Heavy Ion Collisions Phenomenology Overview

The reach of collider energies in heavy-ion collisions has profoundly changed our understanding of QCD under extreme conditions. I review some these new developments and comment on the properties of the produced medium as extracted from experimental data, as well as the exciting new opportunities which will be open at the LHC. QCD is a theory with a very rich dynamical structure but difficult to solve in many situations of phenomenological interest. This structure includes confinement and chiral symmetry breaking as main vacuum properties, a complex phase diagram and hadronic spectrum, asymptotic freedom and others. Among these, only asymptotic freedom has allowed to make extensive experimental tests of the precision of the theory in the short distance regime of the interaction. Lattice calculations allows to test the long distance dynamics giving excellent results for static quantities 1), but with limitations to study out of equilibrium situations. Most of the present phenomenological applications require, however, this real-time dynamics. Two examples arise: the recent interpretations about the structure of resonances on different mass regions of the spectrum – extensively discussed at this conference – or the transport properties of the hot medium created in nuclear collisions. The common question of both topics could be phrased as: what are the relevant building blocks in situations where collective behavior appears and how they organize? The experiments of heavy-ion collisions at high energy attempt to answer this question for the hot part of the phase space diagram. The dynamical properties of the created matter, as the equation of state or different transport coefficients, are accessible experimentally and the findings are being interpreted theoretically. Several questions can be addressed which are normally categorized depending on the time scale as i) initial state of the system, ii) thermalization and evolution, iii) probes of the medium. We follow this classification in the following. 1 The initial state and the Color Glass Condensate The relevant part of the colliding nuclei (or hadrons in general) wave function is dominated at high energies by Lorentz-boosted short-living quantum fluctuations which, with several degrees of sophistication, can be computed perturbatively once some initial condition is provided. This ’small-x gluons’ are produced by sequential splitting in a branching process which makes its number to grow exponentially in rapidity y = − log x, the variable playing the role of time for the evolution. When the density number of gluons is very high, the probability of fusion begins to compensate that of branching and a phenomenon of saturation appears 2) – the corresponding scale when this happens is called the saturation scale Qsat. A successful implementation of this physics is known under the the generic name of Color Glass Condensate 3). It provides a general framework for the whole collision, based on an effective theory separating the fast modes in the wave function from the generated slow modes, associated to small-x gluons, which are treated as classical fields. The quantum evolution equation of this setup is also known and, remarkably, recent attempts exist aiming to provide the link to the subsequent evolution into a thermal system 4). Interestingly, this formalism provides a way of computing multiparticle -4 -2 0 2 4 200 400 600 80