Relativistic heavy-ion collisions

Abstract

The field of relativistic heavy-ion collisions is introduce d to the high-energy physics students with no prior knowledge in this area. The emphasis is on the two most important observables, namely the azimuthal collective flow and jet quenching, and on the role fluid dynamics plays in the inte rpretation of the data. Other important observables described briefly are constituent quark number scaling, ratios of particle abundances, strangeness enhancement, and sequential melting of heavy quarkonia. Comparison is made of some of the basic heavy-ion results obtained at LHC with those obtained at RHIC. Initial findings at LHC which seem to be in apparent conflict with the ac cumulated RHIC data are highlighted. These are exciting times if one is working in the area of relativistic heavy-ion collisions, with two heavyion colliders namely the Relativistic Heavy-Ion Collider (RHIC) at the Brookhaven National Laboratory and the Large Hadron Collider (LHC) at CERN in operation in tandem. Quark-gluon plasma has been discovered at RHIC, but its precise properties are yet to be established. With the phase diagram of strongly interacting matter (QCD phase diagram) also being largely unknown, these are also great times for fresh graduate students to get into this area of research , which is going to remain very active for the next decade at least. The field is maturing as evidenced by the increasing number of text books that are now available [1‐9]. Also available are collected review articles; see e.g., [10‐12]. This is a fascinating inter-disciplinary area of research a t the interface of particle physics and high-energy nuclear physics. It draws heavily from QCD — perturbative, non-perturbative, as well as semiclassical. It has overlaps with thermal field theory, re lativistic fluid dynamics, kinetic or transport theory, quantum collision theory, apart from the standard statistical mechanics and thermodynamics. Quark-Gluon Plasma (QGP) at high temperature, T , and vanishing net baryon number density, nB (or equivalently the corresponding chemical potential, µB), is of cosmological interest, while QGP at low T and large nB is of astrophysical interest. String theorists too have dev eloped interest in this area because of the black hole ‐ fluid dynamics connection. Students of high-energy physics would know that the science of the ‘small’ — the elementary particle physics — is deeply intertwined with the science of the ‘large’ — cosmology — the study of the origin and evolution of the universe. Figure 1 shows the temperature history of the universe starting shortly after the Big Bang. At times ∼ 10 µs after the Big Bang, with T ∼ > 200 MeV, 1 the universe