436 research outputs found
On the Dipole Swing and the Search for Frame Independence in the Dipole Model
Small-x evolution in QCD is conveniently described by Mueller's dipole model
which, however, does not include saturation effects in a way consistent with
boost invariance. In this paper we first show that the recently studied zero
and one dimensional toy models exhibiting saturation and explicit boost
invariance can be interpreted in terms positive definite k-> k+1 dipole
vertices. Such k-> k+1 vertices can in the full model be generated by combining
the usual dipole splitting with k-1 simultaneous dipole swings. We show that,
for a system consisting of N dipoles, one needs to combine the dipole splitting
with at most N-1 simultaneous swings in order to generate all colour
correlations induced by the multiple dipole interactions
Diffractive Excitation in DIS and pp Collisions
We have in earlier papers presented an extension of Mueller's dipole cascade
model, which includes subleading effects from energy conservation and running
coupling as well as colour suppressed effects from pomeron loops via a ``dipole
swing''. The model was applied to describe the total cross sections in pp and
gamma*p collisions. In this paper we present a number of improvements of the
model, in particular related to the confinement mechanism. A consistent
treatment of dipole evolution and dipole--dipole interactions is achieved by
replacing the infinite range Coulomb potential by a screened potential, which
further improves the frame-independence of the model. We then apply the model
to elastic scattering and diffractive excitation, where we specifically study
the effects of different sources for fluctuations. In our formalism we can take
into account contributions from all different sources, from the dipole cascade
evolution, the dipole--dipole scattering, from the impact-parameter dependence,
and from the initial photon and proton wavefunctions. Good agreement is
obtained with data from the Tevatron and from HERA, and we also present some
predictions for the LHC.Comment: correction of titl
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
Numerical solution of the nonlinear evolution equation at small x with impact parameter and beyond the LL approximation
Nonlinear evolution equation at small x with impact parameter dependence is
analyzed numerically. Saturation scales and the radius of expansion in impact
parameter are extracted as functions of rapidity. Running coupling is included
in this evolution, and it is found that the solution is sensitive to the
infrared regularization. Kinematical effects beyond leading logarithmic
approximation are taken partially into account by modifying the kernel which
includes the rapidity dependent cuts. While the local nonlinear evolution is
not very sensitive to these effects, the kinematical constraints cannot be
neglected in the evolution with impact parameter.Comment: 22 pages, 37 figures, RevTe
Small x nonlinear evolution with impact parameter and the structure function data
The nonlinear Balitsky-Kovchegov equation at small x is solved numerically,
incorporating impact parameter dependence. Confinement is modeled by including
effective gluon mass in the dipole evolution kernel, which regulates the
splitting of dipoles with large sizes. It is shown, that the solution is
sensitive to different implementations of the mass in the kernel. In addition,
running coupling effects are taken into account in this analysis. Finally, a
comparison of the calculations using the dipole framework with the inclusive
data from HERA on the structure functions F2 and FL is performed.Comment: 19 pages, 11 figures. Minor revision. One reference added, two
figures update
Fluctuations, Saturation, and Diffractive Excitation in High Energy Collisions
Diffractive excitation is usually described by the Good--Walker formalism for
low masses, and by the triple-Regge formalism for high masses. In the
Good--Walker formalism the cross section is determined by the fluctuations in
the interaction. In this paper we show that by taking the fluctuations in the
BFKL ladder into account, it is possible to describe both low and high mass
excitation by the Good--Walker mechanism. In high energy collisions the
fluctuations are strongly suppressed by saturation, which implies that pomeron
exchange does not factorise between DIS and collisions. The Dipole Cascade
Model reproduces the expected triple-Regge form for the bare pomeron, and the
triple-pomeron coupling is estimated.Comment: 20 pages, 12 figure
Elastic and quasi-elastic and scattering in the Dipole Model
We have in earlier papers presented an extension of Mueller's dipole cascade
model, which includes sub-leading effects from energy conservation and running
coupling as well as colour suppressed saturation effects from pomeron loops via
a ``dipole swing''. The model was applied to describe the total and diffractive
cross sections in and collisions, and also the elastic cross
section in scattering.
In this paper we extend the model to describe the corresponding quasi-elastic
cross sections in , namely the exclusive production of vector mesons
and deeply virtual compton scattering. Also for these reactions we find a good
agrement with measured cross sections. In addition we obtain a reasonable
description of the -dependence of the elastic and quasi-elastic
cross sections
Next-to-leading and resummed BFKL evolution with saturation boundary
We investigate the effects of the saturation boundary on small-x evolution at
the next-to-leading order accuracy and beyond. We demonstrate that the
instabilities of the next-to-leading order BFKL evolution are not cured by the
presence of the nonlinear saturation effects, and a resummation of the higher
order corrections is therefore needed for the nonlinear evolution. The
renormalization group improved resummed equation in the presence of the
saturation boundary is investigated, and the corresponding saturation scale is
extracted. A significant reduction of the saturation scale is found, and we
observe that the onset of the saturation corrections is delayed to higher
rapidities. This seems to be related to the characteristic feature of the
resummed splitting function which at moderately small values of x possesses a
minimum.Comment: 34 page
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