FIRB Project- Future in Research

Phases of QCD, field theoretical models and transport theory:

a unified framework for heavy ion phenomenology at the LHC


The study of the fundamental theory of strong interactions, Quantum Chromo Dynamics (QCD), under extreme conditions of temperature and density has been one of

the most challenging problems in physics during the last 20 years, capturing increasing experimental and theoretical attention.

There are several reasons underlying such a vivid interest. QCD is a quantum field theory with an extremely rich dynamical content; it is the only sector of the Standard Model whose collective behavior is accessible in the laboratory; moreover, the Early Universe was filled by a Quark-Gluon Plasma (QGP), the state of matter created in the present and future heavy-ion facilities.

RHIC experiments, conducted at Brookhaven National Laboratory, have shown indications that such a state of matter behaves like a nearly "perfect fluid", i.e. with very small shear viscosity. At RHIC energies this quantity appears to be close to the lower bound predicted by supersymmetric gauge theories in the infinite coupling limit. Interestingly and surprisingly enough, also in the heavy quark sector there are hints that viscosity is similarly small. Furthermore, some evidence of a new phase of QCD, the color glass condensate (CGC), has been found, while its relevance is envisaged to increase at the upcoming LHC energies at CERN.

The main goal of our research program is to study the microscopic origin of the "nearly perfect fluid" behavior, together with the impact of a finite shear viscosity on various observables. In particular, we will investigate the rich dynamical content of QCD at finite temperature in order to understand if the plasma to be created at LHC energies belongs to a new phase, with respect to the highly non-pertrbative one created at RHIC. Furthermore, the theoretical approach employed will allow to shed light on the implications of a possible primordial CGC phase on the dynamical evolution of the system.

The project will have two sites: the University of Catania and the University of Turin. The key strategy of the scientific collaboration is to combine the expertise that can approach the problem from a more theoretical point of view (mainly through the group coordinated by C. Ratti in Turin) to a more phenomenological one, based on the transport theory (mainly developed in Catania).

More specifically, the development of field-theoretical microscopic models under the guidance of the numerical lattice QCD results that are constantly becoming available, will allow to identify the relevant degrees of freedom in the different temperature/density regimes, thus providing a theoretical ground on which it can be constructed a transport theory incorporating the field interactions, two-three body scatterings, non-equilibrium initial conditions (like CGC) and the hadronization process, . The tool that we will use in this investigation is the Polyakov loop extended Nambu Jona-Lasinio (PNJL) model, which takes into account features of both chiral and deconfinement phase transitions. The role played by color magnetic monopoles to explain the low viscosity in the RHIC temperature regime will also be considered.

Our approach will play a key role to reach conclusive scientific statements in the comprehension of the QGP properties at both RHIC and LHC facilities. In our project the same dynamics will be employed to describe the generation of elliptic flow and its dependence on viscosity, and also to study jet quenching and heavy flavor dynamics as well as hadronization via coalescence and fragmentation.

The development of a transport code alone would lack connections to the progress in the various fields which are expected to develop rather quickly in the upcoming years. Therefore, a collaboration between researchers that have expertise on both transport theory and QCD dynamics at finite T and µ is essential in order to follow the theory related to the experiments at LHC and at RHIC.

This will allow to directly test and verify the effects that the new proposed theoretical ideas have on the experimental observations at RHIC and at the upcoming LHC, together with a comprehensive study of the rich phenomenology of the HIC, relating inclusive observables to more exclusive ones (like the two-three particles correlations triggered by hadronic jets) and including hadronization effects. Furthermore, the study will be extended to heavy flavors. This will be of primary importance at LHC energies, where for the first time heavy quarks will be produced abundantly and hence it will be possible to relate open flavor physics  to the suppression/regeneration of quarkonia.

The theoretical and phenomenological approaches to the physics of the QGP are usually treated separately, thus inevitably reducing their usefulness to understand the pertinent experiments. On the contrary, we will build an interplay from which both approaches will mutually benefit, by maximally exploiting the information and tools that each method can provide.

Finally we remark that the main goal of the present project is the understanding of the properties of the matter created at the RHIC and LHC Colliders that is believed to be the primordial matter present in the Early Universe.

In addition a further outcome of our research program is to create a theoretical background for the upcoming physics at FAIR focused on the search for the QCD critical point. This, indeed, represents the major enterprise in Europe in the field of nuclear and hadron physics as stated in the approved HadronPhysics2 project at the FP7 European program.


Purpose of the project