4,027 research outputs found
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
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