Ultrarelativistic heavy ion collisions - introduction

by Kari J. Eskola

The primary objective of particle physics is to discover the fundamental forces and symmetries, and the elementary particles in Nature. A hierarchy of constituents of matter has been observed: macroscopic matter consists of molecules and atoms, the atoms consist of nucleons which in turn are formed of quarks, antiquarks and gluons (partons). These results have been obtained by scattering experiments at higher and higher energies, as required to achieve information on smaller and smaller objects. At the moment the hierarchy ends at quarks: no substructure has been observed for them, so they are regarded as pointlike particles. Isolated single free quarks have never been observed, and therefore it is conjectured that quarks are confined together with other quarks to form hadrons. The strong (colour) force field between the quarks is intermediated by gluons, and inside the hadrons quark-antiquark pairs form as quantum fluctuations. The fundamental theory describing the mutual strong interactions of quarks, gluons and antiquarks is Quantum Chromodynamics (QCD). QCD is the SU(3) gauge symmetric part of the Standard Model of particle physics.

In the highest-energy lepton-proton and proton-antiproton collisions, the individual scattering processes of quarks and gluons can be directly observed in certain special cases, such as jet production, where the momentum transfer is large and the interaction takes place within a small distance compared with the size of the hadron. These processes can be treated by means of perturbation theory of QCD, i.e. their cross sections can be formulated as an expansion in powers of the strong coupling alpha_s(Q) as long as the coupling stays clearly smaller than unity. This is the case when the typical momentum (or energy) scale Q involved in the process is clearly larger than the inverse size of the hadron, i.e. when Q is much larger than the inherent momentum scale of QCD, Lambda_QCD~ 200 MeV.

In an ideal experiment of high energy particle physics the individual scattered quarks and gluons fly practically freely away, dress with a gluon cloud and rapidly form colour singlet bound states, hadrons. The situation radically changes, however, if sufficiently many partons are made to scatter simultaneously into the same volume element: a dense medium of partons is formed, where the interactions of quarks, antiquarks and gluons are so effectively screened that the formation of bound states is inhibited. This kind of strongly interacting dense matter where the quarks, antiquarks and gluons behave collectively as free, deconfined, particles is called Quark-Gluon Plasma (QGP). Such a new phase of matter can be experimentally produced in ultrarelativistic heavy ion collisions (URHIC).

The phase transition between the confined and deconfined phases of QCD has been studied by the extensive ab initio calculations of lattice QCD. It has been shown that the QGP undoubtedly exists at sufficiently high energy densities. It has also been observed that chiral symmetry is restored (= that the effective mass of the quarks forming a hadron vanishes) at the same critical temperature as where the deconfinement phase transition takes place. For a purely gluonic (SU(3) gauge symmetric) system, for which the Equation of State (EoS) has been computed without approximations, the deconfinement phase transition is of the first order and the critical temperature is T_c ~265 MeV. For the moment, dynamical quarks can only be included in certain approximations, and the order of the phase transition in full QCD is not yet known. The behaviour of the QCD transition at non-zero baryochemical potential (at non-zero net-baryon number) is not yet known from first principles, either.

There are in principle two ways to achieve the QGP phase of matter: if ordinary nuclear matter is compressed to the extent that nucleons overlap, the quarks become deconfined and a cold QGP is formed. This situation may take place inside neutron stars. The experimentally relevant way is to prepare the QGP by ``heating'', i.e. by bringing energy into the system, out of which quark-antiquark pairs then form. Eventually, at sufficient densities and temperature, the quarks, antiquarks and gluons become deconfined and form a hot QGP. This is what happens in the URHIC: the energy for ``heating'' (particle production) is provided by the collision energy, and the scattered quarks, antiquarks and gluons form the QGP. The bigger the nuclei are and the higher collision energy is, the more favourable are the conditions for the formation of the QGP.

The main goal of URHIC is to study the thermodynamics of strongly interacting matter and the QCD-phase transition in particular, i.e. to study condensed elementary particle matter. The field is quite interdisciplinary as it contains elements from particle physics, statistical field theory, particle kinetics, fluid dynamics and nuclear physics. The thermodynamics of strongly interacting matter has also a strong motivation in cosmology: according to the cosmological standard model our Universe underwent a QCD-transition from the QGP to the hadron gas a few microseconds after the Big Bang. In this sense in ultrarelativistic heavy ion collisions one is also studying ``Little Bang'' cosmology in a laboratory.

Experimentally the biggest challenge in colliding heavy nuclei (A~200) at high cms energies (sqrt(s) = 20...5500 AGeV) is to observe the QGP through specific probes sensitive to the presence of the QGP and to the occurrence of the QCD phase transition. In the theory of URHIC, calculations from truly first principles are practically impossible due to the complexity of the scattering dynamics and the non-perturbative features of the produced matter. What is needed from the theory of URHIC, however, is good phenomenology based on QCD. This is also the guiding principle for the research in our URHIC theory group.

First hints of a collective strongly interacting system produced in URHIC were obtained from the fixed target experiments at the Super Proton Synchrotron (SPS) at CERN. The analysis of several observables from independent measurements also suggested that there are indications of the existense of the QGP in central lead-lead collisions at E_beam=158 AGeV (cms-energy sqrt(s)=17 AGeV). The actual properties of the QGP, in any case, clearly remain to be explored in the colliding-beam experiments.

The collider era of URHIC began in July 2000 at the Relativistic Heavy Ion Collider (RHIC) of the Brookhaven National Laboratory (BNL, NY, USA) with the Au-Au collisions at sqrt(s)=56 and 130 AGeV. In July 2001 the planned maximum cms-energy of RHIC, sqrt(s)=200 AGeV, was reached. Since Summer 2000 a truly impressive amount of high-quality data from several experiments at RHIC has been released, making the field more and more exciting.

The experimental URHIC program will be completed by the measurements at the Large Ion Collider Experiment (ALICE) at the Large Hadron Collider (LHC) of CERN, whose operation starts in 2006. ALICE is a large collaboration, involving ~800 scientists from ~70 institutes, and the total cost of the detector is of the order of 120 million CHF.

The ongoing experimental program at the SPS, RHIC and ALICE implies a solid future for the whole URHIC field: RHIC will continue for a decade at least, and similarly also the LHC.

URHIC research in Finland

The ongoing experimental program also provides the main motivation for the research done in the Finnish URHIC theory groups. These groups are located at the Department of Physics of the University of Jyväskylä (JYFL, prof. P.V. Ruuskanen and doc. K.J. Eskola), at the Department of Physics of the University of Helsinki (HYFL, prof. K. Kajantie), and at the Department of Physics at the University of Oulu (prof. E. Suhonen). They have been among the leading ones in this field since 1980. Starting from the beginning of 2002, the URHIC theory groups at JYFL and HYFL have formed a joint effort in the future as well in form of a new project, "Ultrarelativistic heavy ion collisions" (K.J. Eskola), in the Theory program of the Helsinki Institute of Physics (HIP). The total funding for the joint URHIC theory effort comes from HIP, the Academy of Finland, HYFL, JYFL, and the Graduate school for particle and nuclear physics. A research plan of the URHIC-project at HIP can be found here. See also the homepages of the URHIC theory group at JYFL and Kajantie's homepage at HYFL. Close contacts with the experimentalists in the CERN-LHC/ALICE experiment, and in the experiments at the BNL-RHIC, and in particular with the Theory Division of CERN, are emphasized.

On the experimental side, JYFL is participating in the ALICE collaboration through the ALICE-project (W. Trzaska) in the Nuclear Matter program of the Helsinki Institute of Physics.