308 research outputs found
Colour Dipoles and Saturation
We employ values of the colour dipole cross section extracted from
electroproduction and photoproduction data to show that gluon saturation
effects are not required by the current HERA data but will become important in
the THERA energy region.Comment: 3 pages, 2 figures. Talk given by G.S. at DIS 2000(Liverpool, April
2000
Monte Carlo results for the hydrogen Hugoniot
We propose a theoretical Hugoniot obtained by combining results for the
equation of state (EOS) from the Direct Path Integral Monte Carlo technique
(DPIMC) and those from Reaction Ensemble Monte Carlo (REMC) simulations. The
main idea of such proposal is based on the fact that DPMIC provides
first-principle results for a wide range of densities and temperatures
including the region of partially ionized plasmas. On the other hand, for lower
temperatures where the formation of molecules becomes dominant, DPIMC
simulations become cumbersome and inefficient. For this region it is possible
to use accurate REMC simulations where bound states (molecules) are treated on
the Born-Oppenheimer level using a binding potential calculated by Kolos and
Wolniewicz. The remaining interaction is then reduced to the scattering between
neutral particles which is reliably treated classically applying effective
potentials. The resulting Hugoniot is located between the experimental values
of Knudson {\textit{et al.}} \cite{1} and Collins {\textit{et al.}} \cite{2}.Comment: 10 pges, 2 figures, 2 table
The colour dipole approach to small-x processes
We explain why it is possible to formulate a wide variety of high energy
(small-x) photon-proton processes in terms of a universal dipole cross section
and compare and contrast various parameterizations of this function that exist
in the literature.Comment: 6 pages, latex, 2 figures. Contribution to Durham Collider Workshop
(Sept 99) proceeding
Hadron formation in high energy photonuclear reactions
We present a new method to account for coherence length effects in a
semi-classical transport model. This allows us to describe photo- and
electroproduction at large nuclei (A>12) and high energies using a realistic
coupled channel description of the final state interactions that goes beyond
simple Glauber theory. We show that the purely absorptive treatment of the
final state interactions can lead to wrong estimates of color transparency and
formation time effects in particle production. As an example, we discuss
exclusive rho^0 photoproduction on Pb at a photon energy of 7 GeV as well as
K^+ production in the photon energy range 1-7 GeV.Comment: 14 pages, 6 figures, version published in Phys. Rev.
Thermodynamic properties and electrical conductivity of strongly correlated plasma media
We study thermodynamic properties and the electrical conductivity of dense
hydrogen and deuterium using three methods: classical reactive Monte Carlo
(REMC), direct path integral Monte Carlo (PIMC) and a quantum dynamics method
in the Wigner representation of quantum mechanics. We report the calculation of
the deuterium compression quasi-isentrope in good agreement with experiments.
We also solve the Wigner-Liouville equation of dense degenerate hydrogen
calculating the initial equilibrium state by the PIMC method. The obtained
particle trajectories determine the momentum-momentum correlation functions and
the electrical conductivity and are compared with available theories and
simulations
Variational Density Matrix Method for Warm Condensed Matter and Application to Dense Hydrogen
A new variational principle for optimizing thermal density matrices is
introduced. As a first application, the variational many body density matrix is
written as a determinant of one body density matrices, which are approximated
by Gaussians with the mean, width and amplitude as variational parameters. The
method is illustrated for the particle in an external field problem, the
hydrogen molecule and dense hydrogen where the molecular, the dissociated and
the plasma regime are described. Structural and thermodynamic properties
(energy, equation of state and shock Hugoniot) are presented.Comment: 26 pages, 13 figures. submitted to Phys. Rev. E, October 199
Extracting the dipole cross-section from photo- and electro-production total cross-section data
We report on a successful attempt to extract the cross-section for the
high-energy scattering of colour dipoles of fixed transverse size off protons
using electroproduction and photoproduction total cross-section data, subject
to the constraint provided by the ratio of the overall photon dissociation
cross-section to the total cross-section.Comment: LaTeX2e, 29 pages, 11 figures, submitted to Phys Rev D. Because of
error in parameter assignment, one parameter removed and tables of parameter
fits and affected figs 2, 4-9 replaced. Error in figure caption corrected.
Reference update
A Unified Model of Exclusive , and \jpsi Electroproduction
A two-component model is developed for diffractive electroproduction of
, and \jpsi, based on non-perturbative and perturbative
two-gluon exchange. This provides a common kinematical structure for
non-perturbative and perturbative effects, and allows the role of the
vector-meson vertex functions to be explored independently of the production
dynamics. A good global description of the vector-meson data is obtained.Comment: 30 pages, 35 figure
From Deep Inelastic Scattering to Photoproduction: A Unified Approach
The strikingly different high energy behaviours of real photoabsorption
cross-sections with Q^2 = 0 and the low x proton structure function at large
Q^2 are studied from a laboratory frame viewpoint, in which the x and Q^2
dependence reflects the space-time structure of the interaction. This is done
using a simple model which incorporates hadron dominance, but attributes the
striking enhancement observed at HERA at very low x and high Q^2 to
contributions from heavy long-lived fluctuations of the incoming photon.
Earlier published predictions of the model for the then unknown behaviour of
the structure function at small x and intermediate Q^2 are shown to be
strikingly confirmed by recent experimental data. A simultaneous analysis of
real photoabsorption data and structure function data for 0 <= x < 0.1 and 0 <=
Q^2 <= 15 GeV^2 is then reported. An excellent fit is obtained, with all
parameters in the restricted ranges allowed by other physical requirements.Comment: 23 pages, LaTex, 11 figures, Submitted to Physical Revie
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
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