Heavy flavours in heavy-ion collisions: quenching, flow and correlations

We present results for the quenching, elliptic flow and azimuthal correlations of heavy flavour particles in high-energy nucleus-nucleus collisions obtained through the POWLANG transport setup, developed in the past to study the propagation of heavy quarks in the Quark-Gluon Plasma and here extended to include a modeling of their hadronization in the presence of a medium. Hadronization is described as occurring via the fragmentation of strings with endpoints given by the heavy (anti-)quark Q(Qbar) and a thermal parton qbar(q) from the medium. The flow of the light quarks is shown to affect significantly the R_AA and v_2 of the final D mesons, leading to a better agreement with the experimental data. The approach allows also predictions for the angular correlation between heavy-flavour hadrons (or their decay electrons) and the charged particles produced in the fragmentation of the heavy-quark strings

Heavy flavours in AA collisions: production, transport and final spectra

A multi-step setup for heavy-flavour studies in high-energy nucleus-nucleus (AA) collisions --- addressing within a comprehensive framework the initial Q-Qbar production, the propagation in the hot medium until decoupling and the final hadronization and decays --- is presented. The initial hard production of Q-Qbar pairs is simulated using the POWHEG pQCD event generator, interfaced with the PYTHIA parton shower. Outcomes of the calculations are compared to experimental data in pp collisions and are used as a validated benchmark for the study of medium effects. In the AA case, the propagation of the heavy quarks in the medium is described in a framework provided by the relativistic Langevin equation. For the latter, different choices of transport coefficients are explored (either provided by a perturbative calculation or extracted from lattice-QCD simulations) and the corresponding numerical results are compared to experimental data from RHIC and the LHC. In particular, outcomes for the nuclear modification factor R_AA and for the elliptic flow v_2 of D/B mesons, heavy-flavour electrons and non-prompt J/\psi's are displayed.Comment: 16 pages, 21 figure

Langevin dynamics of heavy flavors in relativistic heavy-ion collisions

We study the stochastic dynamics of c and b quarks, produced in hard initial processes, in the hot medium created after the collision of two relativistic heavy ions. This is done through the numerical solution of the relativistic Langevin equation. The latter requires the knowledge of the friction and diffusion coefficients, whose microscopic evaluation is performed treating separately the contribution of soft and hard collisions. The evolution of the background medium is described by ideal/viscous hydrodynamics. Below the critical temperature the heavy quarks are converted into hadrons, whose semileptonic decays provide single-electron spectra to be compared with the current experimental data measured at RHIC. We focus on the nuclear modification factor R_AA and on the elliptic-flow coefficient v_2, getting, for sufficiently large p_T, a reasonable agreement.Comment: Talk given at the workshop "Jets in Proton-Proton and Heavy-Ion Collisions", Prague, 12th-14th August 201

Heavy-quark Langevin dynamics and single-electron spectra in nucleus-nucleus collision

The stochastic dynamics of heavy quarks in the fireball produced in heavy-ion collisions is followed through numerical simulations based on the Langevin equation. The modification of the final p_T spectra (R_AA) of c and b quarks, hadrons and single-electrons with respect to pp collisions is studied. The transport coefficients are evaluated treating separately the contribution of soft and hard collisions. The initial heavy-quark spectra are generated according to NLO-pQCD, accounting for nuclear effects through recent nPDFs. The evolution of the medium is obtained from the output of two hydro-codes (ideal and viscous). The heavy-quark fragmentation into hadrons and their final semileptonic decays are implemented according to up to date experimental data. A comparison with RHIC data for non-photonic electron spectra is given.Comment: 4 pages, 3 figures, Talk given at "Hot Quarks 2010", 21th-26th June 201

Event-shape engineering and heavy-flavour observables in relativistic heavy-ion collisions

Traditionally, events collected at relativistic heavy-ion colliders are classified according to some centrality estimator (e.g. the number of produced charged particles) related to the initial energy density and volume of the system. In a naive picture the latter are directly related to the impact parameter of the two nuclei, which sets also the initial eccentricity of the system: zero in the case of the most central events and getting larger for more peripheral collisions. A more realistic modelling requires to take into account event-by-event fluctuations, in particular in the nucleon positions within the colliding nuclei: collisions belonging to the same centrality class can give rise to systems with different initial eccentricity and hence different flow harmonics for the final hadron distributions. This issue can be addressed by an event-shape-engineering analysis, consisting in selecting events with the same centrality but different magnitude of the average bulk anisotropic flow and therefore of the initial-state eccentricity. In this paper we present the implementation of this analysis in the POWLANG transport model, providing predictions for the transverse-momentum and angular distributions of charm and beauty hadrons for event-shape selected collisions. In this way it is possible to get information on how the heavy quarks propagating (and hadronizing) in a hot environment respond both to its energy density and to its geometric asymmetry, breaking the perfect correlation between eccentricity and impact parameter which characterizes a modelling of the medium based on smooth average initial condition

Heavy-flavor dynamics in nucleus-nucleus collisions: from RHIC to LHC

The stochastic dynamics of c and b quarks in the fireball created in nucleus-nucleus collisions at RHIC and LHC is studied employing a relativistic Langevin equation, based on a picture of multiple uncorrelated random collisions with the medium. Heavy-quark transport coefficients are evaluated within a pQCD approach, with a proper HTL resummation of medium effects for soft scatterings. The Langevin equation is embedded in a multi-step setup developed to study heavy-flavor observables in pp and AA collisions, starting from a NLO pQCD calculation of initial heavy-quark yields, complemented in the nuclear case by shadowing corrections, k_T-broadening and nuclear geometry effects. Then, only for AA collisions, the Langevin equation is solved numerically in a background medium described by relativistic hydrodynamics. Finally, the propagated heavy quarks are made hadronize and decay into electrons. Results for the nuclear modification factor R_AA of heavy-flavor hadrons and electrons from their semi-leptonic decays are provided, both for RHIC and LHC beam energies.Comment: 4 pages, 2 figures (3 eps files); submitted for publication in the proceedings of "Quark Matter 2011", 23-28 May 2011, Annecy (France