Photoproduction of Dijets with High Transverse Momenta at HERA
At HERA the largest cross section is due to photoproduction, where the beam lepton interacts with the proton via the exchange of an almost real photon, i.e. photon with very small virtuality (Q2 ~ 0). It is well-established that the real photon has a partonic structure. This structure can be resolved in the hard photon-proton interactions, when the partons from the photon interact with partons from the proton producing in the final state jets with high transverse energy, i.e. energy projection transverse to the beam axis. The measurement of jet photoproduction (gamma + p -> jets + X) allows tests of the standard theory of strong interactions, Quantum Chromodynamics (QCD) and of our understanding of the proton and the photon in terms of the quarks and gluons from which they are constructed. Two contributions to the jet cross section can be distinguished: direct processes in which the photon itself enters the hard subprocess and the resolved processes in which the parton from the photon participates in the hard scatter. The hadronic structure of the proton and photon are described by their respective parton density functions (PDFs).

Measurements of the parton densities of the photon and proton have been performed in several processes and experiments. The quark densities in the photon have been determined in e+e- collisions. Compared to e+e- data the photoproduction of jets is directly sensitive to the gluon density in the photon. The jet data constrains also the gluon density in the proton at the medium proton momentum fraction xp.

To test predictions of perturbative QCD calculations and PDF parametrisations the paper investigates the photoproduction of dijets using the H1 detector at HERA. The transverse momentum Et of the leading jet ranges between 25 and 80 GeV. The range of photon momentum fraction carried by the parton participating in the hard interaction is 0.1<xgamma<1.0. The proton momentum fraction carried by the interacting parton from the proton is in the range 0.05<xp<0.7.

Fig 3 of paper Compared to the previous H1 publication, this paper presents new measurements with increased statistical precision and an improved understanding of systematic uncertainties. The cross sections for dijet production are measured as a function of the transverse energy of leading jet (Et,max), the momentum fractions of the photon (xgamma) and the proton (xp) participating in the hard interaction, and the angle of the dijets in their centre-of-mass system- cos(theta*).

In order to test the theory as closely as possible, measurements are presented separately for events originating from predominantly direct photon (xgamma>0.8) or resolved photon (xgamma<0.8) processes. The region of xgamma>0.8, in which the photon predominantly interacts directly with the proton, is particularly well suited to test the proton structure as the photon structure plays no significant role there. The data in the region of xgamma<0.8, where the photon behaves like a hadronic object, may also provide additional constrains on the photon parton density function.

In addition the cross sections are investigated at different jet topologies: the case where both jets are in the "backward" direction, where both jets are in the "forward" direction, and where on jet is in the "forward" and one in the "backward" direction.
The data compare well with predictions from Monte Carlo event generators based on leading order (LO) QCD and parton showers and with next-to-leading order (NLO) QCD calculations, except for large xp where there is large uncertainty from the proton parton density function. This uncertainty can be largely reduced using this photoproduction dijet measurement in the global QCD fit analysis.