Dijet Differential Cross Sections Research Articles

Dijet production associated with a leading neutron at HERA

Dijet differential cross sections for the reaction e + p → e ++ jet+ jet+ n+ X in the photoproduction regime have been measured with the ZEUS detector at HERA using an integrated luminosity of 6.4 pb −1. The cross sections are given for jet transverse energies E T jet > 6 GeV, neutron energy E n > 400 GeV, and neutron angle θ n < 0.8 mrad with respect to the proton direction. The fraction of events with a leading neutron in the final state is shown. The predictions of the one pion exchange model are found to be in reasonable agreement with the measurements presented, namely the dijet cross section as a function of the jet transverse energies, the jet pseudorapidities, and the fraction of the momentum of a postulated exchanged pion participating in the production of the dijet system.

Threshold resummation for dijet cross sections

We construct dijet differential cross sections at large momentum transfer, in which threshold logarithms have been summed to all orders in perturbation theory. This extends previous work on heavy quark production, by treating collinear singularities associated with hard, massless partons in the final state. The resummed corrections enable us to define, in the sense of factorization, the underlying color exchange mechanism. The influence of color exchange on the resummed cross section is contained in the eigenvalues and eigenvectors of an anomalous dimension matrix, which describes the factorization of coherent soft gluons from the hard scattering. The precise formulas depend on the partonic scattering angles and energies, as well as on the method used to define the jets in the final state. For cone dijets at fixed invariant mass, we find leading logarithmic corrections that, like those in the Drell-Yan process, are positive, and which grow with increasing dijet invariant mass. Other choices of dijet cross section can give, however, qualitatively different behavior, even at leading logarithm.

Diffractive dijet cross sections in photoproduction at HERA

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon (ϰγOBS) and pomeron (βOBS) momentum participating in the production of the dijet system. The observed ϰγOBS dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section da/dβOBS increases as βOBS increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron.

Diffractive dijet cross sections in photoproduction at HERA

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon (x_gamma^OBS) and pomeron (beta^OBS) momentum participating in the production of the dijet system. The observed x_gamma^OBS dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section d(sigma)/d(beta^OBS) increases as beta^OBS increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron.