1,680 research outputs found
Small-x QCD Effects in Particle Collisions at High Energies
Recent theoretical developments to calculate cross sections of hadronic
objects in the high energy limit are summarised and experimental attempts to
establish the need for new QCD effects connected with a resummation of small
hadron momentum fractions x are reviewed.
The relation between small- parton dynamics and the phenomenon of
diffraction is briefly out-lined. In addition, a search for a novel,
non-perturbative QCD effect, the production of QCD instanton induced events, is
presented.Comment: Invited talk at the XX. Int. Sym. on Lepton and Photon Interactions
at High Energies, Rome, Italy, July 200
Study of proton parton distribution functions at high x using ZEUS data
At large values of x the parton distribution functions (PDFs) of the proton
are poorly constrained and there are considerable variations between different
global fits. Data at such high x have already been published by the ZEUS
Collaboration, but not yet used in PDF extractions. A technique for comparing
predictions based on different PDF sets to the observed number of events in the
ZEUS data is presented. It is applied to compare predictions from the most
commonly used PDFs to published ZEUS data at high Bjorken x. A wide variation
is found in the ability of the PDFs to predict the observed results. A scheme
for including the ZEUS high-x data in future PDF extractions is discussed.Comment: 36 pages, 5 Figures. Additional information showing comparison to
more PDF sets can be found
https://www-zeus.desy.de/zeus_papers/zeus_papers.htm
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
Evidence for a narrow baryonic state decaying to K0Sp and KOSp in deep inelastic scattering at HERA
A resonance search has been made in the K0Sp and KOSp invariant-mass spectrum measured with the ZEUS detector at HERA using an integrated luminosity of 121 pb−1. The search was performed in the central rapidity region of inclusive deep inelastic scattering at an ep centre-of-mass energy of 300–318 GeV for exchanged photon virtuality, Q2, above 1 GeV2. Recent results from fixed-target experiments give evidence for a narrow baryon resonance decaying to K+n and K0Sp, interpreted as a pentaquark. The results presented here support the existence of such state, with a mass of 1521.5±1.5(stat.)+2.8−1.7(syst.) MeV and a Gaussian width consistent with the experimental resolution of 2 MeV. The signal is visible at high Q2 and, for Q2>20 GeV2, contains 221±48 events. The probability of a similar signal anywhere in the range 1500–1560 MeV arising from fluctuations of the background is below 6×10−5
Production of J/psi Mesons at HERA
Inelastic and diffractive production of J/psi mesons at HERA is reviewed. The
data on inelastic photoproduction are described well within errors by the
Colour Singlet Model in next-to-leading order. A search for colour octet
processes predicted within the NRQCD/factorisation approach is conducted in
many regions of phase space. No unambiguous evidence has been found to date.
Diffractive elastic production of J/psi mesons has been measured in the limit
of photoproduction up to the highest photon proton center of mass energies. The
increase of the cross section is described by pQCD models. At larger Q^2, the W
dependence is found to be similar to that observed in photoproduction. First
analyses of data at high t yield a powerlike dependence on |t|. A LO BFKL
calculation gives a good description of the data.Comment: 14 pages, 12 figures, contribution to Ringberg 200
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