt photons and DVCS to probe perturbative QCD and the proton's skewed parton structure, with implications for testing NLO calculations and parton densities.

Abstract

Two recent results in ep physics using the ZEUS detector at HERA are discussed: a measurement of the prompt photon production cross section in photoproduction, and the first observation of Deeply Virtual Compton Scattering in DIS.

Figures (6)

• Figure 1: Examples of (a) direct and (b) resolved prompt-$\gamma$ production at leading order in $ep$ collisions.
• Figure 2: Diagram for DVCS.
• Figure 3: $f_{max}$ distribution for prompt-$\gamma$ candidates.
• Figure 4: Prompt-$\gamma$ cross sections as a function of $E_T^{\gamma}$ and $\eta^{\gamma}$, with NLO predictions overlaid.
• Figure 5: Distribution of $\theta_2$ for the DVCS sample.