2011 QCD and High Energy Interactions - Rencontres de Moriond ...

3 High Density Matter
The one-month dedicated Lead-Lead run performed at LHC end of 2010 at a nucleon-nucleon
centre of mass energy of 2.76 TeV has opened a new era in the study of the deconfined Quark
Gluon Plasma (QGP). LHC investigates the QCD phase transition in the same domain of the
temperature - baryonic potential phase plane as RHIC, but the higher centre of mass energy of the
initial state probes the deconfined medium further away from the phase transition, and dramatically
enhances the yield of hard probes like heavy quarks and electroweak bosons.
Electroweak bosons have been observed for the first time by ATLAS and CMS in Heavy Ion
collisions (figure 12) and their yield, as expected, shows no indication of dependence with the
centrality of the collision. This opens the way to a direct control of nuclear PDF’s and possible
saturation effects at low x.
Figure 12: Z yields measured in Pb-Pb collisions by ATLAS (left) and CMS (right) as a function of the
collision centrality.
The QGP global properties, as measured by ALICE (figure 13) from particle densities, elliptic flow
or Bose-Einstein correlations, are in agreement with the predictions of the standard QGP
thermodynamical model adjusted on RHIC data, with some additional viscous corrections. The
particle flow measured in Heavy Ion collisions however shows unexpected long range rapidity
correlations (“the ridge”). Further investigations of this effect at RHIC shows apparition of a jet-like
structure as the particle transverse momenta increase (figure 14). The “ridge” is confirmed by
ALICE in Pb-Pb collisions and also visible in high multiplicity pp collisions at the LHC energy.