High energy QCD : from RHIC to LHC -- ECT*, Trento - Jan 9-13, 2007
Main topics
- Experimental probes of parton saturation and of thermalization,
Present status and predictions for the LHC
- Color Glass Condensate, Connections with
statistical physics
- Initial particle production in heavy ion collisions,
Recent progress on numerical methods
- Thermalization, Plasma instabilities
Organisers
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Description
The ongoing experiments at the Relativistic Heavy Ion Collider (RHIC, at the Brookhaven National Laboratory), as well as the forthcoming ones at the Large Hadron Collider (LHC, to start at CERN in 2007), have revived the interest in the theory of hadronic scattering at high energy, a longstanding problem for which no satisfactory solution has been given so far. Stimulated by this experimental conjuncture, new physical ideas have emerged, which suggest that the problem of high--energy scattering in Quantum Chromo-Dynamics (QCD) may lie within the realm of weak coupling techniques, and hence can be studied from first principles. Some of the phenomenological consequences of these ideas may have already been observed, such as the geometrical scaling in Deep Inelastic Scattering at HERA or the suppression of p_\perp spectra at forward rapidities in deuteron-nucleus collisions at RHIC.
At the heart of this theoretical progress, is the realization that, at high energy, a hadron wave--function develops a very high gluon density, which introduces a semi--hard scale in the problem --- the so--called ``saturation momentum''. The novelty of this ``saturated'' regime is that it is characterized by strong color fields, contrary to the more conventional situations addressed in perturbative QCD. Therefore, it is the realm of strong non--linear effects, and its theoretical treatment requires elaborated resummations of the high density corrections.
These theoretical ideas have been fueled by the realization that this regime may be reached in ultra--relativistic heavy--ion collisions, where both the large collision energy and the large size of the colliding nuclei tend to increase the saturation scale. Thus, the advent of the LHC, with energies considerably larger than at RHIC, should give a novel dimension to the experimental study of the high--energy hadron--hadron collisions and provide a better playground for testing new ideas.
One should emphasize that the experimental situation in heavy ion collisions is extremely complex, due to the collective effects associated to the large number of intervening particles. A fundamental question in this respect is how to disentangle the `initial state' effects --- by that we mean the properties of the wave-functions of the hadrons before the collision --- from the `final state' interactions among the partons (quarks and gluons) liberated by the collision. Therefore, it seems useful to gather both theorists and experimentalists in order to identify the observables that are best suited to uncovering the physics of the initial state and of gluon saturation.
An equally fundamental issue in this context is that of the thermalization of the matter produced in a heavy--ion collision. Indeed, present experimental results suggest an early formation of an equilibrated quark-gluon plasma. Yet, such an early thermalization seems very difficult to achieve within standard scenarios based on perturbative QCD. There have been interesting developments in recent years on this question, based on the observation that plasmas with an anisotropic distribution of particles develop an instability that may accelerate the thermalization.
In view of this, it seems timely to have a workshop during which one would draw conclusions regarding the initial stages of heavy ion collisions from the first five years of RHIC operation, and pave the way for the forthcoming LHC experiments. Moreover, this workshop is meant to be the ``launching pad'' for a two month program to be held at the Galileo Institute (Florence, Italy) in the spring of 2007 on the same subjects. We expect the talks and discussions to be organized around the following three main topics~:
- Experimental probes of parton saturation and of thermalization, Present status and predictions for the LHC
- Color Glass Condensate, Connections with statistical physics
- Initial particle production in heavy ion collisions, Recent progress on numerical methods
- Thermalization, Plasma instabilities
