A. Beraudo

Heavy-flavour spectra in high-energy nucleus-nucleus collisions

The propagation of the heavy quarks produced in relativistic nucleus–nucleus collisions at RHIC and LHC is studied within the framework of Langevin dynamics in the background of an expanding deconfined medium described by ideal and viscous hydrodynamics. The transport coefficients entering into the relativistic Langevin equation are evaluated by matching the hard-thermal-loop result for soft collisions with a perturbative QCD calculation for hard scatterings. The heavy-quark spectra thus obtained are employed to compute the differential cross sections, the nuclear modification factors RAA and the elliptic-flow coefficients v2 of electrons from heavy-flavor decay.

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

A multi-step setup for heavy-flavor studies in high-energy nucleus-nucleus (AA) collisions—addressing within a comprehensive framework the initial

Relativistic viscous hydrodynamics for heavy-ion collisions with ECHO-QGP

Q\overline{Q}

Heavy-flavour production in high-energy d-Au and p-Pb collisions

production, the propagation in the hot medium until decoupling and the final hadronization and decays—is presented. The initial hard production of

Medium-induced color flow softens hadronization

Q\overline{Q}

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

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 RAA and for the elliptic flow v2 of D/B mesons, heavy-flavor electrons and non-prompt J/ψ’s are displayed.

Potential models and lattice correlators for quarkonia at finite temperature

We present ECHO-QGP, a numerical code for (3+1)-dimensional relativistic viscous hydrodynamics designed for the modeling of the space-time evolution of the matter created in high-energy nuclear collisions. The code has been built on top of the Eulerian Conservative High-Order astrophysical code for general relativistic magneto-hydrodynamics (Del Zanna et al. in Astron. Astrophys. 473:11, 2007] and here it has been upgraded to handle the physics of the Quark–Gluon Plasma. ECHO-QGP features second-order treatment of causal relativistic viscosity effects both in Minkowskian and in Bjorken coordinates; partial or complete chemical equilibrium of hadronic species before kinetic freeze-out; initial conditions based on the Glauber model, including a Monte-Carlo routine for event-by-event fluctuating initial conditions; a freeze-out procedure based on the Cooper–Frye prescription. The code is extensively validated against several test problems and results always appear accurate, as guaranteed by the combination of the conservative (shock-capturing) approach and the high-order methods employed. ECHO-QGP can be extended to include evolution of the electromagnetic fields coupled to the plasma.

Numerical magneto-hydrodynamics for relativistic nuclear collisions

A bstractSoft-hadron measurements in high-energy collisions of small systems like p-Pb and d-Au show peculiar qualitative features (long-range rapidity correlations, flattening of the pT -spectra with increasing hadron mass and centrality, non-vanishing Fourier harmonics in the azimuthal particle distributions) suggestive of the formation of a strongly-interacting medium displaying a collective behaviour, with a hydrodynamic flow as a response to the pressure gradients in the initial conditions. Hard observables (high-pT jet and hadron spectra) on the other hand, within the current large systematic uncertainties, appear only midly modified with the respect to the benchmark case of minimum-bias p-p collisions. What should one expect for heavy-flavour particles, initially produced in hard processes but tending, in the nucleus-nucleus case, to approach kinetic equilibrium with the rest of the medium? This is the issue we address in the present study, showing how the current experimental findings are compatible with a picture in which the formation of a hot medium even in proton-nucleus collisions modifies the propagation and hadronization of heavy-flavour particles.

The contribution of medium-modified color flow to jet quenching

Medium-induced parton energy loss, resulting from gluon exchanges between the QCD matter and partonic projectiles, is expected to underly the strong suppression of jets and high-pT hadron spectra observed in ultra-relativistic heavy ion collisions. Here, we present the first color-differential calculation of parton energy loss. We find that color exchange between medium and projectile enhances the invariant mass of energetic color singlet clusters in the parton shower by a parametrically large factor proportional to the square root of the projectile energy. This effect is seen in more than half of the most energetic color-singlet fragments of medium-modified parton branchings. Applying a standard cluster hadronization model, we find that it leads to a characteristic additional softening of hadronic spectra. A fair description of the nuclear modification factor measured at the LHC may then be obtained for relatively low momentum transfers from the medium. High transverse momentum partons (pT ≫ 10 GeV) produced in heavy ion collisions interact with the QCD matter in the collision region while branching. This interaction is thought to cause the strong medium modification of hadronic spectra and jets measured in heavy ion collisions at the LHC and at RHIC. The modeling of this jet quenching phenomenon has focussed so far on medium-induced parton energy loss prior to hadronization [1], assuming that for sufficiently highpT hadronization occurs time-delayed outside the medium. However, if the color flow of a parton shower is modified within the medium, then hadronization can be affected irrespective of when it occurs. Here, we analyze for the first time the medium-induced color flow in a standard parton energy loss calculation. Compared to the current modeling of parton energy loss [2–7], this will be seen to result in a characteristic softening of the ensuing hadronization. It may thus affect significantly the extraction of medium properties from the measured nuclear modification factor at the LHC [8–10]. We start by considering an elementary building block of a parton shower, the q → q g parton splitting. For a small light-cone energy k + of the gluon compared to the parent parton, x ≡ k + /p + ≪ 1, and for transverse gluon momentum k with K0 ≡ k/k 2 , the gluon spectrum

How (non-)linear is the hydrodynamics of heavy ion collisions?

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.