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S. Fortunato

Publications and source records attributed to S. Fortunato.

14 recordsLinked to original sources
arXiv2003

Predictions for J/Psi Suppression by Parton Percolation

Parton percolation provides geometric deconfinement in the pre-equilibrium stage of nuclear collisions. The resulting parton condensate can lead to charmonium suppression. We formulate a local percolation condition viable for non-uniform collision environments and show that it correctly reproduces the suppression observed for S-U and Pb-Pb collisions at the SPS. Using this formulation, we then determine the behavior of J/Psi suppression for In-In collisions at the SPS and for Au-Au collisions at RHIC.

hep-ph↗
arXiv2003

Heavy Quark Free Energies and Screening in SU(2) Gauge Theory

We study the properties of the free energy of infinitely heavy quark anti-quark pair in SU(2) gauge theory. By means of lattice Monte Carlo simulations we calculated the free energies in the singlet, triplet and color averaged channels, both in the confinement and in the deconfinement phase. The singlet and triplet free energies are defined in Coulomb gauge which is equivalent to their gauge invariant definitions recently introduced by Philipsen. We analyzed the short and the long distance behavior, making comparisons with the zero temperature case. The temperature dependence of the electric screening mass is carefully investigated. The order of the deconfining transition is manifest in the results near T_c and it allows a reliable test of a recently proposed method to renormalize the Polyakov loop.

hep-lat↗
arXiv2003

Critical Droplets and Phase Transitions in Two Dimensions

In two space dimensions, the percolation point of the pure-site clusters of the Ising model coincides with the critical point T_c of the thermal transition and the percolation exponents belong to a special universality class. By introducing a bond probability p_B<1, the corresponding site-bond clusters keep on percolating at T_c and the exponents do not change, until p_B=p_CK=1-exp(-2J/kT): for this special expression of the bond weight the critical percolation exponents switch to the 2D Ising universality class. We show here that the result is valid for a wide class of bidimensional models with a continuous magnetization transition: there is a critical bond probability p_c such that, for any p_B>=p_c, the onset of percolation of the site-bond clusters coincides with the critical point of the thermal transition. The percolation exponents are the same for p_c<p_B<=1 but, for p_B=p_c, they suddenly change to the thermal exponents, so that the corresponding clusters are critical droplets of the phase transition. Our result is based on Monte Carlo simulations of various systems near criticality.

cond-mat.stat-mech↗
arXiv2002

Heavy Quark Free Energies and Screening in SU(2) Gauge Theory

We investigate the singlet, triplet and colour average heavy quark free energies in SU(2) pure gauge theory at various temperatures T. We focus on the long distance behaviour of the free energies, studying in particular the temperature dependence of the string tension and the screening masses. The results are qualitatively similar to the SU(3) scenario, except near the critical temperature Tc of the deconfining transition. Finally we test a recently proposed method to renormalize the Polyakov loop.

hep-lat↗
arXiv2002

A Geometrical Interpretation of Hyperscaling Breaking in the Ising Model

In random percolation one finds that the mean field regime above the upper critical dimension can simply be explained through the coexistence of infinite percolating clusters at the critical point. Because of the mapping between percolation and critical behaviour in the Ising model, one might check whether the breakdown of hyperscaling in the Ising model can also be intepreted as due to an infinite multiplicity of percolating Fortuin-Kasteleyn clusters at the critical temperature T_c. Preliminary results suggest that the scenario is much more involved than expected due to the fact that the percolation variables behave differently on the two sides of T_c.

hep-lat↗
arXiv2002

Parton Percolation and J/Psi Suppression

The geometric clustering of partons in the transverse plane of nuclear collisions leads for increasing A or sqrt(s) to percolation. In the resulting condensate, the partons are deconfined but not yet in thermal equilibrium. We discuss quarkonium dissociation in this precursor of the quark-gluon plasma, with an onset of dissociation when the saturation scale of the parton condensate reaches that of the given quarkonium state.

hep-ph↗
arXiv2001

Cluster Percolation and First Order Phase Transitions in the Potts Model

The q-state Potts model can be formulated in geometric terms, with Fortuin-Kasteleyn (FK) clusters as fundamental objects. If the phase transition of the model is second order, it can be equivalently described as a percolation transition of FK clusters. In this work, we study the percolation structure when the model undergoes a first order phase transition. In particular, we investigate numerically the percolation behaviour along the line of first order phase transitions of the 3d 3-state Potts model in an external field and find that the percolation strength exhibits a discontinuity along the entire line. The endpoint is also a percolation point for the FK clusters, but the corresponding critical exponents are neither in the Ising nor in the random percolation universality class.

hep-ph↗
arXiv2001

Percolation and Magnetization for Generalized Continuous Spin Models

For the Ising model, the spin magnetization transition is equivalent to the percolation transition of Fortuin-Kasteleyn clusters; this result remains valid also for the conventional continuous spin Ising model. The investigation of more general continuous spin models may help to obtain a percolation formulation for the critical behaviour in SU(2) gauge theory. We therefore study a broad class of theories, introducing spin distribution functions, longer range interactions and self-interaction terms. The thermal behaviour of each model turns out to be in the Ising universality class. The corresponding percolation formulations are then obtained by extending the Fortuin-Kasteleyn cluster definition; in several cases they illustrate recent rigorous results.

hep-lat↗
arXiv2000

Percolation and Deconfinement in SU(2) Gauge Theory

Cluster percolation and second order thermal phase transitions show an amazing number of common features: power laws of the variables at criticality, scaling relations of the critical exponents and universality of the critical indices. Because of that, percolation theory seems to be an ideal framework to devise a geometrical picture of a second order phase transition; the leading characters of the phenomenon are ordered domains, whose size increases while approaching the threshold until they fuse into a spanning structure, so that the order from local becomes global. Such a geometrical picture is known to be successful in the Ising model. The clusters are site-bond clusters, i.e. they are built by joining nearest-neighbouring aligned spins with some temperature-dependent bond probability. In this work we extend this result to a wide variety of theories, from continuous Ising-like models to O(n) models. On the grounds of these results, we devise a percolation picture for the confining phase transition of SU(2) pure gauge theory, exploiting its analogies with the Ising model. The cluster definition is obtained by constructing suitable effective theories for SU(2), which admit an equivalent percolation formulation.

hep-lat↗
arXiv2000

Effective Z(2) Spin Models of Deconfinement and Percolation in SU(2) Gauge Theory

Effective theories are helpful tools to gain an intuitive insight into phenomena governed by complex laws. In this work we show by means of Monte Carlo simulations that Z(2) spin models with only spin-spin interactions approximate rather well the critical behavior of SU(2) gauge theory at finite temperature. These models are then used to formulate an effective description of Polyakov loop percolation, which is shown to reproduce the thermal critical behaviour of SU(2) gauge theory.

hep-lat↗
arXiv2000

Percolation and Critical Behaviour in SU(2) Gauge Theory

The paramagnetic-ferromagnetic transition in the Ising model can be described as percolation of suitably defined clusters. We have tried to extend such picture to the confinement-deconfinement transition of SU(2) pure gauge theory, which is in the same universality class of the Ising model. The cluster definition is derived by approximating SU(2) by means of Ising-like effective theories. The geometrical transition of such clusters turns out to describe successfully the thermal counterpart for two different lattice regularizations of (3+1)-d SU(2).

hep-lat↗
arXiv2000

Cluster Percolation in O(n) Spin Models

The spontaneous symmetry breaking in the Ising model can be equivalently described in terms of percolation of Wolff clusters. In O(n) spin models similar clusters can be built in a general way, and they are currently used to update these systems in Monte Carlo simulations. We show that for 3-dimensional O(2), O(3) and O(4) such clusters are indeed the physical `islands' of the systems, i.e., they percolate at the physical threshold and the percolation exponents are in the universality class of the corresponding model. For O(2) and O(3) the result is proven analytically, for O(4) we derived it by numerical simulations.

hep-lat↗
arXiv1999

Percolation and Deconfinement in SU(2) Gauge Theory

We show that deconfinement in SU(2) gauge theory can be described by the percolation of site-bond clusters of like-sign Polyakov loops. In particular, we find that in 2+1 dimensions the percolation variables coincide with those of the 2-dimensional Ising model.

hep-lat↗