3,928 research outputs found
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
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 80 GeV. The
measured leading jet scattering angle distribution is consistent with the
-channel exchange of a massless fermion in 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
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 (
or ), 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 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 , and to study the QCD
dynamics in multi-jet events.Comment: 9 pages, Latex, 5 eps-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
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
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
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