ECT*, Trento, May 30th - June 2nd: Introduction

Standard and novel QCD phenomena at hadron colliders

ECT*, Trento, May 30 - June 2, 2011

Description

Quantum Chromodynamics (QCD), the theory of strong interactions, has been very successful in describing quantitatively the hard phenomena characterized by a large transverse momentum scale. In this regime of QCD parton distributions are dilute, higher twist effects are negligible and the perturbative expansion is successfully predicting quantities such as jet cross-sections.

However, in semi-hard processes occurring in high energy hadronic or nuclear collisions, gluon densities become large and a new phenomenon, known as parton saturation, starts to play an important role. There is a dynamically generated characteristic scale which, if sufficiently above the confinement scale, permits the application of weak coupling techniques. Nevertheless, the study of these semi-hard processes encounters both conceptual and technical difficulties, which so far have prevented one from reaching the same level of accuracy as in the study of hard processes.

Thus, it would be highly desirable to have a QCD framework in which these questions can be addressed more precisely, since they will be central in understanding many aspects of the experiments carried out at the LHC -- from determining the underlying event and the QCD background at sufficiently high collision energy, to the more careful study of phenomena associated with the dense and hot phases of QCD: the Color Glass Condensate (CGC) and the Quark Gluon Plasma (QGP). The exploration of these phases has already started at HERA and RHIC and the important jump in collision energy at the LHC provides new opportunities for a systematic study of these phases and also for new discoveries.

For a more quantitative description of parton saturation one needs to go beyond the leading order non-linear evolution equations or use appropriate Monte Carlo generators which incorporate the correct physics. This provides the initial conditions for ultra-relativistic heavy ion collisions, that is, the wave-functions of the incoming nuclei.

For the intermediate stages of the collision, where the parton densities are rapidly decreasing due to the expansion of the produced medium, the relevant coupling can become large and weak coupling methods might fail. So far the data for ''elliptic flow'' and the observed ''jet quenching'' seem to suggest the formation of a medium which looks like a strongly-coupled fluid. An appropriate tool for first principle calculations of dynamical quantities at strong coupling is the gauge/string duality.

Main objectives

• review the QCD data from previous experiments as well as new data from the LHC (at the time of the workshop, there will be a large corpus of proton-proton data and results from the first nucleus-nucleus run);

• discuss the effects of gluon saturation on particle production; identify novel observables that could single out these effects;

• discuss the next to leading order corrections in the effective theory of the Color Glass Condensate;

• discuss the transition from the early stages of nucleus-nucleus collisions to thermalization;

• discuss the status and prospects of methods based on the AdS/CFT correspondence as an alternative to other non-perturbative approaches in QCD, in relation with the quark-gluon plasma produced in intermediate stages of the nucleus-nucleus collisions.

Organizers