Relativistic heavy-ion collisions

The field of relativistic heavy-ion collisions is introduced 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 interpretation 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 accumulated RHIC data are highlighted.Comment: Updated version of the lectures given at the First Asia-Europe-Pacific School of High-Energy Physics, Fukuoka, Japan, 14-27 October 2012. Published as a CERN Yellow Report (CERN-2014-001) and KEK report (KEK-Proceedings-2013-8), K. Kawagoe and M. Mulders (eds.), 2014, p. 219. Total 21 page

Collectivity in large and small systems formed in ultrarelativistic collisions

Collective flow of the final-state hadrons observed in ultrarelativistic heavy-ion collisions or even in smaller systems formed in high-multiplicity pp and p/d/$^3$He-nucleus collisions is one of the most important diagnostic tools to probe the initial state of the system and to shed light on the properties of the short-lived, strongly-interacting many-body state formed in these collisions. Limited, in the initial years, to the study of mainly the directed and elliptic flows -- the first two Fourier harmonics of the single-particle azimuthal distribution -- this field has evolved in recent years into a much richer area of activity. This includes not only higher Fourier harmonics and multiparticle cumulants, but also a variety of other related observables, such as the ridge seen in two-particle correlations, flow decorrelation, symmetric cumulants and event-plane correlators which measure correlations between the magnitudes or phases of the complex flows in different harmonics, coefficients that measure the nonlinear hydrodynamic response, statistical properties, such as the non-Gaussianity of the flow fluctuations, etc. We present a Tutorial Review of the modern flow picture and the various aspects of the collectivity -- an emergent phenomenon in quantum chromodynamics.Comment: Invited review article for EPJ ST, pedagogical review article, 34 pages, 14 Figs, 14 Exercises, comments welcom

Collective flow in event-by-event partonic transport plus hydrodynamics hybrid approach

Complete evolution of the strongly interacting matter formed in ultrarelativistic heavy-ion collisions is studied within a coupled Boltzmann and relativistic viscous hydrodynamics approach. For the initial nonequilibrium evolution phase, we employ the AMPT model that explicitly includes event-by-event fluctuations in the number and positions of the participating nucleons as well as of the produced partons with subsequent parton transport. The ensuing near-equilibrium evolution of quark-gluon and hadronic matter is modeled within the (2+1)-dimensional viscous hydrodynamics. We probe the role of parton dynamics in generating and maintaining the spatial anisotropy in the preequilibrium phase. Substantial eccentricities epsilon_n are found to be generated in the event-by-event fluctuations in parton production from initial nucleon-nucleon collisions. For ultracentral heavy-ion collisions, the model is able to explain qualitatively the unexpected hierarchy of the harmonic flow coefficients v_n(p_T)(n=2-6) observed at LHC. We find that the results for v_n(p_T) are rather insensitive to the variation (within a range) of the time of switchover from AMPT parton transport to hydrodynamic evolution. The usual Grad and the recently proposed Chapman-Enskog-like (nonequilibrium) single-particle distribution functions are found to give very similar results for v_n(n=2-4). The model describes well both the RHIC and LHC data for v_n(p_T) at various centralities, with a constant shear viscosity to entropy density ratio of 0.08 and 0.12, respectively. The event-by-event distributions of v_{2,3} are in good agreement with the LHC data for midcentral collisions. The linear response relation v_n = k_n epsilon_n is found to be true for n=2,3, except at large values of epsilon_n, where a larger value of k_n is required, suggesting a small admixture of positive nonlinear response even for n=2,3.Comment: 8 pages, 9 figures, v2: Same as the published version. Title change

Event-plane correlators

Correlators between event planes of different harmonics in relativistic heavy-ion collisions have the potential to provide crucial information on the initial state of the matter formed in these collisions. We present a new procedure for analyzing such correlators, which is less demanding in terms of detector acceptance than the one used recently by the ATLAS collaboration to measure various two-plane and three-plane correlators in Pb-Pb collisions at LHC. It can also be used unambiguously for quantitative comparison between theory and data. We use this procedure to carry out realistic simulations within the transport model AMPT. Our theoretical results are in excellent agreement with the ATLAS data, in contrast with previous hydrodynamic calculations which only achieved qualitative agreement. We present predictions for new correlators, in particular four-plane correlators, which can easily be analyzed with our new method.Comment: 6 pages, 3 figures. v2: Further explanations added; results unchange