-
[show abstract]
[hide abstract]
ABSTRACT: 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.
05/2013;
-
[show abstract]
[hide abstract]
ABSTRACT: 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 \emph{Eulerian Conservative High-Order} astrophysical code for
general relativistic magneto-hydrodynamics [\emph{Del Zanna et al., 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 in both Minkowskian or Bjorken coordinates;
partial or complete chemical equilibrium of hadronic species before kinetic
freeze-out; initial conditions based on the optical 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.
05/2013;
-
[show abstract]
[hide abstract]
ABSTRACT: A refined version of a multi-step calculation of heavy-flavor observables in
pp and AA collisions has been developed, based on pQCD at NLO accuracy followed
by parton shower evolution to describe heavy-quark production and on the
relativistic Langevin equation to describe their stochastic evolution in the
QCD plasma. Then, hadronization is modeled through an implementation of
fragmentation functions based on pQCD and constrained by $e^{+}e^{-}$ collider
data. Results of our calculations can be compared with recent measurements
performed at the LHC in Pb--Pb collisions at $\sqrt{s_{NN}}$=2.76 TeV: nuclear
modification factor $R_{AA}$ of the $p_{T}$ spectra at mid-rapidity of
heavy-flavor decay electrons and of exclusively reconstructed open-charm mesons
at different centralities, as well as their elliptic-flow $v_{2}$($p_{T}$) in
semi-central collisions. To test the validity of our setup for such studies,
its predictions are also checked against the $p_{T}$ spectra measured in pp
collisions at $\sqrt{s}$=7 TeV and 2.76 TeV.
08/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
07/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We present a study of the heavy-flavor dynamics in nucleus-nucleus
collisions. The initial (hard) production of c and b quarks is taken from NLO
pQCD predictions. The presence of a hot medium (a Quark Gluon Plasma described
by hydrodynamics) affects the final spectra of open-charm (beauty) hadrons and
their decay electrons with respect to what found in pp collisions. The
propagation of c and b quarks in the plasma is based on a picture of multiple
uncorrelated random collisions, described by a relativistic Langevin equation.
A microscopic evaluation of the transport coefficients is provided within a
pQCD approach (with proper resummation of medium effects). Results for the
final spectra of heavy-flavor hadrons and decay-electrons are given, with
particular emphasis on R_AA and v_2.
05/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: 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 R_AA and the
elliptic flow coefficients v_2 of electrons from heavy-flavour decay.
01/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We study the stochastic dynamics of c and b quarks in the hot plasma produced in nucleus-nucleus collisions at RHIC and LHC, providing results for the nuclear modification factor RAA and the elliptic flow coefficient v2 of the single-electron spectra arising from their semi-leptonic decays. The initial pairs are generated using the POWHEG code, implementing pQCD at NLO. For the propagation in the plasma we develop a relativistic Langevin equation (solved in a medium described by hydrodynamics) whose transport coefficients are evaluated through a first-principle calculation. Finally, at Tc, the heavy quarks are made to hadronize and decay into electrons: the resulting spectra are then compared with RHIC results. Predictions for LHC are also attempted.
Nuclear Physics A. 11/2010; 855(1):404-407.
-
[show abstract]
[hide abstract]
ABSTRACT: As a function of energy E, the average strength function S(E) of a doorway
state is commonly assumed to be Lorentzian in shape and characterized by two
parameters, the peak energy E_0 and the spreading width Gamma. The simple
picture is modified when the density of background states that couple to the
doorway state changes significantly in an energy interval of size Gamma. For
that case we derive an approximate analytical expression for S(E). We test our
result successfully against numerical simulations. Our result may have
important implications for shell--model calculations.
09/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: 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 2010
09/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
07/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: Quark Gluon Plasma transport coefficients for heavy quarks and pairs are computed through an extension of the results obtained for a hot QED plasma by describing the heavy-quark propagation in the eikonal approximation and by weighting the gauge-field configurations with the Hard Thermal Loop effective action. It is shown that such a model allows to correctly reproduce, at leading logarithmic accuracy, the results obtained by other independent approaches. The results are then inserted into a relativistic Langevin equation allowing to follow the evolution of the heavy-quark momentum spectra. Our numerical findings are also compared with the ones obtained in a strongly-coupled scenario, namely with the transport coefficients predicted (though with some limitations and ambiguities) by the AdS/CFT correspondence.
Nuclear Physics A. 02/2009;
-
[show abstract]
[hide abstract]
ABSTRACT: The quark viscosity in the quark–gluon plasma is evaluated in the hard thermal loop (HTL) approximation. The different contributions to the viscosity arising from the various components of the quark spectral function are discussed. The calculation is extended to finite values of the chemical potential.
Journal of Physics G Nuclear and Particle Physics 12/2008; 36(2):025008. · 4.18 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The quark viscosity in the quark gluon plasma is evaluated in HTL approximation. The different contributions to the viscosity arising from the various components of the quark spectral function are discussed. The calculation is extended to finite values of the chemical potential. Comment: 9 pages, 3 figures
08/2007;
-
[show abstract]
[hide abstract]
ABSTRACT: We update our recent calculation of quarkonium Euclidean correlators at finite temperatures in a potential model by including the effect of zero modes in the lattice spectral functions. These contributions cure most of the previously observed discrepancies with lattice calculations, supporting the use of potential models at finite temperature as an important tool to complement lattice studies.
07/2007;
-
[show abstract]
[hide abstract]
ABSTRACT: The Schroedinger equation for the charmonium and bottomonium states at finite temperature is solved by employing an effective temperature dependent potential given by a linear combination of the color singlet free and internal energies obtained on the lattice from the Polyakov loop correlation functions. The melting temperatures and other properties of the quarkonium states are evaluated. The consistency of the potential model approach with the available lattice data on the quarkonium temporal correlators and spectral functions is explored.
01/2007;
-
[show abstract]
[hide abstract]
ABSTRACT: Modelling the chaotic states in terms of the Gaussian Orthogonal Ensemble of random matrices (GOE), we investigate the interaction of the GOE with regular bound states. The eigenvalues of the latter may or may not be embedded in the GOE spectrum. We derive a generalized form of the Pastur equation for the average Green's function. We use that equation to study the average and the variance of the shift of the regular states, their spreading width, and the deformation of the GOE spectrum non-perturbatively. We compare our results with various perturbative approaches.
12/2006;
-
[show abstract]
[hide abstract]
ABSTRACT: Quasielastic electromagnetic and parity-violating electron scattering response functions of relativistic nuclear matter are reviewed. The roles played by the Hartree-Fock field and by nuclear correlations in the Random Phase Approximation (treated within the continued fraction scheme) are illustrated. The parity-violating responses of nuclei to polarized electrons are also revisited, stressing in particular the crucial role played by the pion in the nuclear dynamics. Finally, some issues surrounding scaling and sum rules are addressed.
07/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: A comprehensive parameterization of the colour singlet heavy quark free
energy above $T_c$ is given, using the lattice data in quenched ($N_f=0$) and
unquenched ($N_f=2$ and $N_f=3$) QCD. The corresponding (temperature dependent)
potentials thus obtained are then inserted into the Schr\"odinger equation for
the charmonium and the bottomonium in the deconfined phase of QCD. The solution
of the equation provides an estimate of the melting temperature and of the
radii for the different $c\bar{c}$ and $b\bar{b}$ bound states.
07/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: Following recent studies of inclusive electron scattering from nuclei at high energies which focused on two-nucleon emission mediated by meson-exchange currents, in this work the superscaling behavior of such contributions is investigated. Comparisons are made with existing data below the quasielastic peak where at high momentum transfers scaling of the second kind is known to be excellent and scaling of the first kind is good, in the proximity of the peak where both 1p–1h and 2p–2h contributions come into play, and above the peak where inelasticity becomes important and one finds scaling violations of the two kinds.
Nuclear Physics A. 04/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: The contribution to the nuclear transverse response function R_T arising from two particle-two hole (2p-2h) states excited through the action of electromagnetic meson exchange currents (MEC) is computed in a fully relativistic framework. The MEC considered are those carried by the pion and by Delta degrees of freedom, the latter being viewed as a virtual nucleonic resonance. The calculation is performed in the relativistic Fermi gas model in which Lorentz covariance can be maintained. All 2p-2h many-body diagrams containing two pionic lines that contribute to R_T are taken into account and the relative impact of the various components of the MEC on R_T is addressed. The non-relativistic limit of the MEC contributions is also discussed and compared with the relativistic results to explore the role played by relativity in obtaining the 2p-2h nuclear response. Comment: 27 pages, 12 figures, revtex4; minor modifications in the discussion of the results, references added
04/2003;
