An overview of experimental results from ultra-relativistic heavy-ion collisions at the CERN LHC: Hard probes

Abstract

The first collisions of lead nuclei, delivered by the CERN Large Hadron Collider (LHC) at the end of 2010, at a centreof-mass energy per nucleon pair √ s NN = 2.76 TeV, marked the beginning of a new era in ultra-relativistic heavy-ion physics. The study of the properties of the produced hot and dense strongly-interacting matter at these unprecedented energies is currently experimentally pursued by all four big LHC experiments, ALICE, ATLAS, CMS, and LHCb. The more than a factor 10 increase of collision energy at LHC, relative to the previously achieved maximal energy at other collider facilities, results in an increase of production rates of hard probes. This review presents selected experimental results focusing on observables probing hard processes in heavy-ion collisions delivered during the first three years of the LHC operation. It also presents the first results from Run 2 heavy-ion data at the highest energy, as well as from the studies of the reference pp and p-Pb systems, which are an integral part of the heavy-ion programme. (Małgorzata Anna Janik) 1 In fact, it is a pseudo-critical temperature as 'lattice QCD' calculations indicate a crossover rather than a well defined phase transition [2,3].

Heavy Ion Collisions: The Big Picture and the Big Questions

“…In order to give the "big picture", in this discussion we shall ignore small differences and subtle effects. For useful summaries of and references to RHIC and LHC data we refer the reader to (58,59,60,61) and (62,63,64). See also recent proceedings of Quark Matter conferences and (22, 65) for an overview of theoretical and experimental work.…”

“…The Υ(2S) is an intermediate case.Figure 5is a beautiful example of data which shows that Υ states with different sizes and binding strengths do indeed have different probabilities of surviving in QGP, supporting this picture. J/ψ production in LHC heavy ion collisions is interestingly different(64). These collisions are sufficiently energetic that, on average, about 30 cc pairs are produced in each heavy ion collision(100).…”

Low-energy heavy-ion reactions and the Skyrme effective interaction

“…This review surveys these studies in low energy heavy-ion reactions, finding significant effects on observables from the form of the spin-orbit interaction, the use of the tensor force, and the inclusion of time-odd terms in the density functional.Heavy-ion collisions combine the rich dynamics of a many-body out-of-equilibrium open quantum system with the complexities of the residual part of the strong interaction which leaks out of the small, but neither fundamental or point-like, nucleons, causing them to stick loosely together some of the time, and to fall apart at others. Understanding heavy-ion reactions across all energy scales is necessary to understand stellar nucleosynthesis [1], the synthesis of superheavy nuclei [2,3], the properties of nuclear matter [4][5][6], the QCD phase diagram [7,8] as well as the understanding of reaction mechanisms themselves [9][10][11][12][13].Among the theoretical techniques used to study heavy-ion reactions, methods based on timedependent Hartree-Fock have recently achieved the status of having sufficiently mature implementations free of limiting approximations, and running at a suitable speed, such that systematically varying the effective interaction in the calculations is possible. It is such studies that form the main subject of the present review.…”

“…and V αβγδ = αβ|V |γδ (7) are the two-body interaction matrix elements. Using (5) in (4) and the anticommutation relationships for fermion creation and annihilation operators gives i ρ βα = δ (t βδ ρ δα − ρ βδ t δα )…”

How does the Quark-Gluon Plasma know the collision energy?

“…The novel form of matter produced in heavy ion collisions, the Quark-Gluon Plasma or QGP, is currently under study at two major facilities: the RHIC [1], which typically collides gold nuclei at maximal centre-of-mass energies up to 200 GeV per nucleon pair, and the LHC, which (in the heavy ion runs studied particularly in ALICE) typically collides lead nuclei at maximal centre-of-mass energies around 2.76 TeV per pair, latterly upgraded to 5.02 TeV [2,3]. On this basis, it is often said that the two facilities explore two different regimes for the QGP.…”