arXiv subjects

G. Lugones

Publications and source records attributed to G. Lugones.

At least 37 records · Page 2Linked to original sources

arXiv2018

Finite size effects in strongly interacting matter at zero chemical potential from Polyakov loop Nambu-Jona-Lasinio model in the light of lattice data

We study finite volume effects within the Polyakov loop Nambu-Jona-Lasinio model for two light and one heavy quarks at vanishing baryon chemical potential and finite temperatures. We include three different Polyakov loop potentials and ensure that the predictions of our effective model in bulk are compatible with lattice QCD results. Finite size effects are taken into account by means of the Multiple Reflection Expansion formalism. We analyze several thermodynamic quantities including the interaction measure, the speed of sound, the surface tension, and the curvature energy and find that they are sensitive to finite volume effects, specially for systems with radii below $\sim 10$ fm and temperatures around the crossover one. For all sizes, the system undergoes a smooth crossover. The chiral critical temperature decreases by around $5 \%$ and the deconfinement temperature by less than a $2 \%$ when the radius goes from infinity to 3 fm. Thus, as the drop's size decreases, both temperatures become closer. The surface tension is dominated by the contribution of strange quarks and the curvature energy by $u$ and $d$ quarks. At large temperatures both quantities grow proportionally to $T^{3/2}$ but saturate to a constant value at low $T$.

hep-ph↗

arXiv2018

Surface tension of quark droplets in compact stars and in the early universe

We review the role of the surface tension of quark matter droplets in astrophysical conditions, focusing specifically on the thermodynamic conditions prevailing in cold neutron stars (NSs), in hot lepton rich proto NSs, and in early universe conditions. We analyze quark matter in chemical equilibrium under weak interactions, which is relevant for understanding the internal composition of hybrid stars, as well as just deconfined quark matter out of chemical equilibrium, which is the relevant thermodynamic state for describing the nucleation process of quark matter in NSs. We explore the role of temperature, density, trapped neutrinos, droplet size and magnetic fields within the multiple reflection expansion formalism (MRE). Quark matter is described within the frame of different effective models: the MIT bag model and the $SU(3)_f$ Nambu-Jona-Lasinio model (NJL), including color superconductivity, neutrino trapping and magnetic fields. We also analyze the deconfinement transition at vanishing chemical potential and finite temperature including the Polyakov loop. We explore some astrophysical consequences of our results.

nucl-th↗

arXiv2018

Assessing the role of the Kelvin-Helmholtz instability at the QCD cosmological phase transition

We performed numerical simulations with the PLUTO code in order to analyze the non-linear behavior of the Kelvin-Helmholtz instability in non-magnetized relativistic fluids. The relevance of the instability at the cosmological QCD phase transition was explored using an equation of state based on lattice QCD results with the addition of leptons. The results of the simulations were compared with the theoretical predictions of the linearized theory. For small Mach numbers up to $M_s \sim 0.1$ we find that both results are in good agreement. However, for higher Mach numbers, non-linear effects are significant. In particular, many initial conditions that look stable according to the linear analysis are shown to be unstable according to the full calculation. Since according to lattice calculations the cosmological QCD transition is a smooth crossover, violent fluid motions are not expected. Thus, in order to assess the role of the Kelvin-Helmholtz instability at the QCD epoch, we focus on simulations with low shear velocity and use monochromatic as well as random perturbations to trigger the instability. We find that the Kelvin-Helmholtz instability can strongly amplify turbulence in the primordial plasma and as a consequence it may increase the amount of primordial gravitational radiation. Such turbulence may be relevant for the evolution of the Universe at later stages and may have an impact in the stochastic gravitational wave background.

astro-ph.CO↗

arXiv2017

Constraining color flavor locked strange stars in the gravitational wave era

We perform a detailed analysis of the fundamental mode of non-radial pulsations of color flavor locked strange stars. Solving the general relativistic equations for non-radial pulsations for an equation of state derived within the MIT bag model, we calculate the frequency and the gravitational damping time of the fundamental mode for all the parametrizations of the equation of state that lead to self-bound matter. Our results show that color flavor locked strange stars can emit gravitational radiation in the optimal range for present gravitational wave detectors and that it is possible to constrain the equation of state's parameters if the fundamental oscillation mode is observed and the stellar mass is determined. We also show that the $f$-mode frequency can be fitted as a function of the square root of the average stellar density $\sqrt{M/R^3}$ by a single linear relation that fits quite accurately the results for all parametrizations of the equation of state. All results for the damping time can also be fitted as a function of the compactness $M/R$ by a single empirical relation. Therefore, if a given compact object is identified as a color flavor locked strange star these two relations could be used to determine the mass and the radius from the knowledge of the frequency and the damping time of gravitational waves from the $f$ mode.

astro-ph.HE↗

arXiv2016

Surface tension of highly magnetized degenerate quark matter

We study the surface tension of highly magnetized three flavor quark matter within the formalism of multiple reflection expansion (MRE). Quark matter is described as a mixture of free Fermi gases composed by quarks $u$, $d$, $s$ and electrons, in chemical equilibrium under weak interactions. Due to the presence of strong magnetic fields the particles' transverse motion is quantized into Landau levels, and the surface tension has a different value in the parallel and transverse directions with respect to the magnetic field. We calculate the transverse and longitudinal surface tension for different values of the magnetic field and for quark matter drops with different sizes, from a few fm to the bulk limit. For baryon number densities between $2-10$ times the nuclear saturation density, the surface tension falls in the range $2 - 20$ MeV /fm$^{2}$. The largest contribution comes from strange quarks which have a surface tension an order of magnitude larger than the one for $u$ or $d$ quarks and more than two orders of magnitude larger than for electrons. Our results show that the total surface tension is quite insensitive to the size of the drop. We also find that the surface tensions in the transverse and parallel directions are almost unaffected by the magnetic field if $eB$ is below $\sim 5 \times 10^{-3} $ GeV$^2$. Nevertheless, for higher values of $eB$, there is a significant increase in the parallel surface tension and a significant decrease in the transverse one with respect to the unmagnetized case.

nucl-th↗

arXiv2016

Properties of strongly magnetized ultradense matter and their imprints on magnetar pulsations

We investigate the effect of strong magnetic fields on the adiabatic radial oscillations of hadronic stars. We describe magnetized hadronic matter within the framework of the relativistic nonlinear Walecka model and integrate the equations of relativistic radial oscillations to determine the fundamental pulsation mode. We consider that the magnetic field increases, in a density dependent way, from the surface, where it has a typical magnetar value of $10^{15}$ G, to the interior of the star where it can be as large as $3 \times 10^{18}$ G. We show that magnetic fields of the order of $10^{18}$ G at the stellar core produce a significant change in the frequency of neutron star pulsations with respect to unmagnetized objects. If radial pulsations are excited in magnetar flares, they can leave an imprint in the flare lightcurves and open a new window for the study of highly magnetized ultradense matter.

astro-ph.HE↗

arXiv2013

Surface tension and curvature energy of quark matter in the NJL model

In this paper we study the surface tension and the curvature energy of three-flavor quark matter in equilibrium under weak interactions within the Nambu-Jona-Lasinio model. We include the effect of color superconductivity and describe finite size effects within the multiple reflection expansion (MRE) framework. Our calculations result in large values of the surface tension which disfavor the formation of mixed phases at the hadron-quark inter-phase inside a hybrid star.

hep-ph↗

arXiv2013

Discriminating hadronic and quark stars through gravitational waves of fluid pulsation modes

We investigate non-radial oscillations of hadronic, hybrid and pure self-bound strange quark stars with maximum masses above the mass of the recently observed massive pulsars PSR J1614-2230 and PSR J0348-0432 with $M \approx 2 M_{\odot}$. For the hadronic equation of state we employ different parametrizations of a relativistic mean-field model and for quark matter we use the MIT bag model including the effect of strong interactions and color superconductivity. We find that the first pressure mode for strange quark stars has a very different shape than for hadronic and hybrid stars. For strange quarks stars the frequency of the p1 mode is larger than 6 kHz and diverge at small stellar masses, but for hadronic and hybrid stars it is in the range 4-6 kHz. This allows an observational identification of strange stars even if extra information such as the mass, the radius or the gravitational redshift of the object is unavailable or uncertain. Also, we find as in previous works that the frequency of the g-mode associated with the quark-hadron discontinuity in a hybrid star is in the range 0.4-1 kHz for all masses. Thus, compact objects emitting gravitational waves above 6 kHz should be interpreted as strange quark stars and those emitting a signal within 0.4-1 kHz should be interpreted as hybrid stars.

astro-ph.HE↗

arXiv2013

Unveiling the cosmological QCD phase transition through the eLISA/NGO detector

We study the evolution of turbulence in the early universe at the QCD epoch using a state-of-the-art equation of state derived from lattice QCD simulations. Since the transition is a crossover we assume that temperature and velocity fluctuations were generated by some event in the previous history of the Universe and survive until the QCD epoch due to the extremely large Reynolds number of the primordial fluid. The fluid at the QCD epoch is assumed to be non-viscous, based on the fact that the viscosity per entropy density of the quark gluon plasma obtained from heavy-ion collision experiments at the RHIC and the LHC is extremely small. Our hydrodynamic simulations show that the velocity spectrum is very different from the Kolmogorov power law considered in studies of primordial turbulence that focus on first order phase transitions. This is due to the fact that there is no continuous injection of energy in the system and the viscosity of the fluid is negligible. Thus, as kinetic energy cascades from the larger to the smaller scales, a large amount of kinetic energy is accumulated at the smallest scales due to the lack of dissipation. We have obtained the spectrum of the gravitational radiation emitted by the motion of the fluid finding that, if typical velocity and temperature fluctuations have an amplitude $(Δv) /c \gtrsim 10^{-2}$ and/or $ΔT/T_c \gtrsim 10^{-3}$, they would be detected by eLISA at frequencies larger than $\sim 10^{-4}$ Hz.

astro-ph.CO↗

arXiv2013

Nucleation rate of color superconducting droplets in protoneutron stars

We analyse the nucleation of quark matter droplets under protoneutron star conditions. We adopt a two-phase framework in which the hadronic phase is described through a non-linear Walecka model and the just deconfined matter by the MIT bag model including color superconductivity. Surface tension and curvature energy are calculated self-consistently within the MRE formalism. We impose flavour conservation during the transition, which means that the just deconfined quark droplet is transiently out of equilibrium with respect to weak interactions. Our results show that trapped neutrinos slightly increase the critical density for deconfinement and that color superconductivity significantly decreases such density at low temperatures. We also show that the nucleation rate is negligible for droplets larger than 100-200 fm and is huge for smaller droplets provided that the temperature is low enough. We compare our results with previous calculations using the Nambu-Jona-Lasinio model with color superconductivity and the MIT bag model without color superconductivity. We conclude that the deconfinement transition should be triggered instantaneously when a density slightly larger than the bulk transition density is reached at some layer of a protoneutron star. Since color superconductivity lowers the transition density at low temperatures, the transition is likely to occur after significant cooling in a massive enough protoneutron star.

hep-ph↗

arXiv2012

Hybrid stars in the light of the massive pulsar PSR J1614-2230

We perform a systematic study of hybrid star configurations using several parametrizations of a relativistic mean-field hadronic EoS and the NJL model for three-flavor quark matter. For the hadronic phase we use the stiff GM1 and TM1 parametrizations, as well as the very stiff NL3 model. In the NJL Lagrangian we include scalar, vector and 't Hooft interactions. The vector coupling constant $g_v$ is treated as a free parameter. We also consider that there is a split between the deconfinement and the chiral phase transitions which is controlled by changing the conventional value of the vacuum pressure $- Ω_0$ in the NJL thermodynamic potential by $- (Ω_0 + δΩ_0)$, being $δΩ_0$ a free parameter. We find that, as we increase the value of $δΩ_0$, hybrid stars have a larger maximum mass but are less stable, i.e. hybrid configurations are stable within a smaller range of central densities. For large enough $δΩ_0$, stable hybrid configurations are not possible at all. The effect of increasing the coupling constant $g_v$ is very similar. We show that stable hybrid configurations with a maximum mass larger than the observed mass of the pulsar PSR J1614-2230 are possible for a large region of the parameter space of $g_v$ and $δΩ_0$ provided the hadronic equation of state contains nucleons only. When the baryon octet is included in the hadronic phase, only a very small region of the parameter space allows to explain the mass of PSR J1614-2230. We compare our results with previous calculations of hybrid stars within the NJL model. We show that it is possible to obtain stable hybrid configurations also in the case $δΩ_0=0$ that corresponds to the conventional NJL model for which the pressure and density vanish at zero temperature and chemical potential.

astro-ph.SR↗

arXiv2011

Critical spectrum of fluctuations for deconfinement at proto-neutron star cores

We study the deconfinement of hadronic matter into quark matter in a protoneutron star focusing on the effects of the finite size on the formation of just-deconfined color superconducting quark droplets embedded in the hadronic environment. The hadronic phase is modeled by the non-linear Walecka model at finite temperature including the baryon octet and neutrino trapping. For quark matter we use an $SU(3)_f$ Nambu-Jona-Lasinio model including color superconductivity. The finite size effects on the just deconfined droplets are considered in the frame of the multiple reflection expansion. In addition, we consider that just deconfined quark matter is transitorily out of equilibrium respect to weak interaction, and we impose color neutrality and flavor conservation during the transition. We calculate self-consistently the surface tension and curvature energy density of the quark hadron inter-phase and find that it is larger than the values typically assumed in the literature. The transition density is calculated for drops of different sizes, and at different temperatures and neutrino trapping conditions. Then, we show that energy-density fluctuations are much more relevant for deconfinement than temperature and neutrino density fluctuations. We calculate the critical size spectrum of energy-density fluctuations that allows deconfinement as well as the nucleation rate of each critical bubble. We find that drops with any radii smaller than 800 fm can be formed at a huge rate when matter achieves the bulk transition limit of 5-6 times the nuclear saturation density.

astro-ph.SR↗

arXiv2010

Radial oscillations of color superconducting self-bound quark stars

We investigate the effect of the color-flavor locking pairing pattern on the adiabatic radial oscillations of pure self-bound quark stars using an equation of state in the framework of the MIT Bag model. We integrate the equations of relativistic radial oscillations to determine the fundamental and the first excited oscillation modes for several parameterizations of the equation of state. For low mass stars we find that the period of the fundamental mode is typically $\sim 0.1$ ms and has a small dependence on the parameters of the equation of state. For large mass stars the effect of color-flavor locking is related to the rise of the maximum mass with increasing $Δ$. As for unpaired quark stars, the period of the fundamental mode becomes divergent at the maximum mass but now the divergence is shifted to large masses for large values of the pairing gap $Δ$. As a consequence, the oscillation period is strongly affected by color superconductivity for stars with $M \gtrsim 1.5 \; \textrm{M}_{\odot}$. We fit the period of the fundamental mode with appropriate analytical functions of the gravitational redshift of the star and the pairing gap $Δ$. We further discuss the excitation and damping of the modes and their potential detectability during violent transient phenomena.

astro-ph.HE↗

arXiv2010

Deconfinement transition in protoneutron stars: analysis within the Nambu-Jona-Lasinio model

We study the effect of color superconductivity and neutrino trapping on the deconfinement transition of hadronic matter into quark matter in a protoneutron star. To describe the strongly interacting matter a two-phase picture is adopted. For the hadronic phase we use different parameterizations of a non-linear Walecka model which includes the whole baryon octet. For the quark matter phase we use an $SU(3)_f$ Nambu-Jona-Lasinio effective model which includes color superconductivity. We impose color and flavor conservation during the transition in such a way that just deconfined quark matter is transitorily out of equilibrium with respect to weak interactions. We find that deconfinement is more difficult for small neutrino content and it is easier for lower temperatures although these effects are not too large. In addition they will tend to cancel each other as the protoneutron star cools and deleptonizes, resulting a transition density that is roughly constant along the evolution of the protoneutron star. According to these results the deconfinement transition is favored after substantial cooling and contraction of the protoneutron star.

nucl-th↗

arXiv2009

Deconfinement of neutron star matter within the Nambu-Jona-Lasinio model

We study the deconfinement transition of hadronic matter into quark matter under neutron star conditions assuming color and flavor conservation during the transition. We use a two-phase description. For the hadronic phase we use different parameterizations of a non-linear Walecka model which includes the whole baryon octet. For the quark matter phase we use an SU(3)_f Nambu-Jona-Lasinio effective model including color superconductivity. Deconfinement is considered to be a first order phase transition that conserves color and flavor. It gives a short-lived transitory colorless-quark-phase that is not in beta-equilibrium, and decays to a stable configuration in tau ~ tau_{weak} ~ 10^{-8} s. However, in spite of being very short lived, the transition to this intermediate phase determines the onset of the transition inside neutron stars. We find the transition free-energy density for temperatures typical of neutron star interiors. We also find the critical mass above which compact stars should contain a quark core and below which they are safe with respect to a sudden transition to quark matter. Rather independently on the stiffness of the hadronic equation of state (EOS) we find that the critical mass of hadronic stars (without trapped neutrinos) is in the range of ~ 1.5 - 1.8 solar masses. This is in coincidence with previous results obtained within the MIT Bag model.

astro-ph.HE↗

arXiv2009

Probing Light Pseudoscalar Particles Using Synchrotron Light

The need for purely laboratory-based light pseudoscalar particles searches has been emphasized many times in the literature, since astrophysical bounds on these particles rely on several assumptions to calculate the flux produced in stellar plasmas. In this paper we study the use of light from synchrotron accelerators as a source for a photon regeneration experiment also know as "light shining through a wall". Such an experiment can significantly improve present limits on the pseudoscalar particle mass and the pseudoscalar-photon coupling constant obtained from laser experiments. This is possible even using a small number of powerful magnets (B = 10 T), due to the large incident photon flux. On the other hand, the use of a broadband incident photon-beam instead of infrared or optical lasers allows a significant improvement in the mass reach of the experiment (it is possible to test masses up to 0.01 eV without a drop in sensitivity). Large, but still feasible, configurations can explore in a quite model-independent way a large part of the parameter space examined by solar searches and HB stars in globular clusters. Additionally, the proposal may be useful for testing string motivated effective theories containing light and weakly interacting particles.

hep-ph↗

arXiv2007

A model for giant flares in soft gamma repeaters

We argue that giant flares in SGRs can be associated to the core conversion of an isolated neutron star having a subcritical magnetic field $\sim 10^{12}$ G and a fallback disk around it. We show that, in a timescale of $\lesssim 10^5$ yrs, accretion from the fallback disk can increase the mass of the central object up to the critical mass for the conversion of the core of the star into quark matter. A small fraction of the neutrino-antineutrino emission from the just-converted quark-matter hot core annihilates into $e^+e^-$ pairs above the neutron star surface originating the gamma emission of the spike while the further cooling of the heated neutron star envelope originates the tail of the burst. We show that several characteristics of the giant flare of the SGR 1806-20 of 27 December 2004 (spike and tail energies, timescales, and spectra) can be explained by this mechanism.

astro-ph↗

arXiv2006

Effects of color superconductivity on the nucleation of quark matter in neutron stars

We study the nucleation of quark matter drops at the center of cold deleptonized neutron stars. This is relevant in the determination of the critical mass $M_{cr}$ of hadronic stars above which it is possible a transition to a quark star (strange or hybrid). We investigate the dependence of $M_{cr}$ upon the parameters of the quark model (the Bag constant $B$, the pairing gap $Δ$, and the surface tension $σ$ of the quark-hadron interphase) and for different parametrization of the hadronic equations of state. The dependence of $M_{cr}$ on $B$, $Δ$ and $σ$ is mild if the parameters of the quark model correspond to hybrid stars, and strong if they correspond to strange stars. For a large part of the parameter space corresponding to hybrid stars, the critical mass is very close (but smaller than) the maximum mass of hadronic stars, and therefore compatible with a "mixed" population of compact stars (pure hadronic up to the critical mass and hybrid above the critical mass). For very large $B$ the critical mass is never smaller than the maximum mass of hadronic stars, implying that quark stars cannot form through the here studied mechanism. The energy released in the conversion is $3 \times 10^{52}$ erg - $4 \times 10^{53}$ erg, i.e. sufficient to power a gamma ray burst.

astro-ph↗