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Alessandro Drago

Publications and source records attributed to Alessandro Drago.

At least 73 records · Page 4Linked to original sources
arXiv2007

Softening of the equation of state of matter at large densities and temperatures: chiral symmetry restoration vs. quark deconfinement

We discuss two models for describing the behavior of matter at large densities and intermediate temperatures. In both models a softening of the equation of state takes place due to the appearance of new degrees of freedom. The first is a hadronic model in which the softening is due to chiral symmetry restoration. In the second model the softening is associated with the formation of clusters of quarks in the mixed phase. We show that both models allow a significant softening but, in the first case the bulk modulus is mainly dependent on the density, while in the mixed phase model it also strongly depends on the temperature. We also show that the bulk modulus is not vanishing in the mixed phase due to the presence of two conserved charges, the baryon and the isospin one. Only in a small region of densities and temperatures the incompressibility becomes extremely small. Finally we compare our results with recent analysis of heavy ion collisions at intermediate energies.

hep-ph↗
arXiv2007

Quark matter in compact stars?

Ozel, in a recent reanalysis of EXO 0748-676 observational data (astro-ph/0605106), concluded that quark matter probably does not exist in the center of compact stars. We show that the data is actually consistent with the presence of quark matter in compact stars.

astro-ph↗
arXiv2007

Transition to Quark Matter and long Gamma Ray Bursts

The energy released by the inner engine of GRBs can originate from structural readjustments inside a compact star. In particular, the formation of deconfined quark matter can liberate enough energy to power the burst. We show that the burning of a neutron star into a quark star likely proceeds as a deflagration and not as a detonation. In that way no strong baryon contamination is produced near the surface of the star. It is tempting to associate the temporal structures observed in the light curves with specific processes taking place inside the compact star. The so-called quiescent times, during which no signal is emitted in the highest energy band, correspond to pauses during the processes of readjustment. If the quark (or hybrid) star formed after these transformations is strongly magnetized and rotates rapidly, a prolonged gamma emission can be produced, as proposed by Usov years ago. This can explain the quasi-plateau observed by Swift in several GRBs.

astro-ph↗
arXiv2006

Quiescent times in Gamma-Ray-Bursts: hints of a dormant inner engine

We perform a statistical analysis of the temporal structure of long Gamma-Ray-Bursts (GRBs). First we consider a sample of bursts in which a long quiescent time is present. Comparing the pre-quiescence with the post-quiescence emission we show that they display similar temporal structures, hardness ratios and emitted powers, but, on the average, the post-quiescence emission is roughly twice as long as the pre-quiescence emission. We then consider a sample of long and bright GRBs. We show that the duration of each emission period is compatible with the duration of an active period computed in various inner engine models. At the contrary, if the inner engine is assumed to be always active, i.e. also during the quiescent times, in several cases the total duration of the burst largely exceeds the theoretical durations. Our analysis therefore does not support the interpretation of long quiescent times in terms of stochastic modulation of a continuous wind. Instead the quiescent times can be interpreted as dormancy periods of the inner engine. Before and after a dormancy period the inner engine produces similar emissions.

astro-ph↗
arXiv2006

Burning of an hadronic star into a quark or a hybrid star

We study the hydrodynamical transition from an hadronic star into a quark or a hybrid star. We discuss the possible mode of burning, using a fully relativistic formalism and realistic Equations of State in which hyperons can be present. We take into account the possibility that quarks form a diquark condensate. We also discuss the formation of a mixed phase of hadrons and quarks, and we indicate which region of the star can rapidly convert in various possible scenarios. An estimate of the final temperature of the system is provided. We find that the conversion process always corresponds to a deflagration and never to a detonation. Hydrodynamical instabilities can develop on the front. We estimate the increase in the conversion's velocity due to the formation of wrinkles and we find that, although the increase is significant, it is not sufficient to transform the deflagration into a detonation in essentially all realistic scenarios. Concerning convection, it does not always develop. In particular the system does not develop convection if hyperons are not present in the initial phase and if the newly formed quark phase is made of ungapped (or weakly gapped) quarks. At the contrary, the process of conversion from ungapped quark matter to gapped quarks always allows the formation of a convective layer. Finally, we discuss possible astrophysical implications of our results.

astro-ph↗
arXiv2006

Quark Deconfinement inside Compact Stars and Gamma Ray Bursts Inner Engine

The temporal structure of Gamma Ray Bursts can be interpreted assuming as a inner engine a neutron star which undergoes a progressive compactification via production of strangeness (hyperons and kaons) and quarks. We will propose a tentative identification of various emission periods of the burst with specific structural changes of the star. Each of these modifications of the composition of the compact star takes place as a deflagration and not as a detonation, so the energy released in the transition goes mainly into heat and not into a mechanical wave. This is important in order to avoid an excessive baryonic contamination of the region surrounding the compact star. In this way a ultrarelativistic plasma of electron-positron pairs and of photons can be obtained, powering the Gamma Ray Burst.

astro-ph↗
arXiv2006

Polarized sea measurements at JPARC

Large double spin-asymmetries can be foreseen for Drell-Yan production in $p p$ scattering at JPARC energies. The sign of the asymmetries can be used to discriminate between different model calculations of sea quark distributions.

hep-ph↗
arXiv2005

Gamma Ray Bursts and the transition to Quark Matter in Compact Stars

We discuss a model for long Gamma-Ray-Bursts in which the central engine is associated with the conversion process of a metastable hadronic star into a star containing quark matter. We analyze the observational signatures of the model, i.e. the Supernova-GRB temporal connection and the existence of long quiescent times in the temporal structure of Gamma-Ray-Bursts.

astro-ph↗
arXiv2005

Time-reversal odd distribution functions in chiral models with vector mesons

The so-called time-reversal odd distribution functions are known to be non-vanishing in QCD due to the presence of the link operator in the definition of these quantities. I show that T-odd distributions can be non-vanishing also in chiral models, if vector mesons are introduced as dynamical gauge bosons of a hidden local symmetry. Moreover, since the flavor dependence of these distributions is different in chiral models respect to non chiral ones, the phenomenological analysis of experimental data will be able to distinguish between these two classes of models.

hep-ph↗
arXiv2003

Evaporation of the gluon condensate: a model for pure gauge SU(3)_c phase transition

We interpret lattice data for the equation of state of pure gauge $SU(3)_c$ by an evaporation model. At low temperatures gluons are frozen inside the gluon condensate, whose dynamics is described in terms of a dilaton lagrangian. Above the critical temperature quasi-free gluons evaporate from the condensate: a first order transition is obtained by minimizing the thermodynamical potential of the system. Within the model it is possible to reproduce lattice QCD results at finite temperature for thermodynamical quantities such as pressure and energy. The gluonic longitudinal mass can also be evaluated; it vanishes below the critical temperature, where it shows a discontinuity. At very large temperatures we recover the perturbative scenario and gluons are the only asymptotic degrees of freedom.

hep-ph↗
arXiv2001

From Quark Stars to Hybrid Stars

We show the possible existence of compact stars having a surface composed of a mixed phase of quarks and hadrons. This scenario can be realized both for self-bound stars, satisfying the so-called Witten-Bodmer hypothesis, and for gravitationally bound stars. This class of solutions of the Tolman-Oppenheimer-Volkoff equation can be obtained in all the models we discuss, within a physically acceptable range of values of the model parameters.

hep-ph↗
arXiv2000

Connection between Distribution and Fragmentation Functions

We show that the quark fragmentation function $D(z)$ and the quark distribution function $q(x)$ are connected in the $z \to 1$ limit by the approximate relation $D(z)/z \simeq q(2-1/z)$, where both quantities are in their physical regions. Predictions for proton production in inelastic $e^+ e^-$ annihilation, based on the new relation and standard parametrizations of quark distribution functions, are found to be compatible with the data.

hep-ph↗
arXiv1999

Gamma Ray Bursts via emission of axion-like particles

The Pseudo-Goldstone Boson (PGB) emission could provide a very efficient mechanism for explaining the cosmic Gamma Ray Bursts (GRBs). The PGBs could be produced during the merging of two compact objects like two neutron stars or neutron star - black hole, and then decay into electron-positron pairs and photons at distances ~ 100-1000 km. In this way, a huge energy (up to more than 10^{54} erg) can be deposited into the outer space with low baryon density in the form of ultrarelativistic e^+e^- plasma, the so called fireball, which originates the observed gamma-ray bursts. The needed ranges for the PGB parameters are: mass of order MeV, coupling to nucleons g_{aN} ~ few x 10^{-6} and to electrons g_{ae} ~ few x 10^{-9}. Interestingly enough, the range for coupling constants correspond to that of the invisible axion with the Peccei-Quinn symmetry breaking scale V ~ few x 10^5 GeV, but the mass of the PGB is many orders of magnitude larger than what such a scale would demand to an axion. Neither present experimental data nor astrophysical and cosmological arguments can exclude such an ultramassive axion, however the relevant parameters' window is within the reach of future experiments. Another exciting point is that our mechanism could explain the association of some GRBs with supernovae type Ib/c, as far as their progenitor stars have a radius ~ 10^4 km. And finally, it also could help the supernova type II explosion: PGBs emitted from the core of the collapsing star and decaying in the outer shells would deposit a kinetic energy of the order of 10^{51} erg. In this way, emission of such an axion-like particle could provide an unique theoretical base for understanding the gamma ray bursts and supernova explosions.

hep-ph↗
arXiv1999

Neutron Stars, Supernova Explosions and the Transition to Quark Matter

The transition to quark matter can take place in neutron stars. The structure of a hybrid star, containing a core made of quark matter is discussed. The maximum mass of the non-rotating hybrid star turns out to be 1.6 M_s. Possible signatures of the quark phase as pulsar's timing, cooling rate etc. are briefly analyzed. The deconfinement transition can also take place during the pre-supernova collapse. This possibility is studied by introducing a finite temperature EOS. The dependence of the latter on the proton fraction is shown to be crucial. The softening of the EOS at densities just above nuclear matter saturation density for Z/A = 0.3 helps in obtaining an explosion. At the same time, at larger densities the EOS is stiff enough to support a neutron star compatible with observations.

astro-ph↗
arXiv1998

The Polarized Nucleon in Quark Models

A brief overview of various problems related to the description of a polarized proton in quark models is presented. Structure functions are discussed both for longitudinal and transverse polarization. A recently introduced quantity, relevant in the study of single-spin asymmetries, is shown to be in principle non-vanishing when computed in chiral models.

hep-ph↗
arXiv1997

Recoil effects on nucleon electromagnetic form factors

The electromagnetic form factors are computed using eigenstates of linear momentum for the nucleon. The latter is described in the framework of the chiral color-dielectric model, projecting the hedgehog ansatz on eigenstates of angular momentum and isospin. Form factors are well reproduced, with the exception of the magnetic one for the proton, up to $q^2 \simeq 0.5$ GeV$^2$. The effect of the removal of the spurious center-of-mass contributions shows up mainly in the electric form factor of the proton. A noticeable improvement is obtained with respect to the calculation without linear momentum projection.

nucl-th↗
arXiv1997

Finite Temperature Quark Matter and Supernova Explosion

We study the equation of state of quark matter at finite temperature, using a confinement model in which chiral symmetry remains broken in the deconfined phase. Implications for type II supernova explosion and for the structure and evolution of the proto-neutron star are discussed.

astro-ph↗