1,294 research outputs found
Prompt Photon Production at HERA and LEP
Results on isolated prompt photon production are presented. The measurements
were performed at HERA in deep inelastic ep scattering and photoproduction, as
well as at LEP in photon photon collisions. Differential cross sections are
shown for inclusive prompt photons and those accompanied by a jet. The results
are compared to predictions of perturbative QCD calculations in next to leading
order and to predictions of the event generators PYTHIA and HERWIG.Comment: To appear in the proceedings of 32nd International Conference on
High-Energy Physics (ICHEP 04), Beijing, China, 16-22 Aug 200
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
Charmonium Production via Fragmentation at DESY HERA
The cross section for the photoproduction of large-p_T J/psi mesons at HERA
is calculated at next-to-leading order, adopting a perturbative approach to
describe the fragmentation of charm quarks and gluons into J/psi mesons. We
treat the charm quark according to the massless factorization scheme, where it
is assumed to be one of the active flavours inside the proton and the resolved
photon. We present inclusive distributions in transverse momentum and rapidity,
including the contributions due to direct and resolved photons. The importance
of the colour-octet components of the J/psi wave function, which contribute to
the fragmentation process, is emphasized. In addition to prompt J/psi
production, we consider also the production of chi_{cJ} states followed by
radiative decays to J/psi mesons, both in the colour-singlet and colour-octet
channels.Comment: 32 pages (Latex), 12 figures (Postscript
Photoproduction Processes in Polarized ep - Collisions at HERA
We study various conceivable photoproduction reactions in a polarized ep
collider mode of HERA with respect to their sensitivity to the proton's
polarized gluon distribution. A special emphasis is put on the `resolved' part
of the cross sections which in principle opens the possibility to determine for
the first time also the completely unknown parton content of longitudinally
polarized photons. In the very promising case of dijet production we also
investigate the impact of parton showering, hadronization and jet finding on
the parton level results.Comment: 16 pages, LaTeX, 7 figures, uses epsfig, amssymb, and a41 (included)
styles; Contribution to the proceedings of the 1997 workshop on 'Physics with
Polarized Protons at HERA', Hamburg and Zeuthen, Germany, March-September
199
Tests of QCD: Summary of DIS 2000
This summary of the working group 2 of DIS 2000 encompasses experimental and
theoretical results of jet physics, open and bound state heavy flavour
production, prompt photon production, next-to-leading order QCD calculations
and beyond, instantons, fragmentation, event shapes, and power corrections,
primarily from deep-inelastic scattering and photoproduction at HERA, but also
from the LEP and Tevatron colliders.Comment: 16 pages, LaTeX, including 8 PostScript figures. Talk given at the
8th International Workshop on Deep-Inelastic Scattering (DIS2000), 25th-30th
April 2000, Liverpool, England, to appear in the proceeding
Studies of the diffractive photoproduction of isolated photons at HERA
The photoproduction of isolated photons has been measured in diffractive
events recorded by the ZEUS detector at HERA. Cross sections are evaluated in
the photon transverse-energy and pseudorapidity ranges GeV and
inclusively and also with a jet with transverse energy and
pseudorapidity in the ranges GeV and using a total integrated electron-proton luminosity of 456 pb. A
number of kinematic variables were studied and compared to predictions from the
Rapgap Monte Carlo model. An excess of data is observed above the Rapgap
predictions for where is the
fraction of the longitudinal momentum of the colourless "Pomeron" exchange that
is transferred to the photon-jet final state, giving evidence for direct
Pomeron interactions.Comment: 46 pages, 10 figures. The second version has some textual
clarification
Color-Octet Contributions to J/psi Photoproduction via Fragmentation at HERA
We study J/psi photoproduction via fragmentation at next-to-leading order in
the QCD-improved parton model, using the nonrelativistic factorization
formalism proposed by Bodwin, Braaten, and Lepage. We consider direct and
resolved photoproduction of prompt J/psi mesons and chi_{cJ} mesons radiatively
decaying to J/psi+gamma, taking into account the formation of both
color-singlet and color-octet c anti-c states. Adopting the values of the
long-distance color-octet matrix elements extracted from fits to prompt-J/psi
data recently taken at the Fermilab Tevatron, we predict that measurements of
J/psi photoproduction at DESY HERA should show a distinctive excess over the
expectation based on the color-singlet model at small values of the
inelasticity variable z. This is complementary to the expected enhancement at z
close to 1 due to the color-octet contribution to photon-gluon fusion.Comment: 10 pages (Latex), 3 figures (Postscript
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