Giuseppe Pagliara

Signals of the QCD phase transition in core-collapse supernovae

We explore the implications of the QCD phase transition during the postbounce evolution of core-collapse supernovae. Using the MIT bag model for the description of quark matter, we model phase transitions that occur during the early postbounce evolution. This stage of the evolution can be simulated with general relativistic three-flavor Boltzmann neutrino transport. The phase transition produces a second shock wave that triggers a delayed supernova explosion. If such a phase transition happens in a future galactic supernova, its existence and properties should become observable as a second peak in the neutrino signal that is accompanied by significant changes in the energy of the emitted neutrinos. This second neutrino burst is dominated by the emission of antineutrinos because the electron degeneracy is reduced when the second shock passes through the previously neutronized matter.

QUARK MATTER IN MASSIVE COMPACT STARS

The recent observation of the pulsar PSR J1614-2230 with a mass of 1.97 ± 0.04 M ☉ gives a strong constraint on the quark and nuclear matter equations of state (EoS). We explore the parameter ranges for a parameterized EoS for quark stars. We find that strange stars, made of absolutely stable strange quark matter, comply with the new constraint only if effects from the strong coupling constant and color-superconductivity are taken into account. Hybrid stars, compact stars with a quark matter core and a hadronic outer layer, can be as massive as 2 M ☉, but only for a significantly limited range of parameters. We demonstrate that the appearance of quark matter in massive stars crucially depends on the stiffness of the nuclear matter EoS. We show that the masses of hybrid stars stay below the ones of hadronic and pure quark stars, due to the softening of the EoS at the quark-hadron phase transition.

Astrophysics: Quark matter in compact stars?

Arising from: F. Özel. 441, 1115–1117 (2006)10.1038/nature04858; Özel repliesIn a theoretical interpretation of observational data from the neutron star EXO 0748–676, Özel concludes that quark matter probably does not exist in the centre of neutron stars. However, this conclusion is based on a limited set of possible equations of state for quark matter. Here we compare Özels observational limits with predictions based on a more comprehensive set of proposed quark-matter equations of state from the literature, and conclude that the presence of quark matter in EXO 0748–676 is not ruled out.

Core-collapse supernova explosions triggered by a quark-hadron phase transition during the early post-bounce phase

We explore explosions of massive stars, which are triggered via the quark-hadron phase transition during the early post-bounce phase of core-collapse supernovae. We construct a quark equation of state, based on the bag model for strange quark matter. The transition between the hadronic and the quark phases is constructed applying Gibbs conditions. The resulting quark-hadron hybrid equations of state are used in core-collapse supernova simulations, based on general relativistic radiation hydrodynamics and three-flavor Boltzmann neutrino transport in spherical symmetry. The formation of a mixed phase reduces the adiabatic index, which induces the gravitational collapse of the central protoneutron star (PNS). The collapse halts in the pure quark phase, where the adiabatic index increases. A strong accretion shock forms, which propagates toward the PNS surface. Due to the density decrease of several orders of magnitude, the accretion shock turns into a dynamic shock with matter outflow. This moment defines the onset of the explosion in supernova models that allow for a quark-hadron phase transition, where otherwise no explosions could be obtained. The shock propagation across the neutrinospheres releases a burst of neutrinos. This serves as a strong observable identification for the structural reconfiguration of the stellar core. The ejected matter expands on a short timescale and remains neutron-rich. These conditions might be suitable for the production of heavy elements via the r-process. The neutron-rich material is followed by proton-rich neutrino-driven ejecta in the later cooling phase of the PNS where the νp-process might occur.

Spectral functions of scalar mesons

In this work we study the spectral functions of scalar mesons in one- and two-channel cases by using nonlocal interaction Lagrangian(s). When the propagators satisfy the Kaellen-Lehman representation, a normalized spectral function is obtained, allowing one to take into account finite-width effects in the evaluation of decay rates. In the one-channel case, suitable to the light {sigma} and k mesons, the spectral function can deviate consistently from a Breit-Wigner shape. In the two-channel case with one subthreshold channel, the evaluated spectral function is well approximated by a Flatte distribution; when applying the study to the a{sub 0}(980) and f{sub 0}(980) mesons, the tree-level forbidden KK decay is analyzed.

Can very compact and very massive neutron stars both exist

The existence of neutron stars with masses of ∼2M⊙ requires a stiff equation of state at high densities. On the other hand, the necessary appearance also at high densities of new degrees of freedom, such as hyperons and Δ resonances, can lead to a strong softening of the equation of state with resulting maximum masses of ∼1.5M⊙ and radii smaller than ∼10 km. Hints for the existence of compact stellar objects with very small radii have been found in recent statistical analyses of quiescent low-mass X-ray binaries in globular clusters. We propose an interpretation of these two apparently contradicting measurements, large masses and small radii, in terms of two separate families of compact stars: hadronic stars, whose equation of state is soft, can be very compact, while quark stars, whose equation of state is stiff, can be very massive. In this respect an early appearance of Δ resonances is crucial to guarantee the stability of the branch of hadronic stars. Our proposal could be tested by measurements of radii with an error of ∼1 km, which is within reach of the planned Large Observatory for X-ray Timing satellite, and it would be further strengthened by the discovery of compact stars heavier than ∼2M⊙

EMMI rapid reaction task force meeting on quark matter in compact stars

The recent measurement of two solar mass pulsars has initiated an intense discussion on its impact on our understanding of the high-density matter in the cores of neutron stars. A task force meeting was held from October 7-10, 2013 at the Frankfurt Institute for Advanced Studies to address the presence of quark matter in these massive stars. During this meeting, the recent observational astrophysical data and heavy-ion data was reviewed. The possibility of pure quark stars, hybrid stars and the nature of the QCD phase transition were discussed and their observational signals delineated.

Nuclear matter within a dilatation-invariant parity doublet model: The role of the tetraquark at nonzero density

Abstract We investigate the role of a scalar tetraquark state for the description of nuclear matter within the parity doublet model in the mirror assignment. In the dilatation-invariant version of the model a nucleon–nucleon interaction term mediated by the lightest scalar tetraquark field naturally emerges. At nonzero density one has, beyond the usual chiral condensate, also a tetraquark condensate. The behavior of both condensates and the restoration of chiral symmetry at high density are studied. It is shown that this additional scalar degree of freedom affects non-negligibly the properties of the medium.

The scenario of two families of compact stars. Part 1. Equations of state, mass-radius relations and binary systems

Abstract.We present several arguments which favor the scenario of two coexisting families of compact stars: hadronic stars and quark stars. Besides the well-known hyperon puzzle of the physics of compact stars, a similar puzzle exists also when considering delta resonances. We show that these particles appear at densities close to twice saturation density and must be therefore included in the calculations of the hadronic equation of state. Such an early appearance is strictly related to the value of the L parameter of the symmetry energy that has been found, in recent phenomenological studies, to lie in the range

Stability of color-flavor-locking cores in hybrid stars

40 < L < 62