Measurement of isolated photon production in deep inelastic ep scattering

TL;DR

This work reports a measurement of inclusive isolated-photon production in deep inelastic ep scattering with the ZEUS detector at HERA using 320 pb^-1. Differential cross sections in Q^2, x, E_T^γ, and η^γ are compared to perturbative QCD predictions: the α^3 Gehrmann-De Ridder calculation (QQ, LL, LQ) and MRST’s proton photon content; neither alone fully describes the data, particularly in normalization and Q^2 dependence. An improved description arises from summing MRST with the QQ contribution from GGP, suggesting the need for refined theory to fully exploit the data and potentially constrain the proton’s photon density. Overall, the results provide a stringent test of pQCD in a two-scale DIS regime and illuminate photon production mechanisms in ep collisions.

Abstract

Isolated photon production in deep inelastic ep scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 320pb^-1. Measurements were made in the isolated-photon transverse-energy and pseudorapidity ranges 4 < E_T < 15 GeV and -0.7 < eta < 0.9 for exchanged photon virtualities, Q^2, in the range 10 < Q^2 < 350 GeV and for invariant masses of the hadronic system W_X> 5 GeV. Differential cross sections are presented for inclusive isolated photon production as functions of Q^2, x, E_T and eta. Leading-logarithm parton-shower Monte Carlo simulations and perturbative QCD predictions give a reasonable description of the data over most of the kinematic range.

Figures (5)

• Figure 1: Lowest-order tree-level diagrams for isolated photon production in $ep$ scattering.
• Figure 2: Distributions of $\langle \delta Z \rangle$ and $f_{\mathrm{max}}$. The error bars represent the statistical uncertainties. The light shaded histogram shows a fit to the data of three components with fixed shapes as described in the text. The dark shaded histogram represents the QQ component of the fit, and the white histogram the LL component. The $f_{\mathrm{max}}$ distribution is shown after requiring $\langle \delta Z \rangle < 0.8$.
• Figure 3: Isolated photon differential cross sections in (a) $E_{T}^{\gamma}$, (b) $\eta^{\gamma}$, (c) $Q^{2}$ and (d) $x$. The inner and outer error bars show, respectively, the statistical uncertainty and the statistical and systematic uncertainties added in quadrature. The solid histograms are the Monte Carlo predictions from the sum of QQ photons from Pythia normalised by a factor 1.6 plus Djangoh LL photons. The dashed (dotted) lines show the QQ (LL) contributions.
• Figure 4: Data points as Fig. \ref{['fig:xsec1']}. Theoretical predictions from Gehrmann-De Ridder et al. and Martin et al. are shown with their associated uncertainties indicated by the shaded band and the hatched bands respectively. The dash-dotted line illustrates the combination MRST plus GGP: QQ.
• Figure 5: Isolated photon differential cross-section $\frac{d\sigma}{d\eta^{\gamma}}$, compared to previous measurements at HERA with the additional kinematic restraints $Q^{2} > 35\ \mathrm{GeV}^{2}$ and $5 < E_{T}^{\gamma} < 10\ \mathrm{GeV}$. The histograms show the different binnings used by ZEUS and H1. The symbols are mutually displaced for clarity.