arXiv subjects

F. L. Braghin

Publications and source records attributed to F. L. Braghin.

12 recordsLinked to original sources

arXiv2025

Mixing interactions and effects in the NJL-model

The flavor-dependent quark-antiquark contact interactions, induced by vacuum polarization and recently derived for flavor U(3) Nambu-Jona-Lasinio model, are articulated with the resulting mixing effects emerging from flavor symmetry breaking in view. The formal effects of the explicit mixing interactions, $G_{i\neq j}$, are detailed firstly for the meson mixing problem without the inclusion of 't Hooft interactions induced by instantons. Secondly, it is shown that these mixings, in the scalar channel of quark-antiquark interactions, might give rise to quark mixing in the gap equations. Sixth order quark-antiquark interactions from vacuum polarization, that break $U_A(1)$ symmetry, also contribute.

hep-ph↗

arXiv2023

Charm and beauty content of the pion and kaon in the Flavor U(5) Nambu-Jona-Lasinio model

We consider a U(5)-Nambu-Jona-Lasinio model with flavor-dependent couplings constants obtained from quark-antiquark polarization to investigate the role of the heavy quarks for light constituent quarks and mesons. A quantum mixing, due to the different representations of the flavor group needed to defined quarks and mesons, lead to charm and beauty sea quark contributions to properties of the light quark sector. For a given fitting procedure for the parameters of the model, the charm and beauty quark effective masses are freely varied from very small values to very large values (infinite). The effect of these variations on pion and kaon observables, such as masses, weak decay constants and condensates, is calculated. Although the pion mass can vary (seemingly too much) up to $20\%$ for a very large variation of heavy quark effective masses, their contributions for the kaon mass are at most of the order of $2\%$ and all the other variables and observables, including up and down quark constituent masses, vary at most around $1\%$.

hep-ph↗

arXiv2022

Spontaneous symmetry breakings in the singlet scalar Yukawa model within the auxiliary field method

The aim of this work is to investigate the occurrence of two different spontaneous symmetry breakings {at} two levels of the description of fermion-scalar field model, by means of a set of gap equations and {with} a background field effective action. For that, we consider the Yukawa model, as a toy model for interactions between non-massive fermions intermediated by a self-interacting real scalar field. This model has at stakes two symmetries at the classical level that, as we know, might be spontaneously or dynamically broken with mass generation for the particles. The auxiliary field method is considered and it produces coupled renormalized gap equations. The effective action is then written with quantum contributions by {the} external background {field} method. We brought to light how the renormalization procedure affects the physical gaps, investigate its properties, and discuss the connection between the auxiliary fields not only to define composite states but also to compute the effective action.

hep-th↗

arXiv2022

Constituent quark axial current couplings to light vector mesons in the vacuum and with a weak magnetic field

Unusual constituent quark axial current couplings to light vector mesons, $ρ$ and $ω$, are derived in the vacuum and under weak magnetic field by considering a quark-antiquark interaction mediated by a non perturbative gluon exchange. Similarly, light axial mesons are found to couple anomalously with the constituent quark vector current. These interactions are of the type of the Wess-Zumino-Witten terms, being strongly anisotropic and dependent on the vector (or axial) meson polarization. They also provide axial (vector) form factors for the vector (axial) mesons and are quite small, suppressed nearly by $1/{M^*}^2$ with respect to the vector mesons minimal coupling to the quark vector current. Some three leg meson vertices are also presented: $π-ρ-A_1$ and $V_1V_2 A$ (where $V_1,V_2$ are vector mesons and $A$ an axial meson). A vector and axial-vector mesons mixing is identified at non zero magnetic field which however can contribute only in the presence of a third particle or in a medium. Numerical results are presented for different effective gluon propagators.

hep-ph↗

arXiv2011

Nonlocal effective average action approach to crystalline phantom membranes

We investigate the properties of crystalline phantom membranes, at the crumpling transition and in the flat phase, using a nonperturbative renormalization group approach. We avoid a derivative expansion of the effective average action and instead analyse the full momentum dependence of the elastic coupling functions. This leads to a more accurate determination of the critical exponents and further yields the full momentum dependence of the correlation functions of the in-plane and out-of-plane fluctuation. The flow equations are solved numerically for D=2 dimensional membranes embedded in a d=3 dimensional space. Within our approach we find a crumpling transition of second order which is characterized by an anomalous exponent $η_c\approx 0.63(8)$ and the thermal exponent $ν\approx 0.69$. Near the crumpling transition the order parameter of the flat phase vanishes with a critical exponent $β\approx 0.22$. The flat phase anomalous dimension is $η_f\approx 0.85$ and the Poisson's ratio inside the flat phase is found to be $σ_f\approx -1/3$. At the crumpling transition we find a much larger negative value of the Poisson's ratio $σ_c \approx -0.71(5)$. We discuss further in detail the different regimes of the momentum dependent fluctuations, both in the flat phase and in the vicinity of the crumpling transition, and extract the crossover momentum scales which separate them.

cond-mat.stat-mech↗

arXiv2010

Thermal fluctuations of free standing graphene

We use non-perturbative renormalization group techniques to calculate the momentum dependence of thermal fluctuations of graphene, based on a self-consistent calculation of the momentum dependent elastic constants of a tethered membrane. We find a sharp crossover from the perturbative to the anomalous regime, in excellent agreement with Monte Carlo results for graphene, and give an accurate value for the crossover scale. Our work strongly supports the notion that graphene is well described as a tethered membrane. Ripples emerge naturally from our analysis.

cond-mat.soft↗

arXiv2009

Temperature dependence of nuclear matter generalized isovector symmetry energy with Skyrme-type interactions

The temperature dependence of the nuclear matter isovector symmetry energy coefficient ($\cA_{0,1}$) is investigated in the framework of the generalized nuclear polarizability with Skyrme interactions, as worked out in Refs. \cite{npa,prc}. The variation of $\cA_{0,1}(T)$ is very small (of the order of 1 MeV) for temperatures (T) in the range of 0 and 18 MeV. Different behaviors with temperature are found strongly depending on the Skyrme parameterization, in particular at densities lower than the saturation density $ρ_0$.

nucl-th↗

arXiv2009

Stability of Nagaoka phase, spin effective action and delocalized free holes

The Hubbard model in the limit of infinite $U$ is investigated within a projected slave fermion representation and following a previous work of the author and collaborators \cite{BFK}. The stability of the Nagaoka's phase with respect to a non vanishing concentration of holes ($δ_h$) is analyzed by envisaging the existence of a spin effective action for itinerant magnetism of the Hubbard model. It is considered that,as the hole doping increases away from the half filled insulating limit, free holes are expected to be more delocalized. Depending on treatment for the hopping: a ferromagnetic or anti-ferromagnetic ordering might arise and the Nagaoka's phase might have some stability with respect to $δ_h \neq 0$.

cond-mat.str-el↗

arXiv2003

The Skyrmion Model and the Dynamical Breakdown of Chiral Symmetry

A scalar field, $η({\bf r})$, is coupled to the skyrmion. Its classical value in the vacuum ("condensate") reduces to the pion decay constant, $f_π$, being thus proportional to the chiral condensate $<\bar{q} q>$. A quadratic coupling of the scalar field to the pion kinetic Lagrangian term, reminiscent from the linear sigma model, is considered. Its mass is an additional free parameter in the model associated to the lightest scalar isoscalar meson. Solutions for the two resulting coupled differential equations are found in several cases. The nucleon, as a topological soliton, ``digs a hole'' in the vacuum making the value of the scalar condensate to be close to zero inside the skyrmion. Some of the observables, as for example the baryon masses, may have their values closer to the experimental ones. Derrick's stability analysis is applied to the model.

hep-ph↗

arXiv2001

Self-consistent quantum effects in the quark meson coupling model

We derive the equation of state of nuclear matter including vacuum polarization effects arising from the nucleons and the sigma mesons in the quark-meson coupling model which incorporates explicitly quark degrees of freedom with quark coupled to the scalar and vector mesons. This leads to a softer equation of state for nuclear matter giving a lower value of incompressibility than would be reached without quantum effects. The {\it in-medium} nucleon and sigma meson masses are also calculated in a self-consistent manner.

nucl-th↗

arXiv1995

Isovector response function of hot nuclear matter with Skyrme interactions

We investigate the role of the effective nucleon-nucleon interaction in the description of giant dipole resonances in hot nuclei. For this purpose we calculate the response function of hot nuclear matter to a small isovector external perturbation using various effective Skyrme interactions. We find that for Skyrme forces with an effective mass close to unity an undamped zero sound mode occurs at zero temperature. This mode gives rise in finite nuclei (calculated via the Steinwedel-Jenssen model) to a resonance whose energy agrees with the observed value. We find that zero sound disappears at a temperature of a few MeV, leaving only a broad peak in the dipole strength. For Skyrme forces with a small value of the effective mass (0.4- 0.5), there is no zero sound at zero temperature but only a weak peak located too high in energy. The strength distribution in this case is nearly independent of temperature and shows small collective effects. The relevance of these results for the saturation of photon multiplicities observed in recent experiments is pointed out.

hep-ph↗

arXiv1994

Response Function of Hot Nuclear Matter

We investigate the response function of hot nuclear matter to a small isovector external field using a simplified Skyrme interaction reproducing the value of the symmetry energy coefficient. We consider values of the momentum transfer corresponding to the dipole oscillation in heavy nuclei. We find that while at zero temperature the particle hole interaction is almost repulsive enough to have a sharp (zero sound type) collective oscillation, such is no longer the case at temperatures of a few MeV. As a result a broadening of the dipole resonance occurs, leading to its quasi disappearence by the time the temperature reaches 5 MeV. The sensivity of the temperature evolution of the width when modifying the residual interaction strength is also examined.

nucl-th↗