35 research outputs found
The Linked Dipole Chain Monte Carlo
We present an implementation of the Linked Dipole Chain model for deeply
inelastic ep scattering into the framework of the Ariadne event generator.
Using this implementation we obtain results both for the inclusive structure
function as well as for exclusive properties of the hadronic final state. (The
original publication was based on results from an implementation containing an
error. In this revised version this error has been corrected, some of the
beyond leading-log assumptions have been revised and so have some of the
results.)Comment: 26 page
Investigations into the BFKL Mechanism with a Running QCD Coupling
We present approximations of varying degree of sophistication to the integral
equations for the (gluon) structure functions of a hadron (``the partonic flux
factor'') in a model valid in the Leading Log Approximation with a running
coupling constant. The results are all of the BFKL-type, i.e. a power in the
Bjorken variable x_B^{-\lambda} with the parameter \lambda determined from the
size \alpha_0 of the ``effective'' running coupling \bar{\alpha}\equiv
3\alpha_s/\pi= \alpha_0/\log(k_{\perp}^2) and varying depending upon the
treatment of the transverse momentum pole. We also consider the implications
for the transverse momentum (k_{\perp}) fluctuations along the emission chains
and we obtain an exponential falloff in the relevant \kappa\equiv
\log(k_{\perp}^2)-variable, i.e. an inverse power (k_{\perp}^2)^{-(2+\lambda)}
with the same parameter \lambda. This is different from the BFKL-result for a
fixed coupling, where the distributions are Gaussian in the \kappa-variable
with a width as in a Brownian motion determined by ``the length'' of the
emission chains, i.e. \log(1/x_B). The results are verified by a realistic
Monte Carlo simulation and we provide a simple physics motivation for the
change.Comment: 24 pages, 10 supplementary files, submitted to Physical Review
Gluon Distribution Functions in the kT-factorization Approach
At small x, the effects of finite transverse momenta of partons inside a
hadron become increasingly important, especially in analyses of jets and
heavy-quark production. These effects can be systematically accounted for in a
formalism based on kT-factorization and unintegrated distribution functions. We
present results for the unintegrated distribution function, together with the
corresponding integrated one, obtained within the framework of the Linked
Dipole Chain model. Comparisons are made to results obtained within other
approaches
Uncertainties on Central Exclusive Scalar Luminosities from the unintegrated gluon distributions
In a previous report we used the Linked Dipole Chain model unintegrated gluon
densities to investigate the uncertainties in the predictions for central
exclusive production of scalars at hadron colliders. Here we expand this
investigation by also looking at other parameterizations of the unintegrated
gluon density, and look in more detail on the behavior of these at small k_T.
We confirm our conclusions that the luminosity function for central exclusive
production is very sensitive to this behavior. However, we also conclude that
the available densities based on the CCFM and LDC evolutions are not
constrained enough to give reliable predictions even for inclusive Higgs
production at the LHC
Small-x Dipole Evolution Beyond the Large-N_c Limit
We present a method to include colour-suppressed effects in the Mueller
dipole picture. The model consistently includes saturation effects both in the
evolution of dipoles and in the interactions of dipoles with a target in a
frame-independent way.
When implemented in a Monte Carlo simulation together with our previous model
of energy--momentum conservation and a simple dipole description of initial
state protons and virtual photons, the model is able to reproduce to a
satisfactory degree both the gamma*-p cross sections as measured at HERA as
well as the total p-p cross section all the way from ISR energies to the
Tevatron and beyond
Energy Conservation and Saturation in Small-x Evolution
Important corrections to BFKL evolution are obtained from non-leading
contributions and from non-linear effects due to unitarisation or saturation.
It has been difficult to estimate the relative importance of these effects, as
NLO effects are most easily accounted for in momentum space while unitarisation
and saturation are easier in transverse coordinate space. An essential
component of the NLO contributions is due to energy conservation effects, and
in this paper we present a model for implementing such effects together with
saturation in Mueller's dipole evolution formalism. We find that energy
conservation severely dampens the small-x rise of the gluon density and, as a
consequence, the onset of saturation is delayed. Using a simple model for the
proton we obtain a reasonable qualitative description of the x-dependence of F2
at low Q^2 as measured at HERA even without saturation effects. We also give
qualitative descriptions of the energy dependence of the cross section for
gamma*-gamma* and gamma*-nucleus scattering
The general behavior of unintegrated parton distributions based on the single-scale evolution and the angular ordering constraint
To overcome the complexity of generalized two hard scale (,)
evolution equation, well known as the , , and
() evolution equations, and calculate the unintegrated parton
distribution functions (), , and ()
proposed a procedure based on () the inclusion of single-scale () only
at the last step of evolution and () the angular ordering constraint
() on the terms (the collinear approximation), to bring
the second scale, into the evolution equations. In this work we
intend to use the (Martin et al) parton distribution functions
(PDF) and try to calculate for various values of (the longitudinal
fraction of parton momentum), (the probe scale) and (the parton
transverse momentum) to see the general behavior of three dimensional at
the level up to the working energy scales (. It is shown
that there exits some pronounced peaks for the three dimensional
with respect to the two variables and at various
energies (). These peaks get larger and move to larger values of , as
the energy () is increased. We hope these peaks could be detected in the
experiments at and other laboratories in the less exclusive
processes
Central Exclusive Scalar Luminosities from the Linked Dipole Chain Model gluon densities
We investigate the implication of uncertainties in the unintegrated gluon
distribution for the predictions for central exclusive production of scalars at
hadron colliders. We use parameterizations of the kT-unintegrated gluon density
obtained from the Linked Dipole Chain model, applying different options for the
treatment of non-leading terms. We find that the luminosity function for
central exclusive production is very sensitive to details of the transverse
momentum distribution of the gluon which, contrary to the kT-integrated
distribution, is not very well constrained experimentally
Study of the Linked Dipole Chain Model in heavy quark production at the Tevatron
We present calculations of charm and beauty production at Tevatron within the
framework of kT-factorization, using the unintegrated gluon distributions as
obtained from the Linked Dipole Chain model. The analysis covers transverse
momentum and rapidity distributions and the azimuthal correlations between b
and bbar quarks (or rather muons from their decay) which are powerful tests for
the different unintegrated gluon distributions. We compare the theoretical
results with recent experimental data taken by D0 and CDF collaborations at the
Tevatron Run I and II.Comment: 16 page
kt - factorization and CCFM - the solution for describing the hadronic final states - everywhere ?
The basic ideas of kt-factorization and CCFM parton evolution is discussed.
The unintegrated gluon densities, obtained from CCFM fits to the proton
structure function data at HERA are used to predict hadronic final state cross
sections like jet production at HERA, but also comparisons with recent
measurements of heavy quark production at the Tevatron are presented. Finally,
the kt-factorization approach is applied to Higgs production at high energy
hadron hadron colliders and the transverse momentum spectrum of Higgs
production at the LHC is calculated.Comment: to be published in MPLA, replaced with new reference