Abstract
In this extended analysis using the ZEUS detector at HERA, the photoproduction of isolated photons together with a jet is measured for different ranges of the fractional photon energy, $x_\gamma^{\mathrm{meas}}$, contributing to the photon-jet final state. Cross sections are evaluated in the photon transverse-energy and pseudorapidity ranges $6 < E_T^{\gamma} < 15$ GeV and $-0.7 < \eta^{\gamma} < 0.9$, and for jet transverse-energy and pseudorapidity ranges $4 < E_T^{\rm jet} < 35$ GeV and $-1.5 < \eta^{\rm jet} < 1.8$, for an integrated luminosity of 374 $\mathrm{pb}^{-1}$. The kinematic observables studied comprise the transverse energy and pseudorapidity of the photon and the jet, the azimuthal difference between them, the fraction of proton energy taking part in the interaction, and the difference between the pseudorapidities of the photon and the jet. Higher-order theoretical calculations are compared to the results.
Highlights
G jet p γ (c) jet p (d) include “fragmentation processes” in which a photon is radiated within a jet, illustrated in figure 1
Differential cross sections were calculated for the production of an isolated photon with at least one accompanying jet, in the kinematic region defined by Q2 < 1 GeV2, 0.2 < y < 0.7, −0.7 < ηγ < 0.9, 6 < ETγ < 15 GeV, 4 < ETjet < 35 GeV and −1.5 < ηjet < 1.8
Cross sections in ETjet above 15 GeV are omitted from the tables and figure 4 owing to limited statistics, but this kinematic region is included in the other cross-section measurements
Summary
The measurements are based on a data sample corresponding to an integrated luminosity of 374 ± 7 pb−1, taken during the years 2004 to 2007 with the ZEUS detector at HERA. During this period, HERA ran with an electron or positron beam energy of 27.5 GeV and a proton beam energy of Ep = 920 GeV. At the nominal interaction point, with a cross section approximately 5 × 20 cm, with the finer granularity in the Z direction and the coarser in the (X, Y ) plane. The luminosity was measured [29] using the Bethe-Heitler reaction ep → eγp by a luminosity detector which consisted of two independent systems: a lead-scintillator calorimeter [30,31,32] and a magnetic spectrometer [33]
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