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Register a new userThe First fm/c of Heavy-Ion Collisions
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We present an introductory review of the early time dynamics of high-energy heavy-ion collisions and the kinetics of high temperature QCD. The equilibration mechanisms in the quark-gluon plasma uniquely reflect the non-abelian and ultra-relativistic character of the many body system. Starting with a brief expose of the key theoretical and experimental questions, we provide an overview of the theoretical tools employed in weak coupling studies of the early time non-equilibrium dynamics. We highlight theoretical progress in understanding different thermalization mechanisms in weakly coupled non-abelian plasmas, and discuss their relevance in describing the approach to local thermal equilibrium during the first ${rm fm}/c$ of a heavy-ion collision. Some important connections to the phenomenology of heavy-ion collisions are also briefly discussed.
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We study charm production in ultra-relativistic heavy-ion collisions by using the Parton-Hadron-String Dynamics (PHSD) transport approach. The initial charm quarks are produced by the PYTHIA event generator tuned to fit the transverse momentum spectrum and rapidity distribution of charm quarks from Fixed-Order Next-to-Leading Logarithm (FONLL) calculations. The produced charm quarks scatter in the quark-gluon plasma (QGP) with the off-shell partons whose masses and widths are given by the Dynamical Quasi-Particle Model (DQPM), which reproduces the lattice QCD equation-of-state in thermal equilibrium. The relevant cross sections are calculated in a consistent way by employing the effective propagators and couplings from the DQPM. Close to the critical energy density of the phase transition, the charm quarks are hadronized into $D$ mesons through coalescence and/or fragmentation. The hadronized $D$ mesons then interact with the various hadrons in the hadronic phase with cross sections calculated in an effective lagrangian approach with heavy-quark spin symmetry. The nuclear modification factor $R_{AA}$ and the elliptic flow $v_2$ of $D^0$ mesons from PHSD are compared with the experimental data from the STAR Collaboration for Au+Au collisions at $sqrt{s_{NN}}$ =200 GeV and to the ALICE data for Pb+Pb collisions at $sqrt{s_{NN}}$ =2.76 TeV. We find that in the PHSD the energy loss of $D$ mesons at high $p_T$ can be dominantly attributed to partonic scattering while the actual shape of $R_{AA}$ versus $p_T$ reflects the heavy-quark hadronization scenario, i.e. coalescence versus fragmentation. Also the hadronic rescattering is important for the $R_{AA}$ at low $p_T$ and enhances the $D$-meson elliptic flow $v_2$.
We study the event-by-event generation of flow vorticity in RHIC Au + Au collisions and LHC Pb + Pb collisions by using the HIJING model. Different definitions of the vorticity field and velocity field are considered. A variety of properties of the vorticity are explored, including the impact parameter dependence, the collision energy dependence, the spatial distribution, the event-by-event fluctuation of the magnitude and azimuthal direction, and the time evolution. In addition, the spatial distribution of the flow helicity is also studied.
Discriminating hadronic molecular and multi-quark states is a long standing problem in hadronic physics. We propose here to utilize relativistic heavy ion collisions to resolve this problem, as exotic hadron yields are expected to be strongly affected by their structures. Using the coalescence model, we find that the exotic hadron yield relative to the statistical model result is typically an order of magnitude smaller for a compact multi-quark state, and larger by a factor of two or more for a loosely bound hadronic molecule. We further find that some of the newly proposed heavy exotic states could be produced and realistically measured at RHIC and LHC.
The study of high energy collisions between heavy nuclei is a field unto itself, distinct from nuclear and particle physics. A defining aspect of heavy ion physics is the importance of a bulk, self-interacting system with a rich space-time substructure. I focus on the issue of timescales in heavy ion collisions, starting with proof from low-energy collisions that femtoscopy can, indeed, measure very long timescales. I then discuss the relativistic case, where detailed measurements over three orders of magnitude in energy reveal a timescale increase that might be due to a first-order phase transition. I discuss also consistency in evolution timescales as determined from traditional longitudinal sizes and a novel analysis using shape information.
We review progress in the study of antinuclei, starting from Diracs equation and the discovery of the positron in cosmic-ray events. The development of proton accelerators led to the discovery of antiprotons, followed by the first antideuterons, demonstrating that antinucleons bind into antinuclei. With the development of heavy-ion programs at the Brookhaven AGS and CERN SPS, it was demonstrated that central collisions of heavy nuclei offer a fertile ground for research and discoveries in the area of antinuclei. In this review, we emphasize recent observations at Brookhavens Relativistic Heavy Ion Collider and at CERNs Large Hadron Collider, namely, the antihypertriton and the antihelium-4, as well as measurements of the mass difference between light nuclei and antinuclei, and the interaction between antiprotons. Physics implications of the new observations and different production mechanisms are discussed. We also consider implications for related fields, such as hypernuclear physics and space-based cosmic-ray experiments.
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