Has saturation physics been observed in deuteron-gold collisions at RHIC?

We have addressed the question of whether saturation (CGC) has been observed in deuteron-gold collisions at RHIC. We have made a detailed analysis of the Cronin peak characteristic of the nuclear modification factor measured for d-Au collisions at mid-rapidity. The Cronin peak which is obtained around $p_t\simeq 3$ GeV may be reproduced at the proper height only by boosting the saturation momentum by a huge non-perturbative additional component. At forward rapidity, we get a quantitative agreement with data, reproducing hadron production spectra and the $R_{CP}$ ratio using a recently developed description of the small-x physics.Comment: 4 pages, 2 figures. Talk given at 41st Rencontres de Moriond: QCD and Hadronic Interactions, La Thuile, Italy, 18-25 Mar 200

Interference between initial and final state radiation in a QCD medium

We investigate the color coherence pattern between initial and final state radiation in the presence of a QCD medium. We derive the medium-induced gluon spectrum of an "asymptotic" parton which suffers a hard scattering and subsequently crosses the medium. The angular distribution of the induced gluon spectrum is modified when one includes interference terms between the incoming and the outgoing parton at finite angle between them. The coherent, incoherent and soft limits of the medium-induced gluon spectrum are studied. In the soft limit, we provide a simple and intuitive probabilistic picture which could be of interest for Monte Carlo implementations. The configuration studied here may have phenomenological consequences in high energy nuclear collisions.Comment: 9 pages, 2 figures. Typos corrected and added remarks. Accepted for publication in Phys. Lett.

Jet (de)coherence in Pb-Pb collisions at the LHC

We study the modifications of jets created in heavy-ion collisions at LHC energies. The inherent hierarchy of scales governing the jet evolution allows to distinguish a leading jet structure, which interacts coherently with the medium as a single color charge, from softer sub-structures that will be sensitive to effects of color decoherence. We argue how this separation comes about and show that this picture is consistent with experimental data on reconstructed jets at the LHC, providing a quantitative description simultaneously of the jet nuclear modification factor, the missing energy in di-jet events and the modification of the fragmentation functions. In particular, we demonstrate that effects due to color decoherence are manifest in the excess of soft particles measured in fragmentation functions in Pb-Pb compared to proton-proton collisions.Comment: 5 pages, 2 figure

Radiative energy loss of neighboring subjets

We compute the in-medium energy loss probability distribution of two neighboring subjets at leading order, in the large-$N_c$ approximation. Our result exhibits a gradual onset of color decoherence of the system and accounts for two expected limiting cases. When the angular separation is smaller than the characteristic angle for medium-induced radiation, the two-pronged substructure lose energy coherently as a single color charge, namely that of the parent parton. At large angular separation the two subjets lose energy independently. Our result is a first step towards quantifying effects of energy loss as a result of the fluctuation of the multi-parton jet substructure and therefore goes beyond the standard approach to jet quenching based on single parton energy loss. We briefly discuss applications to jet observables in heavy-ion collisions.Comment: 34 pages, 15 figure

Sudakov suppression of jets in QCD media

We compute modifications to the jet spectrum in the presence of a dense medium. We show that in the large-$N_c$ approximation and at leading logarithmic accuracy the jet nuclear modification factor factorizes into a quenching factor associated to the total jet color charge and a Sudakov suppression factor which accounts for the energy loss of jet substructure fluctuations. This factor, called the jet collimator, implements the fact that subjets, that are not resolved by the medium, lose energy coherently as a single color charge, whereas resolved large angle fluctuations suffer more quenching. For comparison, we show that neglecting color coherence results in a stronger suppression of the jet nuclear modification factor.Comment: 5 pages, 1 figur