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G. Soyez

Publications and source records attributed to G. Soyez.

At least 55 records · Page 3Linked to original sources
arXiv2006

Fluctuations effects in high-energy evolution of QCD

Recently, Iancu and Triantafyllopoulos have proposed a hierarchy of evolution equations in QCD at high energy which generalises previous approaches by including the effects of gluon number fluctuations and thus the pomeron loops. In this paper, we present the first numerical simulations of the Langevin equation which reproduces that hierarchy. This equation is formally the Balitsky-Kovchegov equation supplemented with a noise term accounting for the relevant fluctuations. In agreement with theoretical predictions, we find that the effects of the fluctuations is to reduce the saturation exponent and to induce geometric scaling violations at high energy.

hep-ph↗
arXiv2006

Consequences of strong fluctuations on high-energy QCD evolution

We investigate the behaviour of the QCD evolution towards high-energy, in the diffusive approximation, in the limit where the fluctuation contribution is large. Our solution for the equivalent stochastic Fisher equation predicts the amplitude as well as the whole set of correlators in the strong noise limit. The speed of the front and the diffusion coefficient are obtained. We analyse the consequences on high-energy evolution in QCD.

hep-ph↗
arXiv2006

Saturation in High-energy QCD

In these proceedings, I shall review the basic concepts of perturbative QCD in its high-energy limit, emphasising the approach to the unitarity limit, usually referred to as {\em saturation}. I shall explain the basic framework showing the need for saturation, first, from a simple picture of the high-energy behaviour, then, giving a short derivation of the equation driving this evolution. In the second part, I shall exhibit an analogy with statistical physics and show how this allows to derive {\em geometric scaling} in QCD with saturation. I shall finally consider the effects of gluon-number fluctuations on this picture.

hep-ph↗
arXiv2006

Forward gluon production in hadron-hadron scattering with Pomeron loops

We discuss new physical phenomena expected in particle production in hadron-hadron collisions at high energy, as a consequence of Pomeron loop effects in the evolution equations for the Color Glass Condensate. We focus on gluon production in asymmetric, `dilute-dense', collisions : a dilute projectile scatters off a dense hadronic target, whose gluon distribution is highly evolved. This situation is representative for particle production in proton-proton collisions at forward rapidities (say, at LHC) and admits a dipole factorization similar to that of deep inelastic scattering (DIS). We show that at sufficiently large forward rapidities, where the Pomeron loop effects become important in the evolution of the target wavefunction, gluon production is dominated by `black spots' (saturated gluon configurations) up to very large values of the transverse momentum, well above the average saturation momentum in the target. In this regime, the produced gluon spectrum exhibits diffusive scaling, so like DIS at sufficiently high energy.

hep-ph↗
arXiv2005

On the Probabilistic Interpretation of the Evolution Equations with Pomeron Loops in QCD

We study some structural aspects of the evolution equations with Pomeron loops recently derived in QCD at high energy and for a large number of colors, with the purpose of clarifying their probabilistic interpretation. We show that, in spite of their appealing dipolar structure and of the self-duality of the underlying Hamiltonian, these equations cannot be given a meaningful interpretation in terms of a system of dipoles which evolves through dissociation (one dipole splitting into two) and recombination (two dipoles merging into one). The problem comes from the saturation effects, which cannot be described as dipole recombination, not even effectively. We establish this by showing that a (probabilistically meaningful) dipolar evolution in either the target or the projectile wavefunction cannot reproduce the actual evolution equations in QCD.

hep-ph↗
arXiv2005

QCD traveling waves at non-asymptotic energies

Using consistent truncations of the BFKL kernel, we derive analytical traveling-wave solutions of the Balitsky-Kovchegov saturation equation for both fixed and running coupling. A universal parametrization of the ``interior'' of the wave front is obtained and compares well with numerical simulations of the original Balitsky-Kovchegov equation, even at non-asymptotic energies. Using this universal parametrization, we find evidence for a traveling-wave pattern of the dipole amplitude determined from the gluon distribution extracted from deep inelastic scattering data.

hep-ph↗
arXiv2005

Fluctuation effects in high-energy QCD

The recent high-energy QCD equations including the effects of fluctuations are solved numerically. We discuss their asymptotic properties and compare them with the corresponding behaviour without fluctuations as well as with the statistical-physics model obtained in the saddle point approximation. The potential consequences on phenomenology, particularly on geometric scaling violations, are drawn.

hep-ph↗
arXiv2005

Traveling waves and geometric scaling at non-zero momentum transfer

We extend the search for traveling-wave asymptotic solutions of the non-linear Balitsky-Kovchegov (BK) saturation equation to non-forward dipole-target amplitudes. Making use of conformal invariant properties of the Balitsky-Fadin-Kuraev-Lipatov (BFKL) kernel, we exhibit traveling-wave solutions in momentum space in the region where the momentum transfer q is smaller than the characteristic scale Q of the projectile. We prove geometric scaling in the variable Q/(q Omega_s(Y)) where Omega_s(Y) has the same energy dependence as in the forward analysis.

hep-ph↗
arXiv2005

Geometric scaling in high-energy QCD at nonzero momentum transfer

We show how one can obtain geometric scaling properties from the Balitsky-Kovchegov (BK) equation. We start by explaining how, this property arises for the b-independent BK equation. We show that it is possible to extend this model to the full BK equation including momentum transfer. The saturation scale behaves like max(q,Q_T) where q is the momentum transfer and Q_T a typical scale of the target.

hep-ph↗
arXiv2005

New insight in global QCD fits using Regge theory

In global QCD fits, one has to choose an initial parton distribution at Q^2=Q_0^2. I shall argue that the initial condition choses in usual standard sets is inconsistent with analytic S-matrix theory. I shall show how one can combine these two approaches, leading to a Regge-compatible next-to-leading order global QCD fit. This allows one to extend the parametrisation in the low-Q^2 region. Finally, I shall discuss how it it possible to use the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) equation to obtain information on Regge models at high Q^2.

hep-ph↗
arXiv2004

Small-$Q^2$ extension of DGLAP-constrained Regge residues

In a previous paper, we have shown that it was possible to use the DGLAP evolution equatio n to constrain the high-$Q^2$ ($Q^2 \ge 10$ GeV$^2$) behaviour of the residues of a high-e nergy Regge model, and we applied the developed method to the triple-pole pomeron model. W e show here that one can obtain a description of the low-$Q^2$ $γ^{(*)} p$ data match ing the high-$Q^2$ results at $Q^2 = 10$ GeV$^2$.

hep-ph↗
arXiv2003

t-channel unitarity and photon cross sections

We analyse the consequences of t-channel unitarity for the photon hadronic cross sections. We show that gamma(*)p and gamma(*) gamma(*) elastic amplitudes must include the complex-j plane singularities associated with hadronic elastic amplitudes, and give the characteristics of possible new singularities. We also show that several different models of hadronic total cross sections can be used to predict the LEP data from the HERA measurements.

hep-ph↗
arXiv2003

Log(1/x) Gluon Distribution and Structure Functions in the Loop-Loop Correlation Model

We consider the interaction of the partonic fluctuation of a scalar ``photon'' with an external color field to calculate the leading and next-to-leading order gluon distribution of the proton following the work done by Dosch-Hebecker-Metz-Pirner. We relate these gluon distributions to the short and long distance behavior of the cross section of an adjoint dipole scattering off a proton. The leading order result is a constant while the next-to-leading order result shows a ln(1/x) enhancement at small x. To get numerical results for the gluon distributions at the initial scale Q^2_0=1.8 GeV^2, we compute the adjoint dipole-proton cross section in the loop-loop correlation model. Quark distributions at the same initial scale are parametrized according to Regge theory. We evolve quark and gluon distributions to higher Q^2 values using the DGLAP equation and compute charm and proton structure functions in the small-x region for different Q^2 values.

hep-ph↗
arXiv2003

The triple-pole pomeron: Regge theory and DGLAP evolution

We will explain how it is possible to link Regge theory with DGLAP evolution using a triple-pole pomeron model. We will first show that Regge theory can be used to constrain the initial condition for DGLAP evolution. We will then spell out a method to extract Regge couplings at high Q^2 using DGLAP evolution.

hep-ph↗
arXiv2003

DGLAP evolution extends the triple pole pomeron fit

We show that the triple pole pomeron model \cite{CMS} provides an initial condition for a DGLAP evolution \cite{DGLAP} that produces a fit to high $Q^2$ experimental DIS data. We obtain good $χ^2$ for initial scales down to 3 GeV$^2$. Values of the initial scale smaller than 1.45 GeV$^2$ are ruled out at the 90% confidence level.

hep-ph↗
arXiv2003

Regge residues from DGLAP evolution

We show that combining forward and backward evolution allows to extract the residues of the triple-pole pomeron and of the other singularities for 10 GeV$^2 \le Q^2 \le 1000$ GeV$^2$. In this approach, the essential singularity generated by the DGLAP evolution is considered as a numerical approximation to a triple-pole pomeron. Using an analytical expression for the form factors, we reproduce the experimental data with a $χ^2/dof$ of 1.02. This proves the compatibility between Regge theory and DGLAP evolution. The method used here enables us to evaluate the uncertainties on the gluon distribution which prove to be large at small $x$ and small $Q^2$.

hep-ph↗
arXiv2002

Consequences of t-channel unitarity for the interaction of real and virtual photons at high energies

We analyze the consequences of t-channel unitarity for photon cross sections and show what assumptions are necessary to allow for the existence of new singularities at $ Q^{2}=0 $ for the $ γp $ and $ γγ$ total cross sections. For virtual photons, such singularities can in general be present, but we show that, apart from the perturbative singularity associated with $ γ^{*}γ^{*}\to q\bar q $, no new ingredient is needed to reproduce the data from LEP and HERA, in the Regge region.

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