JIMWLK evolution: from color charges to rapidity correlations

We study multi-particle production with rapidity correlations in high-energy p+A collisions. In the context of the Color Glass Condensate, the evolution for such correlations is governed by a generalization of the JIMWLK equation which evolves the strong nuclear fields both in the amplitude and in the complex conjugate one. We give the equivalent Langevin formulation, whose main ingredient is the color charge density linked to a projectile parton (a Wilson line).Comment: 4 pages, 2 figures, based on talk given at Hard Probes 2013, 4 - 8 Nov 2013, Cape Town, South Afric

JIMWLK evolution for multi-particle production with rapidity correlations

We study multi-particle production with rapidity correlations in proton-nucleus collisions at high energy in the Color Glass Condensate framework. The high-energy evolution responsible for such correlations is governed by a generalization of the JIMWLK equation describing the simultaneous evolution of the strong nuclear color fields in the direct amplitude and the complex conjugate amplitude. This functional equation can be used to derive ordinary evolution equations for the cross-sections for particle production, but the ensuing equations appear to be too complicated to be useful in practice, including in the limit of a large number of colors Nc. We propose an alternative formulation based on a Langevin process, which is valid for generic Nc and is better suited for numerical implementations. For illustration, we present the stochastic equations which govern two gluon production with arbitrary rapidity separation.Comment: 8 pages, 6 figures, based on talk given at IS 2013, 8 - 14 Sep 2013, Illa da Toxa, Spai

Running coupling effects in the evolution of jet quenching

We study the consequences of including the running of the QCD coupling in the equation describing the evolution of the jet quenching parameter $\hat q$ in the double logarithmic approximation. To start with, we revisit the case of a fixed coupling, for which we obtain exact solutions valid for generic values of the transverse momentum (above the medium saturation scale) and corresponding to various initial conditions. In the case of a running coupling, we construct approximate solutions in the form of truncated series obtained via successive iterations, whose convergence is well under control. We thus deduce the dominant asymptotic behavior of the renormalized $\hat q$ in the limit of a large evolution time $Y\equiv\ln(L/\lambda)$, with $L$ the size of the medium and $\lambda$ the typical wavelength of a medium constituent. We show that the asymptotic expansion is universal with respect to the choice of the initial condition at $Y=0$ and, moreover, it is remarkably similar to the corresponding expansion for the saturation momentum of a shockwave (a large nucleus). As expected, the running of the coupling significantly slows down the increase of $\hat q$ with $Y$ in the asymptotic regime at $Y\gg 1$. For the phenomenologically interesting value $Y\simeq 3$, we find an enhancement factor close to 3, independently of the initial condition and for both fixed and running coupling.Comment: 25 pages, 5 figure

JIMWLK evolution for multi-particle production in Langevin form

Within the effective theory for the Color Glass Condensate, we study multi-particle production with rapidity correlations in proton-nucleus collisions at high energy. The high-energy evolution responsible for such correlations is governed by a generalization of the JIMWLK equation which describes the simultaneous evolution of the (strong) nuclear color fields in the direct amplitude and the complex conjugate amplitude. This functional equation can be used to derive ordinary evolution equations for the cross-sections for particle production (a generalization of the Balitsky hierarchy). However, the ensuing equations appear to be too complicated to be useful in practice, including in the limit where the number of colors is large. To circumvent this problem, we propose an alternative formulation of the high-energy evolution as a Langevin process, which is better suited for numerical implementations. This process is directly oriented towards the calculation of the cross-sections, so its detailed structure depends upon the nature of the final state. We present the stochastic equations appropriate for two gluon production, and also for three gluon production, with generic rapidity differences.Comment: 28 pages, 5 figure

UV Cascade in Classical Yang-Mills Theory

We study the real-time behavior of classical Yang-Mills theory under initial conditions with nonperturbatively large, infrared field amplitudes. Our lattice study confirms the cascade of energy towards higher momenta and lower occupancy, which occurs via a scaling solution $f[p,t_1] = (t_0/t_1)^{4/7}\, f[p (t_0/t_1)^{1/7},t_0]$. Above a characteristic scale p_{max}, f falls exponentially; below p_{max}, $f[p] \propto p^{-4/3}$. We find no evidence for different infrared exponents or for infrared occupancies in excess of those described by this scaling solution. We also investigate what the fate of large occupancies would be, both in the electric and the magnetic sector.Comment: 24 pages with 13 color figure

Saturation momentum at fixed and running QCD coupling

A relationship, linking the saturation momentum in the case of fixed and running QCD coupling, respectively, is derived from the Balitsky-Kovchegov equation. It relies on the linear instability of the evolution equation in the dilute regime. The leading orders of the saturation momenta are mapped onto each other exactly. For subleading terms a qualitative correspondence is achieved with a relative error going to zero for large rapidities. The relationship can also be derived for the Balitsky-Kovchegov equation with a cutoff accounting for low-density effects and is satisfied by the corresponding isoline functions. Further implications arise for the existence of travelling-wave solutions in the two situations.Comment: revised version, 6 pages, no figure

From Classical to Quantum Saturation in the Nuclear Gluon Distribution

We study the gluon content of a large nucleus (i) in the semi-classical McLerran-Venugopalan model and (ii) in the high energy limit as given by the quantum evolution of the Color Glass Condensate. We give a simple and qualitative description of the Cronin effect and high-pT suppression in proton-nucleus collisions.Comment: 4 pages, 5 figures, To appear in the Proceedings of International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions (HP2004), Ericeira, Portugal, 4-10 Nov, 200