arXiv subjects

Vincenzo Greco

Publications and source records attributed to Vincenzo Greco.

85 records · Page 5Linked to original sources

arXiv2013

Renormalized vs Nonrenormalized Chiral Transition in a Magnetic Background

We study analytically the chiral phase transition for hot quark matter in presence of a strong magnetic background, focusing on the existence of a critical point at zero baryon chemical potential and nonzero magnetic field. We build up a Ginzburg-Landau effective potential for the chiral condensate at finite temperature, computing the coefficients of the expansion within a chiral quark-meson model. Our conclusion is that the existence of the critical point at finite $\bm B$ is very sensitive to the way the ultraviolet divergences of the model are treated. In particular, we find that after renormalization, no chiral critical point is present in the phase diagram. On the other hand, such a critical point there exists when the ultraviolet divergences are not removed by a proper renormalization of the thermodynamic potential.

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arXiv2012

Elliptic flow at varying energy heavy ion collisions: Partonic vs hadronic dynamics

We examine whether the breakdown in elliptic flow quark number scaling observed at the Relativistic Heavy Ion Collider (RHIC) energy scan is related to the turning off of deconfinement by testing the hypothesis that hydrodynamics and parton coalescence always apply, but are obscured, at lower energies, by variations in the widths of quark and anti-quark rapidity distribution. We find that this effect is enough to spoil quark number scaling in elliptic flow. A lack of scaling in data therefore does not signal the absence of partonic degrees of freedom and hadronization by coalescence. In a coalescing partonic fluid, however, elliptic flow of anti-baryons should be the greater than that of baryons, since antibaryons contain a greater admixture of partons from the highly flowing midrapidity region. Intriguingly, purely hadronic dynamics has a similar dependence of baryon-anti-baryons $v_2$ as purely partonic dynamics, again because antibaryons tend to come from regions where the deviation of the system from hydrodynamic behavior is at its smallest. The opposite trend observed in experiment is therefore an indication that we might be misunderstanding the origin of $v_2$. We finish by discussing possible explanations of this, and suggest experimental measurements capable of clarifying the situation.

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arXiv2012

Quasiparticles and $Z(N)-$ lines in Hot Yang-Mills theories

In this talk we review, the quasiparticle description of the hot Yang-Mills theories, in which the quasiparticles propagate in (and interact with) a background field related to Z(N)-lines. We compare the present description with a more common one in which the effects of the Z(N)-lines are neglected. We show that it is possible to take into account the nonperturbative effects at the confinement transition temperature even without a divergent quasiparticle mass.

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arXiv2012

Shear viscosity and chemical equilibration of the QGP

We have investigated, in the frame work of the transport approach, different aspects of the QGP created in Heavy Ion Collisions at RHIC and LHC energies. The shear viscosity $η$ has been calculated by using the Green-Kubo relation at the cascade level. We have compared the numerical results for $η$ obtained from the Green-Kubo correlator with the analytical formula in both the Relaxation Time Approximation (RTA) and the Chapman-Enskog approximation (CE). From this comparison we show that in the range of temperature explored in a Heavy Ion collision the RTA underestimates the viscosity by about a factor of 2, while a good agreement is found between the CE approximation and Gree-Kubo relation already at first order of approximation. The agreement with the CE approximation supplies an analytical formula that allows to develop kinetic transport theory at fixed shear viscosity to entropy density ratio, $η/s$. We show some results for the build up of anisotropic flows $v_{2}$ in a transport approach at fixed shear viscosity to entropy density ratio, $η/s$. We study the impact of a T-dependent $η/s(T)$ on the generation of the elliptic flows at both RHIC and LHC. We show that the transport approach provides, in a unified way, a tool able to naturally describe the $v_{2}(p_{T})$ in a wide range of $p_{T}$, including also the description of the rise and fall and saturation of the $v_{2}(p_{T})$ observed at LHC. Finally, we have studied the evolution of the quark-gluon composition employing a Boltzmann-Vlasov transport approach that include: the mean fields dynamics, associated to the quasi-particle model, and the elastic and inelastic collisions for massive quarks and gluons. Following the chemical evolution from an initial gluon dominated plasma we predict a quark dominance close to $T_{C}$ paving the way to an hadronization via quark coalescence.

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arXiv2011

Recent thermodynamic results from lattice QCD analyzed within a quasi-particle model

The thermodynamic behavior of QCD matter at high temperature is currently studied by lattice QCD theory. The main features are the fast rise of the energy density $ε$ around the critical temperature $T_c$ and the large trace anomaly of the energy momentum tensor $< Θ_μ^μ>=ε- 3 P$ which hints at a strongly interacting system. Such features can be accounted for by employing a massive quasi-particle model with a temperature-dependent bag constant. Recent lattice QCD calculations with physical quark masses by the Wuppertal-Budapest group have shown a slower increase of $ε$ and a smaller $<Θ_μ^μ>$ peak with respect to previous results from the hotQCD collaboration. We investigate the implications of such differences from the point of view of a quasi-particle model, also discussing light and strange quark number susceptibilities. Furthermore, we predict the impact of these discrepancies on the temperature-dependence of the transport properties of matter, like the shear and bulk viscosities.

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arXiv2010

Sensitivity of the Jet Quenching Observables to the Temperature Dependence of the Energy Loss

The quenching of minijet (particles with $p_T>> T, Λ_{QCD}$) 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 $T_c$ and the $q\leftrightarrow g$ conversion is significant. The use of an equation of state fitted to lattice QCD calculations, slowing down the cooling as $T \to T_c$, seems to contribute to both the enhancement of $v_2$ and the efficiency of the conversion mechanism.

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arXiv2009

The effect of quark coalescence on conical signals

We explore the effect of hadronization by partonic coalescence on a "conical" signal at the partonic level. We show that, by transferring partons from a lower to a higher $p_T$, coalescence makes the conical signal stronger and hence less susceptible to thermal smearing, provided the signal is integrated over a large momentum bin and effects such as non-collinearity and a finite Wigner function width are taken into account. We explore the role of this effect in baryon/meson scaling and calculate the effect of resonances decays on such a conical signal.

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arXiv2008

T-matrix approach to heavy quark diffusion in the QGP

We assess transport properties of heavy quarks in the Quark-Gluon Plasma (QGP) using static heavy-quark (HQ) potentials from lattice-QCD calculations in a Brueckner many-body T-matrix approach to evaluate elastic heavy-quark-light-quark scattering amplitudes. In the attractive meson and diquark channels resonance states are formed for temperatures up to ~1.5 T_c, increasing pertinent drag and diffusion coefficients for heavy-quark rescattering in the QGP beyond the expectations from perturbative-QCD calculations. We use these transport coefficients, complemented with perturbative elastic HQ gluon scattering, in a relativistic Langevin simulation to obtain HQ p_t distributions and elliptic flow (v_2) under conditions relevant for the hot and dense medium created in ultrarelativistic heavy-ion collisions. The heavy quarks are hadronized to open-charm and -bottom mesons within a combined quark-coalescence fragmentation scheme. The resulting single-electron spectra from their semileptonic decays are confronted with recent data on "non-photonic electrons" in 100 AGeV Au-Au collisions at the Relativistic Heavy-Ion Collider (RHIC).

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arXiv2008

Coalescence Models For Hadron Formation From Quark Gluon Plasma

We review hadron formation from a deconfined quark gluon plasma (QGP) via coalescence or recombination of quarks and gluons. We discuss the abundant experimental evidence for coalescence from the Relativistic Heavy Ion Collider (RHIC) and compare the various coalescence models advocated in the literature. We comment on the underlying assumptions and remaining challenges as well as the merits of the models. We conclude with a discussion of some recent developments in the field.

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arXiv2006

Heavy-Quark Probes of the Quark-Gluon Plasma at RHIC

Thermalization and collective flow of charm (c) and bottom (b) quarks in ultra-relativistic heavy-ion collisions are evaluated based on elastic parton rescattering in an expanding quark-gluon plasma (QGP). We show that resonant interactions in a strongly interacting QGP (sQGP), as well as the effects of parton coalescence, can play an essential role in the interpretation of recent data from the Relativistic Heavy-Ion Collider (RHIC), and thus illuminate the nature of the sQGP and its hadronization. Our main assumption, motivated by recent findings in lattice computations of Quantum Chromodynamics, is the existence of D- and B-meson states in the sQGP, providing resonant cross sections for heavy quarks up to temperatures of sim 2 T_c. Pertinent drag and diffusion coefficients are implemented into a relativistic Langevin simulation to compute transverse-momentum spectra and azimuthal asymmetries (v_2) of b- and c-quarks in Au-Au collisions at RHIC. Hadronization into D- and B-mesons is calculated from a combination of coalescence with light quarks and fragmentation, and associated electron-decay spectra and v_2 are compared to recent RHIC data. We also comment on the relative importance of radiative and elastic energy loss of heavy quarks in the QGP.

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arXiv2004

Pseudorapidity dependence of anisotropic flows in relativistic heavy-ion collisions

The pseudorapidity dependence of anisotropic flows $v_{1}$, $v_{2}$, $v_{3}$ , and $v_{4}$ of charged hadrons in heavy-ion collisions at the Relativistic Heavy Ion Collider is studied in a multi-phase transport model. We find that while the string melting scenario, in which hadrons that are expected to be formed from initial strings are converted to their valence quarks and antiquarks, can explain the measured $p_{T}$-dependence of $v_{2}$ and $ v_{4} $ of charged hadrons at midrapidity with a parton scattering cross section of about 10 \textrm{mb}, the scenario without string melting reproduces better the recent data on $v_{1}$ and $v_{2}$ of charged hadrons at large pseudorapidity in Au + Au collisions at $\sqrt{s}=200$ AGeV . Our results thus suggest that a partonic matter is formed during early stage of relativistic heavy ion collisions only around midrapidity and that strings remain dominant at large rapidities. The $p_{T}$-dependence of $v_{1} $, $v_{2}$, $v_{3}$ and $v_{4}$ for charged hadrons at forward pseudorapidity is also predicted, and we find that while $v_{1}$ and $v_{2}$ are appreciable at large pseudorapidity the higher-order anisotropic flows $v_{3}$ and $v_{4} $ are essentially zero.

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arXiv2004

Momentum anisotropies in the quark coalescence model

Based on the quark coalescence model, we derive relations among the momentum anisotropies of mesons and baryons in relativistic heavy ion collisions from a given, but arbitrary azimuthal distribution for the partons. Besides the familiar even Fourier coefficients such as the elliptic flow, we also pay attention to odd Fourier coefficients such as the directed flow, which has been observed at finite rapidity even at RHIC energies.

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arXiv2003

Effect of symmetry energy on two-nucleon correlation functions in heavy-ion collisions induced by neutron-rich nuclei

Using an isospin-dependent transport model, we study the effects of nuclear symmetry energy on two-nucleon correlation functions in heavy ion collisions induced by neutron-rich nuclei. We find that the density dependence of the nuclear symmetry energy affects significantly the nucleon emission times in these collisions, leading to larger values of two-nucleon correlation functions for a symmetry energy that has a stronger density dependence. Two-nucleon correlation functions are thus useful tools for extracting information about the nuclear symmetry energy from heavy ion collisions.

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