Hovik Grigorian — Page 2

arXiv2006

Parametrization of a nonlocal, chiral quark model in the instantaneous three-flavor case. Basic formulas and tables

In these notes we describe the basic formulation of the parametrization scheme for the instantaneous nonlocal chiral quark model in the three flavor case. We choose to discuss the Gaussian, Lorentzian-type, Woods-Saxon and sharp cutoff (NJL) functional forms of the momentum dependence for the formfactor of the separable interaction. The four parameters: light and strange quark mass, coupling strength (G_S) and range of the interaction (Lambda) have been fixed by the same phenomenological inputs: pion and kaon mass, pion decay constant and light quark mass in vacuum. The Wood-Saxon and Lorentzian-type formfactor is suitable for an interpolation between sharp cutoff and soft momentum dependence. Results are tabulated for applications in models of hadron structure and quark matter at finite temperatures and chemical potentials where separable models have proven successful.

hep-ph↗

arXiv2005

Cooling of Neutron Stars with Color Superconducting Quark Cores

We show that within a recently developed nonlocal chiral quark model the critical density for a phase transition to color superconducting quark matter under neutron star conditions can be low enough for these phases to occur in compact star configurations with masses below 1.3 M_sun. We study the cooling of these objects in isolation for different values of the gravitational mass and argue that, if the quark matter phase would allow unpaired quarks, the corresponding hybrid stars would cool too fast. The comparison with observational data puts tight constraints on possible color superconducting quark matter phases. Possible candidates with diquark gaps of the order of 10 keV - 1 MeV such as the "2SC+X" and the color spin locking (CSL) phase are presented.

hep-ph↗

arXiv2005

Hot Neutron and Quark Star Evolution

The physics of compact objects is one of the very actively developing branches of theoretical investigations, since the careful analysis of different models for the internal structure of these objects, their evolutionary characteristics and the comparison with modern observational data give access to discriminate among various speculations about the state of matter under extreme conditions. The lecture provides an overview to the problem of neutron star cooling evolution. We discuss the scheme and necessary inputs for neutron star cooling simulations from the background of a microscopic modeling in order to present the surface temperature - age characteristics of hybrid neutron stars including the determination of main regulators of the cooling process, the question of neutrino production and diffusion.

astro-ph↗

arXiv2004

Cooling of Neutron Stars with Color Superconducting Quark Cores

We show that within a recently developed nonlocal chiral quark model the critical density for a phase transition to color superconducting quark matter under neutron star conditions can be low enough for these phases to occur in compact star configurations with masses below 1.3 M_solar. We study the cooling of these objects in isolation for different values of the gravitational mass. Our equation of state (EoS) allows for 2SC quark matter with a large quark gap \~100 MeV for u and d quarks of two colors that coexists with normal quark matter within a mixed phase in the hybrid star interior. We argue that, if the phases with unpaired quarks were allowed, the corresponding hybrid stars would cool too fast. If they occured for M < 1.3 M_solar, as it follows from our EoS, one could not appropriately describe the neutron star cooling data existing by today. We discuss a "2SC+X" phase, as a possibility to have all quarks paired in two-flavor quark matter under neutron star constraints, where the X-gap is of the order of 10 keV - 1 MeV. Density independent gaps do not allow to fit the cooling data. Only the presence of an X-gap that decreases with increase of the density could allow to appropriately fit the data in a similar compact star mass interval to that following from a purely hadronic model. This scenario is suggested as an alternative explanation of the cooling data in the framework of a hybrid star model.

astro-ph↗

arXiv2002

Equation of State for Strange Quark Matter in a Separable Model

We present the thermodynamics of a nonlocal chiral quark model with separable 4-fermion interaction for the case of U(3) flavor symmetry within a functional integral approach. The four free parameters of the model are fixed by the chiral condensate, and by the pseudoscalar meson properties (pion mass, kaon mass, pion decay constant). We discuss the T=0 equation of state (EoS) which describes quark confinement (zero quark matter pressure) below the critical chemical potential mu_c=333 MeV. The new result of the present approach is that the strange quark deconfinement is separated from the light quark one and occurs only at a higher chemical potential of mu_{c,s}=492 MeV. We compare the resulting EoS to bag model ones for two and three quark flavors, which have the phase transition to the vacuum with zero pressure also at mu_c. We study quark matter stars in general relativity theory assuming beta-equilibrium with electrons and show that for configurations with masses close to the maximum of stability at M=1.62 ... 1.64 M_{solar} strange quark matter can occur.

hep-ph↗