Factorization breaking in single-diffractive dijet production at the Tevatron

We perform a NLO QCD analysis of single-diffractive dijet production in proton-antiproton collisions. By comparing the ratio of single- and non-diffractive cross sections to data from the Tevatron, the rapidity-gap survival probability is determined as a function of the momentum fraction of the parton in the antiproton. Assuming Regge factorization, this probability can be interpreted as a suppression factor for the diffractive structure function measured in deep-inelastic scattering at HERA. In contrast to the observations for photoproduction, the suppression factor in proton-antiproton collisions depends on the momentum fraction of the parton in the Pomeron even at NLO.Comment: 4 pages, 2 figures, contribution to the 15th international QCD conference (QCD 10), Montpellier, France (2010

Factorization Breaking in Diffractive Dijet Photoproduction

We have calculated the diffractive dijet cross section in low-Q^2 ep scattering in the HERA regime. The results of the calculation in LO and NLO are compared to recent experimental data of the H1 collaboration. We find that in LO the calculated cross sections are in reasonable agreement with the experimental results. In NLO, however, some of the cross sections disagree, showing that factorization breaking occurs in that order. By suppressing the resolved contribution by a factor of approximately three, good agreement with all the data is found. The size of the factorization breaking effects in diffractive dijet photoproduction agrees well with absorptive model predictions.Comment: 13 pages, 8 figure

Jet Photoproduction at THERA

We demonstrate that a future high-energy electron-proton collider like THERA could largely extend the current HERA program in jet photoproduction of testing QCD and determining the partonic structure of the proton and the photon. Depending on the electron beam energy (250-500 GeV) and the collider mode ($ep$ or $\gamma p$), the range in the hard transverse energy scale of the jets could be increased by a factor of 2-3 and the reach in the momentum fraction $x$ of the partons in the proton or photon by at least one order of magnitude. It would thus become possible to check the determinations of the gluon density in the proton obtained in deep-inelastic scattering experiments, to measure the gluon density in the photon down to low values of $x$, and to study the QCD dynamics in multi-jet events.Comment: 9 pages, Latex, 5 eps-figure

Three jet cross sections in photoproduction at HERA

We calculate three jet cross sections in photoproduction using exact matrix elements for the direct and resolved contributions. Numerical distributions are presented in a generic, irreducible set of variables that allows to disentangle the dynamics of partonic QCD subprocesses from each other and from pure phase space distributions. The results are compared to preliminary data from the ZEUS collaboration at HERA. It is found that the largest contribution comes from photon-gluon fusion in the mass range 36 GeV $< M_{\rm 3-jet} <$ 80 GeV. The measured leading jet scattering angle distribution is consistent with the $t$-channel exchange of a massless fermion in $2\to 2$ scattering, where the third parton is assumed to arise from soft bremsstrahlung. The data are inconsistent with pure phase space and Rutherford scattering distributions.Comment: 9 pages, 8 figure

Status Report of NNLO QCD Calculations

We review recent progress in next-to-next-to-leading order (NNLO) perturbative QCD calculations with special emphasis on results ready for phenomenological applications. Important examples are new results on structure functions and jet or Higgs boson production. In addition, we describe new calculational techniques based on twistors and their potential for efficient calculations of multiparticle amplitudes.Comment: Talk presented at DIS 2005, 4 pages, 2 figure

Status of Hard Interactions (Jets and Heavy Flavor)

We review the status of hard interactions, in particular of jet and heavy flavor production, at HERA and LEP. Emphasis is given to recent theoretical developments. Instantons, event shapes, and prompt photons are also briefly discussed.Comment: Opening plenary talk presented at DIS 2001, Bologna, April 2001. 12 pages, 9 figure

High-energy photon collisions at the LHC - dream or reality?

We discuss the potential of high-energy photon collisions at the LHC for improving our understanding of QCD and studying the physics beyond the Standard Model. After reviewing briefly the legacy of past photoproduction experiments at LEP and HERA, we examine the gold-plated channels proposed for a photon collider at the ILC for their potential in a hadron collider environment. We stress that initial-state photon interactions have indeed been observed at RHIC and at the Tevatron. Three promising channels at the LHC are then presented in some detail: exclusive vector-meson production, measurements of possibly anomalous electroweak gauge-boson or top-quark couplings, and slepton production.Comment: 12 pages, 2 tables, 3 figures. Summary talk given at the Workshop on "High-energy photon collisions at the LHC", April 22-25, 2008, CERN, Switzerland. To appear in the proceeding

Hard Photoproduction at HERA

In view of possible photoproduction studies in ultraperipheral heavy-ion collisions at the LHC, we briefly review the present theoretical understanding of photons and hard photoproduction processes at HERA, discussing the production of jets, light and heavy hadrons, quarkonia, and prompt photons. We address in particular the extraction of the strong coupling constant from photon structure function and inclusive jet measurements, the infrared safety and computing time of jet definitions, the sensitivity of dijet cross sections on the parton densities in the photon, factorization breaking in diffractive dijet production, the treatment of the heavy-quark mass in charm production, the relevance of the color-octet mechanism for quarkonium production, and isolation criteria for prompt photons.Comment: 10 pages, 9 figures, contribution to the CERN Yellow Report on ``The Physics of Ultraperipheral Collisions at the LHC'' [eds. K. Hencken, R. Vogt and P. Yepes