arXiv subjects

D. Zappala'

Publications and source records attributed to D. Zappala'.

16 recordsLinked to original sources

arXiv2016

Hybrid star structure with the Field Correlator Method

We explore the relevance of the color-flavor locking phase in the equation of state (EoS) built with the Field Correlator Method (FCM) for the description of the quark matter core of hybrid stars. For the hadronic phase, we use the microscopic Brueckner-Hartree-Fock (BHF) many-body theory, and its relativistic counterpart, i.e. the Dirac-Brueckner (DBHF). We find that the main features of the phase transition are directly related to the values of the quark-antiquark potential $V_1$, the gluon condensate $G_2$ and the color-flavor superconducting gap $Δ$. We confirm that the mapping between the FCM and the CSS (constant speed of sound) parameterization holds true even in the case of paired quark matter. The inclusion of hyperons in the hadronic phase and its effect on the mass-radius relation of hybrid stars is also investigated.

nucl-th↗

arXiv2009

Testing the Field Correlator Method with astrophysical constraints

We study the structure of hybrid stars with the Field Correlator Method, extended to the zero temperature limit, for the quark phase. For the hadronic phase, we use the microscopic Brueckner-Hartree- Fock many-body theory. The comparison with the neutron star mass phenomenology puts serious constraints on the currently adopted values of the gluon condensate $G_2 \simeq 0.006-0.007 \rm {GeV^4}$, and the large distance static $Q \bar Q$ potential.

hep-ph↗

arXiv2008

Astrophysical constraints on the confining models : the Field Correlator Method

We explore the relevance of confinement in quark matter models for the possible quark core of neutron stars. For the quark phase, we adopt the equation of state (EoS) derived with the Field Correlator Method, extended to the zero temperature limit. For the hadronic phase, we use the microscopic Brueckner-Hartree-Fock many-body theory. We find that the currently adopted value of the gluon condensate $G_2 \simeq 0.006-0.007 \rm {GeV^4}$, which gives a critical temperature $T_c \simeq 170 \rm MeV$, produces maximum masses which are only marginally consistent with the observational limit, while larger masses are possible if the gluon condensate is increased.

hep-ph↗

arXiv2007

Quark matter in neutron stars within the Nambu - Jona-Lasinio model and confinement

The quark matter equation of state (EOS) derived from the standard Nambu - Jona-Lasinio (NJL) model is soft enough to render neutron stars (NS) unstable at the onset of the deconfined phase, and no pure quark matter can be actually present in its interior. Since this is a peculiarity of the NJL model, we have studied a modified NJL model with a momentum cut-off which depends on the density. This procedure, which improves the agreement between QCD and NJL model at large density, modifies the standard NJL equation of state, and then it is potentially relevant for the stability analysis of neutron stars. We show that also within this approach, the NS instability still persists, and that the vacuum pressure, as a signal of quark confinement, has a fundamental role for the NS stability. In this respect, our conclusions point to a relationship between confinement and NS stability.

hep-ph↗

arXiv2004

Gluon condensation and deconfinement critical density in nuclear matter

An upper limit to the critical density for the transition to the deconfined phase, at zero temperature, has been evaluated by analyzing the behavior of the gluon condensate in nuclear matter. Due to the non linear baryon density effects, the upper limit to the critical density, ρ_c turns out about nine times the saturation density, rho_0 for the value of the gluon condensate in vacuum <(α_s/π) G^2>=0.012 GeV^4. For neutron matter ρ_c \simeq 8.5 ρ_0. The dependence of the critical density on the value of the gluon condensate in vacuum is studied.

nucl-th↗

arXiv2004

Violation of Lorentz invariance and dynamical effects in high energy gamma rays

The relation between the violation of Lorentz invariance and the dynamical effects in high energy gamma rays production is discussed. By using the framework of noncommutative classical electrodynamics, it is shown that full dynamical calculations are required to put bounds on the Lorentz violating scale by the phenomenological analysis of these processes as, for example, the synchrotron radiation from the CRAB nebula. It is observed that an improvement of the present bound on the scale of noncommutativity can be obtained only by astrophysical observations of gamma ray spectra in strong magnetic fields such as pulsars.

hep-ph↗

arXiv2003

Renormalization group analysis of the three-dimensional Gross-Neveu model at finite temperature and density

The Renormalization Group flow equations obtained by means of a proper time regulator are used to analyze the restoration of the discrete chiral symmetry at non-zero density and temperature in the Gross-Neveu model in d=2+1 dimensions. The effects of the wave function renormalization of the auxiliary scalar field on the transition have been studied. The analysis is performed for a number of fermion flavors N_f=12 and the limit of large N_f is also considered. The results are compared with those coming from lattice simulations.

hep-th↗

arXiv2003

Nonuniform symmetry breaking in noncommutative $λΦ^4$ theory

The spontaneous symmetry breaking in noncommutative $λΦ^4$ theory has been analyzed by using the formalism of the effective action for composite operators in the Hartree-Fock approximation. It turns out that there is no phase transition to a constant vacuum expectation of the field and the broken phase corresponds to a nonuniform background. By considering $<ϕ(x)>=A \cos(\vec Q \cdot \vec x)$ the generated mass gap depends on the angles among the momenta $\vec k$ and $\vec Q$ and the noncommutativity parameter $\vecθ$. The order of the transition is not easily determinable in our approximation.

hep-th↗

arXiv2002

Perturbative and non-perturbative aspects of the proper time renormalization group

The renormalization group flow equation obtained by means of a proper time regulator is used to calculate the two loop beta function and anomalous dimension eta of the field for the O(N) symmetric scalar theory. The standard perturbative analysis of the flow equation does not yield the correct results for both beta and eta. We also show that it is still possible to extract the correct beta and eta from the flow equation in a particular limit of the infrared scale. A modification of the derivation of the Exact Renormalization Group flow, which involves a more general class of regulators, to recover the proper time renormalization group flow is analyzed.

hep-th↗

arXiv2002

Comment on "Feynman Effective Classical Potential in the Schrodinger Formulation"

We comment on the paper "Feynman Effective Classical Potential in the Schrodinger Formulation"[Phys. Rev. Lett. 81, 3303 (1998)]. We show that the results in this paper about the time evolution of a wave packet in a double well potential can be properly explained by resorting to a variational principle for the effective action. A way to improve on these results is also discussed.

quant-ph↗

arXiv2001

Improving the Renormalization Group approach to the quantum-mechanical double well potential

The gap between ground and first excited state of the quantum-mechanical double well is calculated using the Renormalization Group equations to the second order in the derivative expansion, obtained within a class of proper time regulators. Agreement with the exact results is obtained both in the strong and weak coupling regime.

quant-ph↗

arXiv2001

Proper time regulator and Renormalization Group flow

We consider some applications of the Renormalization Group flow equations obtained by resorting to a specific class of proper time regulators. Within this class a particular limit that corresponds to a sharpening of the effective width of the regulator is investigated and a procedure to analytically implement this limit on the flow equations is shown. We focus on the critical exponents determination for the O(N) symmetric scalar theory in three dimensions. The large N limit and some perturbative features in four dimensions are also analysed. In all problems examined the results are optimized when the mentioned limit of the proper time regulator is taken.

hep-th↗

arXiv2000

Chiral symmetry breaking in the Wegner-Houghton approach

The Wegner-Houghton formulation of the exact renormalization group evolution equation is used in order to study the chiral symmetry breaking of the linear sigma model coupled to an isospin doublet of quarks. A numerical investigation for a particular truncation of the equation which includes the scalar field renormalization function is presented.

hep-ph↗

arXiv1999

Wegner-Houghton equation and derivative expansion

We study the derivative expansion for the effective action in the framework of the Exact Renormalization Group for a single component scalar theory. By truncating the expansion to the first two terms, the potential $U_k$ and the kinetic coefficient $Z_k$, our analysis suggests that a set of coupled differential equations for these two functions can be established under certain smoothness conditions for the background field and that sharp and smooth cut-off give the same result. In addition we find that, differently from the case of the potential, a further expansion is needed to obtain the differential equation for $Z_k$, according to the relative weight between the kinetic and the potential terms. As a result, two different approximations to the $Z_k$ equation are obtained. Finally a numerical analysis of the coupled equations for $U_k$ and $Z_k$ is performed at the non-gaussian fixed point in $D<4$ dimensions to determine the anomalous dimension of the field.

hep-th↗

arXiv1997

Fixed point analysis of a scalar theory with an external field

A momentum dependent projection of the Wegner-Hougton equation is derived for a scalar theory coupled to an external field. This formalism is useful to discuss the phase diagram of the theory. In particular we study some properties of the Gaussian fixed point.

hep-th↗

arXiv1995

Remarks on the Cosmological Constant and the $λΦ^4$ Phase Transition

We reanalyze the problem of the cosmological constant associated with the phase transition in a self-interacting scalar theory. It is pointed out that the generally accepted ``triviality'' of $(λΦ^4)_4$ implies a first-order phase transition. As a consequence, Spontaneous Symmetry Breaking can be consistent with zero cosmological constant if one assumes that it vanishes in the symmetric phase $<Φ>=0$.

hep-ph↗