Observation of isolated high-E_T photons in deep inelastic scattering

TL;DR

This study reports the first measurements of isolated prompt photon production in deep inelastic ep scattering with the ZEUS detector, spanning inclusive photons and photons accompanied by a jet. It tests QCD in a two-scale regime defined by the photon virtuality $Q^2$ and photon transverse energy $E_T^{\gamma}$, comparing data to both leading-log MC models (Pythia/Herwig) and to a full ${O}(\alpha^3\alpha_s)$ calculation for the photon+jet final state. While MC models capture some kinematic features, they systematically underpredict the cross sections, especially in the inclusive channel, and the ${O}(\alpha^3\alpha_s)$ calculation provides a compatible description for the photon+jet channel within theoretical uncertainties. The results demonstrate QCD dynamics in DIS with two hard scales and emphasize areas where higher-order effects and electron-wide-angle radiation must be incorporated in MC models.

Abstract

First measurements of cross sections for isolated prompt photon production in deep inelastic ep scattering have been made using the ZEUS detector at the HERA electron-proton collider using an integrated luminosity of 121 pb^-1. A signal for isolated photons in the transverse energy and rapidity ranges 5 < E_T^gamma < 10 GeV and -0.7 < eta^gamma < 0.9 was observed for virtualities of the exchanged photon of Q^2 > 35 GeV^2. Cross sections are presented for inclusive prompt photons and for those accompanied by a single jet in the range E_T^jet \geq 6 GeV and -1.5 \leq eta^jet < 1.8. Calculations at order alpha^3alpha_s describe the data reasonably well.

Figures (5)

• Figure 1: The lowest-order tree-level diagrams for prompt photon production in $ep$ scattering. Vertex corrections enter at the same order.
• Figure 2: (a) Distribution of $\langle \delta Z \rangle$ for prompt photon candidates in selected events. (b) Distribution of $f_{\rm{max}}$ after a cut on $\langle \delta Z \rangle <0.65$. Also given are fitted distributions for Monte Carlo $\eta$ mesons, $\pi^0 + \eta$ and $\pi^0 + \eta + \gamma$ (where the $\gamma$ is taken from DVCS data), with similar selection criteria and $E_T^\gamma$ spectrum to the observed candidates.
• Figure 3: Inclusive prompt-photon differential cross section (a) in rapidity, (b) in transverse energy, in the range $-0.7 < \eta^\gamma < 0.9$ and $5 < E_T^\gamma < 10 {\rm{\,\text{Ge}\text{V}}}$. The inner error bars are statistical while the outer represent systematic uncertainties added in quadrature. (c) Distribution of $Q^2$. In each case the histograms show MC predictions, normalised to data.
• Figure 4: Cross section for prompt-photon-plus-jet production differential in (a) photon rapidity, (b) photon transverse energy, (c) jet pseudorapidity, (d) jet transverse energy, for events with a photon in the range $-0.7 < \eta^\gamma < 0.9$ and $5 < E_T^\gamma < 10 {\rm{\,\text{Ge}\text{V}}}$ and one jet in the range $-1.5 < \eta^{\rm{jet}} < 1.8$ and $E_T^{\rm{jet}} > 6 {\rm{\,\text{Ge}\text{V}}}$. The inner error bars are statistical and the outer represent systematic uncertainties added in quadrature. The band around the data points shows the effect of calorimeter energy-scale uncertainty. The histograms show Monte Carlo predictions, normalised to the data.
• Figure 5: Cross section for prompt-photon-plus-jet production differential in (a) photon rapidity, (b) photon transverse energy, (c) jet pseudorapidity, (d) jet transverse energy, for events with a photon in the range $-0.7 < \eta^\gamma < 0.9$ and $5 < E_T^\gamma < 10 {\rm{\,\text{Ge}\text{V}}}$ and one jet in the range $-1.5 < \eta^{\rm{jet}} < 1.8$ and $E_T^{\rm{jet}} > 6 {\rm{\,\text{Ge}\text{V}}}$. The inner error bars are statistical while the outer represent systematic uncertainties added in quadrature. The band around the data points shows the effect of calorimeter energy scale uncertainty. The boxed band shows the parton-level predictions of Kramer and Spiesberger including the effect of renormalisation scale uncertainty. The single line indicates their prediction of the contribution of photons radiated from the quark line.