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Fabio L. Braghin

Publications and source records attributed to Fabio L. Braghin.

35 records · Page 2Linked to original sources
arXiv2016

Quark and pion effective couplings from polarization effects

A flavor SU(2) effective model for pions and quarks is derived by considering polarization effects departing from the usual quark-quark effective interaction induced by dressed gluon exchange, i.e. a global color model for QCD. For that, the quark field is decomposed into a component that yields light mesons and the quark-antiquark condensate, being integrated out by means of the auxiliary field method, and another component which yields constituent quarks, which is basically a background quark field. Within a longwavelength and weak quark field expansion (or large quark effective mass expansion) of a quark determinant, the leading terms are found up to the second order in a zero order derivative expansion, by neglecting vector mesons that are considerably heavier than the pion. Pions are considered in the structureless limit and, besides the chiral invariant terms that reproduce previously derived expressions, symmetry breaking terms are also presented. The leading chiral quark-quark effective couplings are also found corresponding to a NJL and a vector-NJL couplings. All the resulting effective coupling constants and parameters are expressed in terms of the current and constituent quark masses and of the coupling $g$.

hep-ph↗
arXiv2015

Spontaneous Symmetry Breakings in the Linear Sigma Model at finite baryonic density

The linear sigma model with pions and sigma coupled to baryons and a (massive) gauge vector meson is considered for the description of a finite baryonic density system. The stability equation for the bound system is studied in the homogeneous limit considering quantum fluctuations for the scalar and pseudo scalar fields within a variational method with truncations. Nearly exact solutions for the stability equation which also solve the field equations are sought semi-analytically. Bosonic fields are found to have non zero expected classical values at finite density corresponding to condensates of dynamical symmetry breakings. The two components of baryon field, neutron and proton for example, acquire different masses due to the isospin symmetry breaking and these states may even oscillate in the medium. There is a non trivial solution for the (massive) vector meson field indicating the occurrence of a gauge dynamical symmetry breaking at finite density typical of a superconductor.

hep-ph↗
arXiv2015

Higher order effective interactions and effective bosonized model for 2-N particle states

In this work, an effective fermion model with particular higher order interactions given by: $I_{II} = \sum_n^N g_{2^n} (\barψ_a ψ_a)^{2^n}$, for finite $N$, is investigated by means of the auxiliary field method by taking into account 2$n$-particle states as proposed in Ref. [1]. The bosonized model was found to exhibit an approximate symmetry when expanded for weak field fluctuations around the mean field solutions. In the present work, the role of the mean field solutions for the corresponding auxiliary fields is investigated. With the integration of fermions, the resulting determinant is expanded in a polynomial boson model in the weak field approximation and the approximate symmetry found for this series does not appear if the boson mean fields are set to zero.

hep-ph↗
arXiv2015

Higher order fermion effective polynomial interactions

Three different fermion effective potentials given by series of bilinears, $\sum_j^N (\barψ_a ψ_a)^{2^j}$ , $\sum_j^N (\barψ_a ψ_a)^{j}$ and also $\sum_j^N (\barψ_a γ_μψ_a)^{2j}$ where $a=1,...N_r$ and integer $j$ are investigated by introducing sets of auxiliary fields. A mininal procedure is adopted to deal with the auxiliary fields and an effective bosonized model in each case is found by assuming weak field fluctuations, i.e. weak enough when compared to (normalized) coupling constants. Different fermion condensates are considered for the ground state in the first two series analysed and the factorization of all higher order condensates into the lowest order one is found in most cases, i.e. in general $<(\overlineψ_a ψ_a)^n> \propto <\barψ_a ψ_a>^n$. For the case of the third series built with vector-type bilinears no condensation is assumed to occur. The corresponding (weak) scalar fields effective models for the three cases are expanded in polynomial interactions. The resulting low energy effective boson model may a exhibit new approximate symmetry depending on the terms present in the original series-model and on the values of the coupling constants.

hep-th↗
arXiv2014

Factorization breaking of four-quark condensates in the Nambu Jona Lasinio model

We investigate the factorization hypothesis of the four-quark condensate $\langle q \bar{q} q \bar{q} \rangle = \, A \, \langle q \bar{q} \rangle^2$ with the help of the Nambu Jona-Lasinio Model supplemented with eighth order interactions. For that purpose we use the bosonization method with multiple auxiliary variables. We find that in a simplified U(1) version of the model factorization holds, whereas in the full SU(3)-flavor version of the model factorization is broken by terms which are related to the 't Hooft interactions.

hep-ph↗
arXiv2014

Vacuum polarization corrections to low energy quark effective couplings

In this work corrections to low energy punctual effective quark couplings up to the eighth order are calculated by considering vacuum polarization effects with the scalar quark-antiquark condensate. The departing point is a QCD-based NJL model. By separating the quark field into two components, one that condenses and another one for interacting quarks, the former is integrated out with the help of usual auxiliary fields and an effective action in terms for interacting quark fields is found. The scalar auxiliary field reduces to the quark-antiquark condensate in the vacuum and the determinant is expanded in powers of the quark-antiquark bilinears generating chiral invariant effective 2N-quark interactions (N=2, 3...). The corresponding coupling constants and effective masses are estimated, and the general trend is that for increasing the effective gluon mass the values of the effective coupling constants decrease. All the values are in good agreement with phenomenological fits.

hep-ph↗
arXiv2008

The breakdown of the mean-field description of the Nagaoka phase

We discuss the relevance of the improved mean-field slave-fermion theory to describe the Nagaoka ($U=\infty$) limit of the Hubbard model. In this theory the crucial on-site constraint of no double electron occupancy is taken into account rigorously prior to the mean-field approximation. At one-loop approximation the effective mean-field action shows a long-range ferromagnetic order over the whole doping range. This indicates that the slave-fermion mean-field theory does not constitute an appropriate framework to describe the physics of the Nagaoka phase. We discuss the drawbacks of this mean-field theory and present some results on the derivation of a low-energy effective spin action to describe the Nagaoka phase beyond the mean-field approximation.

cond-mat.str-el↗
arXiv2006

Linear Sigma Model at finite baryonic density and symmetry breakings

The linear sigma model at finite baryonic density with a massive vector field is investigated considering that all the bosonic fields develop non zero expected classical values, eventually associated to condensates and corresponding to dynamical symmetry breakings which might occur in the QCD phase diagram. A modified equation for the classical vector field is proposed with its respective solution. Some in medium properties of the model (mainly masses) are investigated within reasonable prescriptions. In particular the behavior of the {\it in medium} pion and sigma masses and a particular way of calculating {\it in medium} coupling to baryons is investigated. A symmetry radius for finite baryonic densities is proposed and calculated in different ways in terms of the other variables of the model and these different ways of calculating it agree quite well. However, assuming that the pion and sigma masses go to zero close to the restoration of chiral symmetry a too high value for the critical density is obtained $ρ_c \simeq 4.3 ρ_0$.

hep-ph↗
arXiv2006

Di-Antiquarks condensation in Color Superconductivity

Some consequences of a classical vector field (chromo-electromagnetic field) coupled to quarks, which undergo to superfluid and/or superconductive states with diquark / diantiquark condensation, are investigated. For this, one scalar field exchange is considered in the lines investigated by Pisarski and Rischke \cite{PISARSKI-RISCHKE} in the mean field approach. Some effects and possible consequences are discussed.

hep-ph↗
arXiv2005

Nuclear matter symmetry energy from generalized polarizabilities: dependences on momentum, isospin, density and temperature

Symmetry energy terms from macroscopic mass formulae are investigated as generalized polarizabilities of nuclear matter. Besides the neutron-proton (n-p) symmetry energy the spin dependent symmetry energies and a scalar one are also defined. They depend on the nuclear densities ($ρ$), neutron-proton asymmetry ($b$), temperature ($T$) and exchanged energy and momentum ($q$). Based on a standard expression for the generalized polarizabilities, a differential equation is proposed to constrain the dependence of the symmetry energy on the n-p asymmetry and on the density. Some solutions are discussed. The q-dependence (zero frequence) of the symmetry energy coefficients with Skyrme-type forces is investigated in the four channels of the particle-hole interaction. Spin dependent symmetry energies are also investigated indicating much stronger differences in behavior with $q$ for each Skyrme force than the results for the neutron-proton one.

nucl-th↗
arXiv2005

Free parameters in quantum field theories: an analysis of the variational approach

The usual renormalization procedure for the variational approximation with a trial Gaussian ansatz for the $\lap$ model in 3+1 dimensions is re-analysed as a departing framework for the investigation of the parameters of the model. The so-called asymmetric phase of the model (where $ \neq 0$) is considered for the search of privileged values of these parameters (mass and coupling constant) and possible conditions they may be expected to satisfy. This also may yield a suitable approach for the investigation of the reliability and stability of the approximation. The extremization of the renormalized energy density with relation to the renormalized mass, coupling and $\bphi$ is done. The minimizations of the renormalized energy with relation to the mass and $\bphi$ provide different expressions from the ones obtained by the usual variational procedure for the regularized theory. Sort of ``energy scale'' invariances in expressions for the renormalized mass and coupling constant are found. A different view on the restoration of symmetry issue is discussed. The transcendental character of the GAP equation may be reduced or even eliminated by placing some variables in the complex plane.

hep-ph↗
arXiv2003

Pion Mass Dependence of Nucleon Magnetic Moments

The relevance of the pion mass, provenient from a term which explicitely breaks chiral symmetry in the Lagrangian, for nucleon magnetic moment in the frame of the Skyrmion model in two different versions: the usual Skyrme model and a modified one which includes a coupling to a light scalar meson field, the sigma $σ(\simeq 500-600$ MeV). The results are compared to other calculations. Our main motivation comes from usual extrapolations for values of low energy QCD observables obtained in lattices with large values of pion/quark masses toward realistic value of $m_π$. which do not allow it. We do a comparison with results from the Cloudy Bag Model and a chiral hadronic model from chiral perturbation theory. There are several resulting extrapolations from the region of large pion mass to the realistic value depending on the considered model for low energy QCD.

hep-lat↗
arXiv2003

Isospin and density dependences of nuclear matter symmetry energy coefficients II

Symmetry energy coefficients of explicitly isospin asymmetric nuclear matter at variable densities (from .5$ρ_0$ up to 2 $ρ_0$) are studied as generalized screening functions. An extended stability condition for asymmetric nuclear matter is proposed. We find the possibility of obtaining stable asymmetric nuclear matter even in some cases for which the symmetric nuclear matter limit is unstable. Skyrme-type forces are extensively used in analytical expressions of the symmetry energy coefficients derived as generalized screening functions in the four channels of the particle hole interaction producing alternative behaviors at different $ρ$ and $b$ (respectively the density and the asymmetry coefficient). The spin and spin-isospin coefficients, with corrections to the usual Landau Migdal parameters, indicate the possibility of occurring instabilities with common features depending on the nuclear density and n-p asymmetry. Possible relevance for high energy heavy ions collisions and astrophysical objects is discussed.

nucl-th↗
arXiv2001

Expanding non homogeneous configurations of the $λϕ^4$ model

A time dependent variational approach is considered to derive the equations of movement for the $λϕ^4$ model. The temporal evolution of the model is performed numerically in the frame of the Gaussian approximation in a lattice of 1+1 dimensions given non homogeneous initial conditions (like bubbles) for the classical and quantum parts of the field which expands. A schematic model for the initial conditions is presented considering the model at finite fermionic density. The non zero fermionic density may lead either to the restoration of the symmetry or to an even more asymmetric phase. Both kinds of situations are considered as initial conditions and the eventual differences in early time dynamics are discussed. In the early time evolution there is strong energy exchange between the classical and quantum parts of the field as the initial configuration expands. The contribution of the quantum fluctuations is discussed especially in the strong coupling constant limit. The continuum limit is analyzed.

hep-ph↗
arXiv2001

Isospin dependence of nuclear matter symmetry energy coefficients

Generalized symmetry energy coefficients of asymmetric nuclear matter are obtained as screening functions. The dependence of the isospin symmetry energy coefficient on the neutron proton (n-p) asymmetry may be determined unless by a constant (exponent) $Z$ which depend on microscopic properties. The dependence of the generalized symmetry energy coefficients with Skyrme forces on the n-p asymmetry and on the density, only from .5 up to 1.5 $ρ_0$, are investigated in the isospin and scalar channels. The use of Skyrme-type effective forces allows us to obtain analytical expressions for these parameters as well as their dependences on the neutron-proton (n-p) asymmetry, density and even temperature. Whereas the density dependence of these coefficients obtained with Skyrme forces is not necessarily realistic the dependence on the n-p asymmetry exhibit a more consistent behaviour. The isospin symmetry energy coefficient (s.e.c.) increases as the n-p asymmetry acquires higher values whereas the isoscalar s.e.c. decreases. Some consequences for the Supernovae mechanism are discussed.

nucl-th↗
arXiv1995

The giant resonances in hot nuclei: linear response calculations

We calculate the isovector response function of hot nuclear matter using various effective Skyrme interactions. For Skyrme forces with a small effective mass the strength distribution is found to be nearly independent of temperature, and shows little collective effects. In contrast effective forces with an effective mass close to unity produce sizeable collective effects at zero temperature which disappear at temperatures of a few MeV. We discuss the relevance of these results for the saturation of the multiplicity of photons emitted by the giant dipole resonance in hot nuclei beyond $T$=3 MeV observed in recent experiments.

nucl-th↗