arXiv subjects

Giuseppe Pagliara

Publications and source records attributed to Giuseppe Pagliara.

At least 19 recordsLinked to original sources

arXiv2026

Coexistence of strange quark stars and neutron stars: metastability and nucleation in proto-neutron stars

If strange quark matter (SQM) is absolutely stable, hadronic neutron stars (NSs) and strange quark stars (QSs) may coexist in the so-called two-families scenario. A key issue is explaining how hadronic NSs can survive as long-lived metastable objects, rather than promptly converting into stable QSs. Since all NSs are born as hot proto-neutron stars (PNSs), a necessary condition for their existence is that the conditions for conversion, triggered by the nucleation of the first critical SQM droplet, are not reached in every PNS. We investigate the thermal nucleation of critical SQM droplets under representative PNS conditions, accounting for flavor-composition fluctuations and finite-size effects on color superconductivity by suppressing pairing in droplets smaller than the diquark coherence length. We define the nucleation conditions as the thermodynamic state at which a critical SQM droplet is expected to nucleate statistically within the characteristic dynamical timescale of the system. Under these conditions, the reduced nucleation barrier is nearly universal, with $W_*/T\simeq167$ to within a few units. Exploring the SQM parameter space, we show that requiring a canonical $1.4~\rm{M}_{\odot}$ NS to survive the PNS stage sets a lower bound on the hadron-quark surface tension $σ\gtrsim 67$ MeVfm$^{-2}$.

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arXiv2026

Is the coexistence of strange quark stars and hadronic stars favored by astrophysical data? A Bayesian analysis

Hadronic stars and strange quark stars could coexist within the so-called two-families scenario. In this respect, hadronic matter and strange quark matter correspond to two distinct equilibrium phases described by two different equations of state. We perform here the first detailed Bayesian analysis that makes use of astrophysical and laboratory data in order to constrain the equations of state adopted within the two-families scenario for hadronic and strange quark matter. In particular, in hadronic matter we consider the possible formation of hyperons and delta resonances (beside nucleons) within a class of non linear relativistic mean field models and in quark matter we consider the possible formation of a color-superconducting phase within a bag-like model. Results of the analysis indicate that while at the moment both the one-family and the two-families scenarios are compatible with the data, by comparing the Bayes factors of both models, the two-families scenario is favored with respect to the one-family scenario. Specifically, the two-families framework naturally relieves the tension between the intermediate-density softness of the equation of state required by small-radius objects and the high-density stiffness needed to support massive pulsars. Ultimately, future detections of even more massive compact objects, very compact ordinary-mass objects, or precise measurements of two distinct masses with the same radius, will provide strong indications in favor of the two-families scenario.

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arXiv2025

Role of thermal fluctuations in nucleation of three-flavor quark matter

We present a framework that aims to investigate the role of thermal fluctuations of the matter composition and color-superconductivity in the nucleation of three-flavor deconfined quark matter in the typical conditions of high-energy astrophysical systems related to compact stars. It is usually assumed that the flavor composition is locally fixed during the formation of the first seed of deconfined quark matter since weak interaction acts too slowly to re-equilibrate flavors. However, the matter composition fluctuates around its average equilibrium values at the typical temperatures of high-energy astrophysical processes. Here, we extend our previous two-flavor nucleation formalism to a three-flavor case. We develop a thermodynamic framework incorporating finite-size effects and thermal fluctuations of local composition to compute the nucleation probability as the product of droplet formation and composition fluctuation rates. Moreover, we discuss the role of color-superconductivity in nucleation, arguing that it can play a role only in systems larger than the typical coherence length of diquark pairs. We found that thermal fluctuations of the matter composition lead to lowering the potential barrier between the metastable hadronic phase and the stable quark phase. Moreover, the formation of diquark pairs reduces the critical radius and thus the potential barrier in the low baryon density and temperature regime.

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arXiv2025

Relativistic mean-field predictions for dense matter equation of state and application to neutron stars

Relativistic mean-field models (RMF) based on the exchange of $σ$, $ω$, and $ρ$ mesons including non-linear nucleon-$σ$ couplings and density-dependent $ρ$ coupling, are considered. A large set of models is generated using the Markov chain Monte Carlo approach and Bayesian statistics to reproduce nuclear physics knowledge encoded in terms of the nuclear empirical parameters and $χ$EFT predictions for low-density neutron matter. These models are filtered, in a second step, using astrophysical constraints: the tidal deformability obtained from GW170817 parameter estimation and the observational masses deduced from radio-astronomy. We then obtain a set of selected RMF models that are compatible with present nuclear and astrophysical constraints and that can be employed to make predictions and to quantity their uncertainties. Predictions for masses and radii are compared to NICER masses-radii analyses for PSR J0030+0451 and PSR J0740+6620. We find that RMF models can be made soft enough to predict low values for neutron star radii compatible with GW170817 and, at larger densities, stiff enough to be compatible with NICER analyses for massive neutron stars. Our models can also reach large values for the maximum mass, up to 2.6$M_\odot$. In addition, for the core composition, we obtain a large distribution of the proton fraction for canonical mass neutron stars, some of them allowing the direct URCA fast cooling process. For massive neutron stars, however, most of our models suggest a large proton fraction in the core allowing direct URCA fast cooling process.

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arXiv2024

Thermal fluctuations of matter composition and quark nucleation in compact stars

At the extreme densities reached in the core of neutron stars, it is possible that quark deconfined matter is produced. The formation of this new phase of strongly interacting matter is likely to occur via a first-order phase transition for the typical temperatures reached in astrophysical processes. The first seeds of quark matter would then form through a process of nucleation within the metastable hadronic phase. Here we address the role of the thermal fluctuations in the hadronic composition on the nucleation of two-flavour quark matter. At finite temperature, thermodynamic quantities in a system fluctuate around average values. Being nucleation a local process, it is possible that it occurs in a subsystem whose composition makes the nucleation easier. We will consider the total probability of the nucleation as the product between the probability that a subsystem has a certain hadronic composition different from the average in the bulk, and the nucleation probability in that subsystem. We will show how those fluctuations of the hadronic composition can increase the efficiency of nucleation already for temperatures $\sim (0.1-1)$ keV. However, for temperatures $\lesssim (1-10)$ MeV, the needed overpressure exceeds the maximum pressure reached in compact stars. Finally, for even larger temperatures the process of nucleation can take place, even taking into account finite size effects.

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arXiv2024

Strange Dwarfs: a review on the (in)stability

White dwarfs are the remnants of stars not massive enough to become supernovae. This review explores the concept of strange dwarfs, a unique class of white dwarfs which contain cores of strange quark matter. Strange dwarfs have different sizes, masses, and evolutionary paths with respect to white dwarfs. They might form through the accumulation of normal matter on strange quark stars or by capture of strangelets. The stability of strange dwarfs has been debated, with initial studies suggesting stability, while later analyses indicated potential instability. This review revisits these discussions, focusing on the critical role of boundary conditions between nuclear and quark matter in determining stability. It also offers insights into their formation, structure, and possible detection in the universe.

astro-ph.SR↗

arXiv2024

Is the compact object associated with HESS J1731-347 a strange quark star?

The analysis of the central compact object within the supernova remnant HESS J1731-347 suggests that it has a small radius and, even more interestingly, a mass of the order or smaller than one solar mass. This raises the question of which astrophysical process could lead to such a small mass, since the analysis of various types of SN explosions indicate that is it not possible to produce a neutron star with a mass smaller than about $1.17 M_\odot$. Here we show that masses of the order or smaller than one solar mass can be obtained in the case of strange quark stars and that it is possible to build a coherent model explaining not only the mass and the radius of that object, but also its slow cooling suggested in various analyses. We also show that an astrophysical path exists which leads to the formation of such an object, and we discuss the role played in that scenario by strangelets assumed to constitute the dark matter.

astro-ph.HE↗

arXiv2023

Stability and instability of strange dwarfs

More than 20 years ago, Glendenning, Kettner and Weber proposed the existence of stable white dwarfs with a core of strange quark matter. More recently, by studying radial modes, Alford, Harris and Sachdeva concluded instead that those objects are unstable. We aim to clarify this issue. We investigate the stability of these objects by looking at their radial oscillations while incorporating boundary conditions at the quark-hadron interface, corresponding to either a rapid or a slow conversion of hadrons into quarks. Our analysis shows that objects of this type are stable if the star is not strongly perturbed, and ordinary matter cannot transform into strange quark matter because of the Coulomb barrier separating the two components. On the other hand, ordinary matter can be transformed into strange quark matter if the star undergoes a violent process, as in the preliminary stages of a type Ia supernova, and this causes the system to become unstable and to collapse into a strange quark star. In this way, accretion-induced collapse of strange dwarfs can be facilitated, and km-sized objects with subsolar masses can be produced.

astro-ph.SR↗

arXiv2022

Formation and evaporation of strangelets during the merger of two compact stars

We study the partial fragmentation of a strange quark star into strangelets during the process of merger of two strange quark stars. We discuss the fate of the fragments considering their possible evaporation into nucleons. We show that only a rather small amount of large size strangelets, ejected from the spiral arms in the post-merger, survives a total evaporation into nucleons. In this way we demonstrate that: 1) the density of strangelets in the galaxy is too low to trigger the conversion of all neutron stars into strange quark stars and it allows the co-existence of both types of compact objects; 2) the probability of direct detection of a strangelet is negligible and therefore its nondetection is compatible with the strange quark matter hypothesis; 3) most of the matter ejected during and after the merger of two strange quark stars evaporates into nucleons and therefore it can generate a kilonova-like signal.

astro-ph.HE↗

arXiv2022

Merger of a Neutron Star with a Black Hole: one-family vs. two-families scenario

A kilonova signal is generally expected after a Black Hole - Neutron Star merger. The strength of the signal is related to the equation of state of neutron star matter and it increases with the stiffness of the latter. The recent results obtained by NICER from the analyses of PSR J0740+6620 suggest a rather stiff equation of state and the expected kilonova signal is therefore strong, at least if the mass of the Black Hole does not exceed $\sim 10 \mathrm{\,M}_\odot$ and if the adimensional spin parameter is not too small and the orbit is prograde. We compare the predictions obtained by considering equations of state of neutron star matter satisfying the most recent observations and assuming that only one family of compact stars exists with the results predicted in the two-families scenario. In the latter a soft hadronic equation of state produces very compact stellar objects while a rather stiff quark matter equation of state produces massive strange quark stars, satisfying NICER results. The expected kilonova signal in the two-families scenario is very weak: in particular the Hadronic Star - Black Hole merger produces a much weaker signal than in the one-family scenario because the hadronic equation of state is very soft. Moreover, according to the only existing simulation, the Strange Quark Star - Black Hole merger does not produce a kilonova signal because the amount of mass ejected is negligible. These predictions will be easily tested with the new generation of detectors if Black Holes with an adimensional spin parameter $χ_\mathrm{BH}\gtrsim 0.2$ or a mass $\mathrm M_\mathrm{BH} \lesssim 4 \mathrm M_\odot$ can be present in the merger.

astro-ph.HE↗

arXiv2021

Leggett-Garg inequalities and decays of unstable systems

We apply the Leggett-Garg inequalities (LGI) to the case of classical and quantum unstable systems. For classical systems the two assumptions of macroscopic realism and non-invasive measurements imply that the three-measurement string $K_3$ is identically equal to one. Also for quantum mechanical systems -- for which the two assumptions are in general not valid -- we find that $K_3=1$ for purely exponential decays ($K_3\leq1$ is the general LGI). On the other hand, the necessary deviations from the exponential decay law at short and long times predicted by quantum mechanics lead to values of $K_3 \neq 1$. Moreover, a strict violations $K_3>1$ occurs typically at short times. Thus, we conclude that experiments in which such deviations from the exponential decay law have been observed should also find in their data violations of the LGIs.

quant-ph↗

arXiv2021

Speed of sound in dense matter and two families of compact stars

The existence of massive compact stars $(M\gtrsim 2.1 M_{\odot})$ implies that the conformal limit of the speed of sound $c_s^2=1/3$ is violated if those stars have a crust of ordinary nuclear matter. Here we show that, if the most massive objects are strange quark stars, i.e. stars entirely composed of quarks, the conformal limit can be respected while observational limits on those objects are also satisfied. By using astrophysical data associated with those massive stars, derived from electromagnetic and gravitational wave signals, we show, within a Bayesian analysis framework and by adopting a constant speed of sound equation of state, that the posterior distribution of $c_s^2$ is peaked around 0.3, and the maximum mass of the most probable equation of state is $\sim 2.13 M_{\odot}$. We discuss which new data would require a violation of the conformal limit even when considering strange quark stars, in particular we analyze the possibility that the maximum mass of compact stars is larger than $2.5M_{\odot}$, as it would be if the secondary component of GW190814 is a compact star and not a black hole. Finally, we discuss how the new data for PSR J0740+6620 obtained by the NICER collaboration compare with our analysis (not based on them) and with other possible interpretations.

astro-ph.HE↗

arXiv2020

Why can hadronic stars convert into strange quark stars with larger radii

The total binding energy of compact stars is the sum of the gravitational binding energy $(BE)_g$ and the nuclear binding energy $(BE)_n$, the last being related to the microphysics of the interactions. While the first is positive (binding) both for hadronic stars and for strange quark stars, the second is large and negative for hadronic stars (anti-binding) and either small and negative (anti-binding) or positive (binding) for strange quark stars. A hadronic star can convert into a strange quark star with a larger radius because the consequent reduction of $(BE)_g$ is over-compensated by the large increase in $(BE)_n$. Thus, the total binding energy increases due to the conversion and the process is exothermic. Depending on the equations of state of hadronic matter and quark matter and on the baryonic mass of the star, the contrary is obviously also possible, namely the conversion of hadronic stars into strange quark stars having smaller radii, a situation more often discussed in the literature. We provide a condition that is sufficient and in most of the phenomenologically relevant cases also necessary in order to form strange quark stars with larger radii while satisfying the exothermicity request. Finally, we compare the two schemes in which quark stars are produced (one having large quark stars and the other having small quark stars) among themselves and with the third-family scenario and we discuss how present and future data can discriminate among them.

nucl-th↗

arXiv2020

Bayesian Inference of Dense Matter Equation of State within Relativistic Mean Field Models using Astrophysical Measurements

We present a Bayesian analysis to constrain the equation of state of dense nucleonic matter by exploiting the available data from symmetric nuclear matter at saturation and from observations of compact X-ray sources and from the gravitational wave event GW170817. For the first time, such analysis is performed by using a class of models, the relativistic mean field models, which allow to consistently construct an equation of state in a wide range of densities, isospin asymmetries and temperatures. The selected class of models contains five nuclear physics empirical parameters at saturation for which we construct the joint posterior distributions. By exploring different types of priors, we find that the equations of state with the largest evidence are the ones featuring a strong reduction of the effective mass of the nucleons in dense matter which can be interpreted as an indication of a phase transition to a chiral symmetry restored phase. Those equations of state in turn predict $R_{1.4} \sim 12$ km. Finally, we present a preliminary investigation on the effect of including $Λ$ hyperons showing that they appear in stars more massive than about $1.6 M_{\odot}$ and lead to radii larger than about $R_{1.4} \sim 14$ km. Within the model here explored, the formation of such particles provide a poor agreement with the constraints from GW170817.

astro-ph.HE↗

arXiv2020

Measurement of the neutron lifetime and inverse quantum Zeno effect

Quantum mechanics predicts that the decay rate of unstable systems could be effectively modified by the process of the measurement of the survival probability. Depending on the intrinsic properties of the unstable system and the experimental setup for the observation, one could obtain the so called (direct) quantum Zeno and inverse quantum Zeno effects corresponding to a slowing down or a speeding up of the decay, respectively. We argue that the inverse quantum Zeno effect is in principle detectable at a percent level for the neutron decay in experiments with trapped ultracold neutrons. Conversely, this effect is basically undetectable in experiments in which the neutron lifetime is measured by measuring the decays of beams of neutrons. As a test of our claim, we propose a simple qualitative correlation between the number of neutrons in the trap and the neutron lifetime: the larger the number, the faster the decay. Finally we discuss also the presently available measurements of the neutron lifetime and address the issue of the possible discrepancy that has been reported among the results obtained by the different experimental techniques.

hep-ph↗

arXiv2019

How to Test the Two-Families Scenario

We shortly summarize the two-families scenario in which both hadronic stars and strange quark stars can exist and we describe the main predictions one can obtain from it. We then concentrate on the observables that most likely will be measured in the near future, i.e. masses, radii, tidal deformabilities and moments of inertia and we present a list of objects that are candidate strange quark stars in this scheme. We show that the estimates of the radii derived up to now from observations are all compatible with the two-families scenario and in particular all the objects having large radii can easily be interpreted as strange quark stars.

astro-ph.HE↗

arXiv2019

Merger of compact stars in the two-families scenario

We analyse the phenomenological implications of the two-families scenario on the merger of compact stars. That scenario is based on the coexistence of both hadronic stars and strange quark stars. After discussing the classification of the possible mergers, we turn to detailed numerical simulations of the merger of two hadronic stars, i.e., "first family" stars in which delta resonances and hyperons are present, and we show results for the threshold mass of such binaries, for the mass dynamically ejected and the mass of the disk surrounding the post-merger object. We compare these results with those obtained within the one-family scenario and we conclude that relevant signatures of the two-families scenario can be suggested, in particular: the possibility of a rapid collapse to a black hole for masses even smaller than the ones associated to GW170817; during the first milliseconds, oscillations of the postmerger remnant at frequencies higher than the ones obtained in the one-family scenario; a large value of the mass dynamically ejected and a small mass of the disk, for binaries of low total mass. Finally, based on a population synthesis analysis, we present estimates of the number of mergers for: two hadronic stars; hadronic star - strange quark star; two strange quark stars. We show that for unequal mass systems and intermediate values of the total mass, the merger of a hadronic star and a strange quark star is very likely (GW170817 has a possible interpretation into this category of mergers). On the other hand, mergers of two strange quark stars are strongly suppressed.

astro-ph.HE↗

arXiv2018

Wheeler-DeWitt Universe Wave Function in the presence of stiff matter

We study the Wheeler-DeWitt (WDW) equation close to the Big-Bang. We argue that an interaction dominated fluid (speed of sound equal to the speed of light), if present, would dominate during such an early phase. Such a fluid with $p=ρ\propto1/a^{6}$ generates a term in the potential of the wave function of the WDW equation proportional to $-1/a^{2}.\ $This very peculiar quantum potential, which embodies a spontaneous breaking of dilatation invariance, has some very remarkable consequences for the wave function of the Universe: $Ψ(a)$ vanishes at the Big-Bang: $Ψ(0)=0$; the wave function $Ψ(a)$ is always real; a superselection rule assures that the system is confined to $a\geq0$ without the need of imposing any additional artificial barrier for unphysical negative $a$. These results do not depend on the operator-ordering problem of the WDW equation.

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