433 research outputs found
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
Recent Low x and Diffractive Collider Data
Selected recent data from collider experiments pertaining to the
understanding of QCD at low Bjorken-x are reviewed. The status of QCD and Regge
factorisation in hard diffractive interactions is discussed in terms of data
from HERA and the Tevatron. The possibility of anomalous behaviour in the
total cross section is confronted with the most recent
measurements from LEP. Data from all three colliders that are sensitive to
possible BFKL effects are presented and different interpretations are
discussed.Comment: 9 pages, introductory talk from the 1999 Durham Phenomenology
Workshop on Collider Physic
Momentum Spectra in the Current Region of the Breit Frame in Deep Inelastic Scattering at HERA
The production of charged particles has been measured in deep inelastic
scattering (DIS) with the ZEUS detector. The evolution of the moments of the
scaled momenta distributions in and has been investigated in the
current fragmentation region of the Breit frame. The results in the current
region are compared to data and QCD analytical calculations. The
results are consistent with the universality of single-particle spectra in DIS
and annihilation at high The results at low disagree with
analytical calculations based on the modified leading log approximation (MLLA)
and local parton hadron duality (LPHD).Comment: 3 pages. 1 postscript figure + 1 postscript preprint logo + 1 LaTeX
file + 1 style fil
High energy factorization predictions for the charm structure function F2^c at HERA
High energy factorization predictions for F2^c are derived using BFKL
descriptions of the proton structure function F2 at HERA. The model parameters
are fixed by a fit of F2 at small x. Two different approaches of the non
perturbative proton input are shown to correspond to the factorization at the
gluon or quark level, respectively. The predictions for F2^c are in agreement
with the data within the present error bars. However, the photon wave-function
formulation (factorization at quark level) predicts significantly higher F2^c
than both gluon factorization and a next-leading order DGLAP model.Comment: latex file + 6 encapsulated figures, 28 page
Nuclear shadowing in deep inelastic scattering on nuclei: leading twist versus eikonal approaches
We use several diverse parameterizations of diffractive parton distributions,
extracted in leading twist QCD analyses of the HERA diffractive deep inelastic
scattering (DIS) data, to make predictions for leading twist nuclear shadowing
of nuclear quark and gluon distributions in DIS on nuclei. We find that the
HERA diffractive data are sufficiently precise to allow us to predict large
nuclear shadowing for gluons and quarks, unambiguously. We performed detailed
studies of nuclear shadowing for up and charm sea quarks and gluons within
several scenarios of shadowing and diffractive slopes, as well as at central
impact parameters. We compare these leading twist results with those obtained
from the eikonal approach to nuclear shadowing (which is based on a very
different space-time picture) and observe sharply contrasting predictions for
the size and Q^2-dependence of nuclear shadowing. The most striking differences
arise for the interaction of small dipoles with nuclei, in particular for the
longitudinal structure function F_{L}^{A}.Comment: 43 pages, 16 figures, requires JHEP style fil
Jet shapes in ep collisions at HERA
New measurements of the jet shape in ep collisions at HERA using the
k_T-cluster jet algorithm are presented.Comment: 7 pages, 3 figures; plenary talk given at the 3rd UK Phenomenology
Workshop on HERA Physics, Durham, UK, September 199
Deep Inelastic Scattering at HERA
Results from the H1 and ZEUS experiments at HERA on deep inelastic scattering
are reviewed. The data lead to a consistent picture of a steep rise in the F_2
structure function and in the gluon density within the proton. Important new
information on the partonic structure of diffraction is emerging from H1 and
ZEUS. The space-like region in which the weak and electromagnetic interactions
become of equal strength is being explored for the first time. A possible
excess of events at high x and Q^2 compared to the expectations of the Standard
Model has been observed in both experiments.Comment: 80 pages, 42 figure
Gaps between Jets in the High Energy Limit
We use perturbative QCD to calculate the parton level cross section for the
production of two jets that are far apart in rapidity, subject to a limitation
on the total transverse momentum Q0 in the interjet region. We specifically
address the question of how to combine the approach which sums all leading
logarithms in Q/Q0 (where Q is the jet transverse momentum) with the BFKL
approach, in which leading logarithms of the scattering energy are summed. This
paper constitutes progress towards the simultaneous summation of all important
logarithms. Using an "all orders" matching, we are able to obtain results for
the cross section which correctly reproduce the two approaches in the
appropriate limits.Comment: 29 pages, 9 figures, minor corrections to text and improved figure
Open charm production in diffractive ep collisions
The cross section for the diffractive reaction gamma* + p -> c cbar + p with
a real or virtual photon is calculated in the nonperturbative two-gluon
exchange model of Landshoff and Nachtmann. Numerical predictions are given for
cross sections and spectra at typical HERA values of c.m. energy and photon
virtuality. The contribution of charm to the diffractive structure function is
evaluated and found to be rather small in the model, and the ratio between the
production rates for b bbar and c cbar is tiny.Comment: 16 pages including 7 figures, Latex2e, uses epsf.st
Diffractive upsilon production at the LHC
We compute the rate for diffractive upsilon meson production at the Tevatron
and the LHC. The upsilon is produced diffractively via the subprocess gamma + p
-> upsilon + p where the initial photon is radiated off an incoming proton (or
antiproton). We consider the possibility to use low angle proton detectors to
make a measurement of the gamma p cross-section and conclude that a measurement
of the cross-section at a centre of mass energy in excess of 1 TeV is possible
at the LHC. This is in the region where saturation effects are likely to reveal
themselves.Comment: 14 page
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