4,472 research outputs found

### A zero-dimensional model for high-energy scattering in QCD

We investigate a zero-dimensional toy model originally introduced by Mueller
and Salam which mimics high-energy scattering in QCD in the presence of both
gluon saturation and gluon number fluctuations, and hence of Pomeron loops.
Unlike other toy models of the reaction-diffusion type, the model studied in
this paper is consistent with boost invariance and, related to that, it
exhibits a mechanism for particle saturation close to that of the JIMWLK
equation in QCD, namely the saturation of the emission rate due to high-density
effects. Within this model, we establish the dominant high-energy behaviour of
the S-matrix element for the scattering between a target obtained by
evolving one particle and a projectile made with exactly n particles.
Remarkably, we find that all such matrix elements approach the black disk limit
S=0 at high rapidity Y, with the same exponential law: ~ exp(-Y) for all
values of n. This is so because the S-matrix is dominated by rare target
configurations which involve only few particles. We also find that the bulk
distribution for a saturated system is of the Poisson type.Comment: 34 pages, 9 figures. Some explanations added on the frame-dependence
of the relevant configurations (new section 3.3

### Non-linear QCD evolution with improved triple-pomeron vertices

In a previous publication, we have constructed a set of non-linear evolution
equations for dipole scattering amplitudes in QCD at high energy, which extends
the Balitsky-JIMWLK hierarchy by including the effects of fluctuations in the
gluon number in the target wavefunction. In doing so, we have relied on the
color dipole picture, valid in the limit where the number of colors is large,
and we have made some further approximations on the relation between scattering
amplitudes and dipole densities, which amount to neglecting the non-locality of
the two-gluon exchanges. In this Letter, we relax the latter approximations,
and thus restore the correct structure of the `triple-pomeron vertex' which
describes the splitting of one BFKL pomeron into two within the terms
responsible for fluctuations. The ensuing triple-pomeron vertex coincides with
the one previously derived by Braun and Vacca within perturbative QCD. The
evolution equations can be recast in a Langevin form, but with a multivariable
noise term with off-diagonal correlations. Our equations are shown to be
equivalent with the modified version of the JIMWLK equation recently proposed
by Mueller, Shoshi, and Wong.Comment: 15 page

### Gluon Distributions and Color Charge Correlations in a Saturated Light-cone Wavefunction

We describe the light-cone wavefunction in the saturation regime in terms of
the density of gluons per unit of transverse phase space, the occupation
number, and in terms of the color charge correlator. The simple McLerran-
Venugopalan model gives what are claimed to be general results for the phase
space gluon density, but it does not well describe the general case for the
charge correlator. We derive the general momentum dependence and rapidity
dependence of the color charge correlator which exhibits strong color
shielding. A simplel physical picture which leads to these general results is
described.Comment: 17 pages, Latex, 7 figure

### Di-jet asymmetry and wave turbulence

We describe a new physical picture for the fragmentation of an energetic jet
propagating through a dense QCD medium, which emerges from perturbative QCD and
has the potential to explain the di-jet asymmetry observed in Pb-Pb collisions
at the LHC. The central ingredient in this picture is the phenomenon of wave
turbulence, which provides a very efficient mechanism for the transport of
energy towards the medium, via many soft particles which propagate at large
angles with respect to the jet axis.Comment: 6 pages, 3 figures. Invited plenary talk at the 6th International
Conference on Hard and Electromagnetic Probes of High-Energy Nuclear
Collisions (Hard Probes 2013), Stellenbosch, South Africa, Nov. 4-8, 201

### The non-linear evolution of jet quenching

We construct a generalization of the JIMWLK Hamiltonian, going beyond the
eikonal approximation, which governs the high-energy evolution of the
scattering between a dilute projectile and a dense target with an arbitrary
longitudinal extent (a nucleus, or a slice of quark-gluon plasma). Different
physical regimes refer to the ratio $L/\tau$ between the longitudinal size $L$
of the target and the lifetime $\tau$ of the gluon fluctuations. When $L/\tau
\ll 1$, meaning that the target can be effectively treated as a shockwave, we
recover the JIMWLK Hamiltonian, as expected. When $L/\tau \gg 1$, meaning that
the fluctuations live inside the target, the new Hamiltonian governs phenomena
like the transverse momentum broadening and the radiative energy loss, which
accompany the propagation of an energetic parton through a dense QCD medium.
Using this Hamiltonian, we derive a non-linear equation for the dipole
amplitude (a generalization of the BK equation), which describes the
high-energy evolution of jet quenching. As compared to the original BK-JIMWLK
evolution, the new evolution is remarkably different: the plasma saturation
momentum evolves much faster with increasing energy (or decreasing Bjorken's
$x$) than the corresponding scale for a shockwave (nucleus). This widely opens
the transverse phase-space for the evolution and implies the existence of large
radiative corrections, enhanced by the double logarithm $\ln^2(LT)$, with $T$
the temperature of the medium. This confirms and explains from a physical
perspective a recent result by Liou, Mueller, and Wu (arXiv:1304.7677). The
dominant corrections are smooth enough to be absorbed into a renormalization of
the jet quenching parameter $\hat q$. This renormalization is controlled by a
linear equation supplemented with a saturation boundary, which emerges via
controlled approximations from the generalized BK equation alluded to above.Comment: 54 pages plus 4 appendices, 6 figure

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