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Dijet angular distributions in direct and resolved photoproduction at HERA
Abstract
Jet photoproduction, where the two highest transverse energy (ETjet) jets have ETjet above 6 GeV and a jet-jet invariant mass above 23 GeV, has been studied with the ZEUS detector at the HERA ep collider. Resolved and direct photoproduction samples have been separated. The cross section as a function of the angle between the jet-jet axis and the beam direction in the dijet rest frame has been measured for the two samples. The measured angular distributions differ markedly from each other. They agree with the predictions of QCD calculations, where the different angular distributions reflect the different spins of the quark and gluon exchanged in the hard subprocess.
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.
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