339 research outputs found
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
Measurement of D*+/- diffractive cross sections in photoproduction at HERA
The first measurement of D*+/- meson diffractive photoproduction cross
sections has been performed with the ZEUS detector at the HERA collider,
using an integrated luminosity of 38 pb^-1. The measurement has been performed
for photon--proton center-of-mass energies in the range 130 < W < 280 GeV and
photon virtualities Q^2 < 1 GeV^2. D*+/- mesons have been reconstructed with
p_T(D*)>2 GeV and -1.5 D^0 \pi^+_s
with D^0 --> K-\pi^+ (+c.c.). The diffractive component has been selected with
0.001<x_Pom<0.018. The measured diffractive cross section in this kinematic
range is: \sigma_(ep --> e'D*Xp') =
0.74 +/- 0.21 (stat.)^{+0.27}_{-0.18} (syst.) +/-0.16 (p. diss.) ~nb (ZEUS
preliminary). Measured integrated and differential cross sections have been
compared to theoretical expectations.Comment: LaTeX2e, 10 pages, 6 Postscript figures. Talk given at the Russian
Academy of Science (RAS) Nuclear Physics 2000 Conference, Moscow, Russia,
November 27-December 2, 2000. To be published in Rus. Nucl. Phys. (Ya.F.),
Proceedings of RAS Nucl. Phys. 2000 Con
A characteristic plot of pomeron-exchanged processes in diffractive DIS
The dependence of the fractal behaviors of the pomeron induced system in deep
inelastic lepton-nucleon scattering upon the diffractive kinematic variables is
found rather robust and not sensitive to the distinct parameterization of the
pomeron flux factor and structure function. A feasible experimental test of the
phenomenological pomeron-exchanged model based on the fractal measurement in
DESY collider HERA is proposed.Comment: 14 pages, 2 Postscript figure
On the dbar/ubar Asymmetry and Parton Distributions
We discuss the impact of different measurements of the dbar/ubar asymmetry in
the extraction of parameterizations of parton distribution functions.Comment: Contributed paper to LP01, 13 pages, 4 figure
A Unified Treatment of High Energy Interactions
It is well known that high energy interactions as different as
electron-positron annihilation, deep inelastic lepton-nucleon scattering,
proton-proton interactions, and nucleus-nucleus collisions have many features
in common. Based upon this observation, we construct a model for all these
interactions, which relies on the fundamental hypothesis that the behavior of
high energy interactions is universal.Comment: 4 pages, LaTeX created with LyX, 8 figures, more figures at
http://www-subatech.in2p3.fr/~theo/nexus/example
Soft Photoproduction Physics
Several topics of interest in soft photoproduction physics are discussed.
These include jet universality issues (particle flavour composition), the
subdivision into event classes, the buildup of the total photoproduction cross
section and the effects of multiple interactions.Comment: 10 pages, LaTeX2e, no figures, to appear in the proceedings of the
Durham Workshop on HERA Physics, ``Proton, Photon and Pomeron Structure'',
17--23 September 1995, Durham, U.
QCD: Quantum Chromodynamic Diffraction
The first measurements of the diffractive structure function at
HERA are discussed. A factorisable interpretation in which a partonic structure
is assigned to the pomeron is investigated through QCD analyses in which both
the quark and gluon densities are permitted to vary freely. A method of
measuring the longitudinal structure function of the pomeron without changing
the centre of mass energy is presented. The possibility that the pomeron
structure may receive a large contribution from gluons, relative to quarks, at
high is highlighted, and the experimental signatures which may support
such a structure are reviewed.Comment: 8 Latex pages, 4 figures, from talks given by the authors at the
Workshop on Proton, Photon and Pomeron Structure, Durham, September 199
Structure Functions of the Nucleon and their Interpretation
The current status of measurements of the nucleon structure functions and
their understanding is reviewed. The fixed target experiments E665, CCFR and
NMC and the HERA experiments H1 and ZEUS are discussed in some detail. The
extraction of parton momentum distribution functions from global fits is
described, with particular attention paid to much improved information on the
gluon momentum distribution. The status of alpha_s measurements from deep
inelastic data is reviewed. Models and non-perturbative approaches for the
parton input distributions are outlined. The impact on the phenomenology of QCD
of the data at very low values of the Bjorken x variable is discussed in
detail. Recent advances in the understanding of the transition from deep
inelastic scattering to photoproduction are summarised. Some brief comments are
made on the recent HERA measurements of the ep NC and CC cross-sections at very
high Q2.Comment: 196 pages, 79 figures, uses ijmpa.sty and psfig.tex (included
Has HERA reached a new QCD regime?
These notes are a summary of our efforts to answer the question in the title.
Our answer is in the affirmative as: (i) HERA data indicate a large value of
the gluon structure function; (ii) no contradictions with the asymptotic
predictions of high density QCD have been observed; and (iii) the numerical
estimates of our model give a natural description of the size of deviation from
the routine DGLAP explanation. We discuss the alternative approaches and
possible new experiments.Comment: 29 pages, 37 figures in eps file
Resolved Photon Processes
We review the present level of knowledge of the hadronic structure of the
photon, as revealed in interactions involving quarks and gluons ``in" the
photon. The concept of photon structure functions is introduced in the
description of deep--inelastic scattering, and existing
parametrizations of the parton densities in the photon are reviewed. We then
turn to hard \gamp\ and \gaga\ collisions, where we treat the production of
jets, heavy quarks, hard (direct) photons, \jpsi\ mesons, and lepton pairs. We
also comment on issues that go beyond perturbation theory, including recent
attempts at a comprehensive description of both hard and soft \gamp\ and \gaga\
interactions. We conclude with a list of open problems.Comment: LaTeX with equation.sty, 85 pages, 29 figures (not included). A
complete PS file of the paper, including figures, can be obtained via
anonymous ftp from
ftp://phenom.physics.wisc.edu/pub/preprints/1995/madph-95-898.ps.
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