96 research outputs found
Hadrons from Coalescence plus Fragmentation in AA collisions from RHIC to LHC energy
In a coalescence plus independent fragmentation approach we calculate the
spectra of the main hadrons: in a wide
range of transverse momentum from low up to about 10 GeV. The approach in
its main features was developed several years ago at RHIC energy. Augmenting
the model with the inclusion of some more main resonance decays, we show that
the approach correctly predicts the evolution of the spectra from RHIC to
LHC energy and in particular the baryon-to-meson ratios that reach a value of the order of unit at . This is achieved without any change of the coalescence parameters. The
more recent availability of experimental data up to for
spectrum as well as for and shows some lack of
yield in a limited range around 6 GeV. This indicates that the baryons
spectra from AKK fragmentation functions are too flat at . We also show that in a coalescence plus fragmentation approach one
predicts a nearly independent ratio up to
followed by a significant decrease at higher . Such a behavior is driven
by a similar radial flow effect at and the dominance of
fragmentation for at larger .Comment: 11 pages, 12 figures Added the figure and discussion about the
proton/phi ratio, as in the accepted version for pubblicatio
Sensitivity of the Jet Quenching Observables to the Temperature Dependence of the Energy Loss
The quenching of minijet (particles with ) in
ultra-relativistic heavy-ion collisions has been one of the main prediction and
discovery at RHIC. We analyze the correlation between different observables
like the nuclear modification factor \Rapt, the elliptic flow and the ratio
of quark to gluon suppressions. We show that the temperature (or entropy
density) dependence of the in-medium energy loss strongly affects the relation
among these observables. In particular the large elliptic flow and the nearly
equal \Rapt of quarks and gluons can be accounted for only if the energy loss
occurs mainly around and the conversion is
significant. The use of an equation of state fitted to lattice QCD
calculations, slowing down the cooling as , seems to contribute to
both the enhancement of and the efficiency of the conversion mechanism.Comment: 12 pages, 10 figure
Thermalization, Isotropization and Elliptic Flow from Nonequilibrium Initial Conditions with a Saturation Scale
In this article we report on our results about the computation of the
elliptic flow of the quark-gluon-plasma produced in relativistic heavy ion
collisions, simulating the expansion of the fireball by solving the
relativistic Boltzmann equation for the parton distribution function tuned at a
fixed shear viscosity to entropy density ratio . Our main goal is to
put emphasis on the role of a saturation scale in the initial gluon spectrum,
which makes the initial distribution far from a thermalized one. We find that
the presence of the saturation scale reduces the efficiency in building-up the
elliptic flow, even if the thermalization process is quite fast and the pressure isotropization even faster
. The impact of the non-equilibrium
implied by the saturation scale manifests for non-central collisions and can
modify the estimate of the viscosity respect to the assumption of full
thermalization in -space. We find that the estimate of is
modified from to at RHIC and
from to at LHC. We complete our
investigation by a study of the thermalization and isotropization times of the
fireball for different initial conditions and values of showing how
the latter affects both isotropization and thermalization. Lastly, we have seen
that the range of values explored by the phase-space distribution function
is such that at the inner part of the fireball stays with
occupation number significantly larger than unity despite the fast longitudinal
expansion, which might suggest the possibility of the formation of a transient
Bose-Einstein Condensate.Comment: Published versio
Elliptic Flow and Shear Viscosity of the Shattered Color Glass Condensate
In this talk, we report on our results about the computation of the elliptic
flow of the quark-gluon-plasma produced in relativistic heavy ion collisions,
simulating the expansion of the fireball by solving the relativistic Boltzmann
equation for the parton distribution function tuned at a fixed shear viscosity
to entropy density ratio . We emphasize the role of saturation in the
initial gluon spectrum modelling the shattering of the color glass condensate,
causing the initial distribution to be out of equilibrium. We find that the
saturation reduces the efficiency in building-up the elliptic flow, even if the
thermalization process is quite fast . and
the pressure isotropization even faster .
The impact of the initial non-equilibrium manifests for non-central collisions
and can modify the estimate of the viscosity respect to the assumption of full
thermalization in -space.Comment: 8 pages, 3 figures. Talk given at XIV Convegno su Problemi di Fisica
Nucleare Teorica, 29-31 October 2013, Cortona, Ital
Heavy Flavor Suppression: Boltzmann vs Langevin
The propagation of heavy flavor through the quark gluon plasma has been
treated commonly within the framework of Langevin dynamics, i.e. assuming the
heavy flavor momentum transfer is much smaller than the light one. On the other
hand a similar suppression factor has been observed experimentally for
light and heavy flavors. We present a thorough study of the approximations
involved by Langevin equation by mean of a direct comparison with the full
collisional integral within the framework of Boltzmann transport equation. We
have compared the results obtained in both approaches which can differ
substantially for charm quark leading to quite different values extracted for
the heavy quark diffusion coefficient. In the case of bottom quark the
approximation appears to be quite reasonable.Comment: 4 pages, 3 figures, Talk given at SQM-2013 (Birmingham, UK
Initial state fluctuations from mid-peripheral to ultra-central collisions in a event-by-event transport approach
We have developed a relativistic kinetic transport approach that incorporates
initial state fluctuations allowing to study the build up of elliptic flow
and high order harmonics , and for a fluid at fixed
. We study the effect of the ratio and its T dependence on
the build up of the for two different beam energies: RHIC for Au+Au
at and LHC for at . We find
that for the two different beam energies considered the suppression of the
due to the viscosity of the medium have different contributions
coming from the cross over or QGP phase. Our study reveals that only in
ultra-central collisions () the have a stronger
sensitivity to the T dependence of in the QGP phase and this
sensitivity increases with the order of the harmonic n. Moreover, the study of
the correlations between the initial spatial anisotropies and the
final flow coefficients shows that at LHC energies there is more
correlation than at RHIC energies. The degree of correlation increases from
peripheral to central collisions, but only in ultra-central collisions at LHC,
we find that the linear correlation coefficient for
and . This suggests that the final correlations in the
(,) space reflect the initial correlations in the
(,) space.Comment: Minor changes. Published on Phys. Rev.
Heavy Flavor in Medium Momentum Evolution: Langevin vs Boltzmann
The propagation of heavy quarks in the quark-gluon plasma (QGP) has been
often treated within the framework of the Langevin equation (LV), i.e. assuming
the momentum transfer is small or the scatterings are sufficiently forward
peaked, small screening mass . We address a direct comparison between the
Langevin dynamics and the Boltzmann collisional integral (BM) when a bulk
medium is in equilibrium at fixed temperature. We show that unless the cross
section is quite forward peaked () or the mass to temperature
ratio is quite large () there are significant
differences in the evolution of the spectra and consequently on nuclear
modification factor . However for charm quark we find that very
similar between the LV and BM can be obtained, but with a
modified diffusion coefficient by about depending on the angular
dependence of the cross section which regulates the momentum transfer. Studying
also the momentum spread suffered by a single heavy quarks we see that at
temperatures the dynamics of the scatterings is far
from being of Brownian type for charm quarks. In the case of bottom quarks we
essentially find no differences in the time evolution of the momentum spectra
between the LV and the BM dynamics independently of the angular dependence of
the cross section, at least in the range of temperature relevant for
ultra-relativistic heavy-ion collisions. Finally, we have shown the possible
impact of this study on and for a realistic simulation
of relativistic HIC. For larger the elliptic flow can be about
larger for the Boltzmann dynamics with respect to the Langevin. This is helpful
for a simultaneous reproduction of and .Comment: Accepted for publication in Phys. Rev. C . New figs have been
included to describe the experimental result
Toward a simultaneous description of and for heavy quarks
The two key observables related to heavy quarks that have been measured in
RHIC and LHC energies are the nuclear suppression factor and the
elliptic flow . Simultaneous description of these two observables is a top
challenge for all the existing models. We have highlighted how a consistent
combination of four ingredients i.e the temperature dependence of the energy
loss, full solution of the Boltzmann collision integral for the momentum
evolution of heavy quark, hadronization by coalescence, then the hadronic
rescattering, responsible to address a large part of such a puzzle. We have
considered four different models to evaluate the temperature dependence of drag
coefficients of the heavy quark in QGP. All these four different models are set
to reproduce the same as of the experiments. We have shown that for
the same , the could be quite different depending on the
interaction dynamics as well as other ingredients.Comment: 4 pages, To appear in the proceedings of SQM-201
Estimating the Charm Quark Diffusion Coefficient and thermalization time from D meson spectra at RHIC and LHC
We describe the propagation of charm quarks in the quark-gluon plasma (QGP)
by means of a Boltzmann transport approach. Non-perturbative interaction
between heavy quarks and light quarks have been taken into account through a
quasi-particle approach in which light partons are dressed with thermal masses
tuned to lQCD thermodynamics. Such a model is able to describe the main feature
of the non-perturbative dynamics: the enhancement of the interaction strength
near . We show that the resulting charm in-medium evolution is able to
correctly predict simultaneously the nuclear suppression factor, , and
the elliptic flow, , at both RHIC and LHC energies and at different
centralities. The hadronization of charm quarks is described by mean of an
hybrid model of fragmentation plus coalescence and plays a key role toward the
agreeement with experimental data.
We also performed calculations within the Langevin approach which can lead to
very similar as Boltzmann, but the charm drag coefficient as to
be reduced by about a and also generates an elliptic flow is
about a smaller. We finally compare the space diffusion coefficient
extracted by our phenomenological approach to lattice QCD results,
finding a satisfying agreement within the present systematic uncertainties. Our
analysis implies a charm thermalization time, in the limit, of
about which is smaller than the QGP lifetime at LHC energy.Comment: 13 pages, 12 figure
Toward an understanding of the and puzzle for heavy quarks
One of the primary aims of the ongoing nuclear collisions at Relativistic
Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies is to create
a Quark Gluon Plasma (QGP). The heavy quarks constitutes a unique probe of the
QGP properties. Both at RHIC and LHC energies a puzzling relation between the
nuclear modification factor and the elliptic flow
related to heavy quark has been observed which challenged all the existing
models.\\ We discuss how the temperature dependence of the heavy quark drag
coefficient can address for a large part of such a puzzle. We have considered
four different models to evaluate the temperature dependence of drag and
diffusion coefficients propagating through a quark gluon plasma (QGP). All the
four different models are set to reproduce the same
experimentally observed at RHIC energy. We have found that for the same
one can generate times more depending on the
temperature dependence of the heavy quark drag coefficient.Comment: proceedings of the 2015 Hard Probes conferenc
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