arXiv subjects

William R. Tavares

Publications and source records attributed to William R. Tavares.

At least 19 recordsLinked to original sources

arXiv2026

From Threshold Crossing to Wave-function Renormalization: Defining the Pion Mott Temperature in a Magnetic Field

We investigate the dissociation of the neutral pion in hot magnetized quark matter within the two-flavor Nambu--Jona-Lasinio model. At zero magnetic field, the Mott temperature is conventionally determined by $m_{π^0}(T_{\rm Mott})=2M(T_{\rm Mott})$, above which a real pole ceases to exist. At finite magnetic field, Landau quantization replaces this single threshold by a hierarchy of quark--antiquark continua and generates multiple solutions of the pion pole equation, rendering a direct threshold-crossing criterion ambiguous since a real pion solution below the lowest nominal threshold always exists. We therefore propose to define the magnetic Mott temperature through the inflection point of the pion wave-function renormalization factor $Z_{π^0}(T,eB)$ of that lowest pole, corresponding to the fastest loss of its spectral function strength. The prescription reproduces the conventional Mott temperature as $eB\to 0$ and tracks the chiral pseudocritical temperature for both constant and magnetic-field-dependent couplings. Our results characterize pion dissociation in a magnetic field as a spectral crossover rather than a simple threshold crossing.

hep-ph↗

arXiv2026

Robust Finite-Momentum Instabilities in Dense Matter

The predicted extent of inhomogeneous chiral phases in effective models of quantum chromodynamics is notoriously sensitive to ultraviolet regularization. We show that this sensitivity is largely artificial. In the two-flavor Nambu--Jona-Lasinio model, conventional implementations of three-dimensional cutoff, Pauli--Villars, and proper-time regularization produce strongly different finite-momentum instability regions. Once ultraviolet regulators are restricted to genuinely divergent vacuum contributions, however, all three prescriptions yield nearly identical stability diagrams. Both the onset of the moat regime and the subsequent finite-momentum instability become quantitatively robust. The apparent scheme dependence originates from regulating ultraviolet-finite medium contributions associated with the Fermi-surface response. Our results identify spatially modulated chiral correlations as a genuine property of the dense medium rather than an artifact of the ultraviolet prescription.

hep-ph↗

arXiv2026

Magnetic catalysis and Hall conductivity of excitonic insulators in a planar four-Fermi model

We study the effects of a perpendicular magnetic field on the excitonic insulator (EI) phase in the semiconductor regime using an extended planar four-Fermi model. Within the large-$N$ approximation, we determine the coupled scalar and excitonic condensates at finite temperature, chemical potential, and magnetic field. The field enhances the EI condensate and raises its critical temperature, providing an excitonic realization of magnetic catalysis, while the scalar condensate remains constant throughout the EI phase. By contrast, the critical chemical potential depends nonmonotonically on the field because of the successive occupation of Landau levels. The magnetic field also shifts the mean-field tricritical point and enlarges the first-order region of the temperature--chemical-potential phase diagram. We further analyze the Hall conductivity and find that increasing the field reduces the number of plateaus and modifies the threshold for a finite Hall response. For the parameters considered, the emergence of the EI condensate is accompanied by a characteristic change in the Hall conductivity, including a field-dependent change of slope near a continuous transition. These results show that the combined phase structure and Hall response can provide complementary signatures of excitonic ordering in planar fermionic systems.

cond-mat.mes-hall↗

arXiv2026

Dense and Cold Magnetized Quark Matter: A Review of Magnetic-Field-Independent Regularization and the Medium Separation Scheme

We present a comprehensive review of regularization schemes for magnetized dense quark matter within effective models of quantum chromodynamics, focusing on the Magnetic-Field-Independent Regularization (MFIR) and the Medium Separation Scheme (MSS) at finite chemical potential and magnetic field. In nonrenormalizable frameworks such as the Nambu-Jona-Lasinio model, the treatment of ultraviolet divergences is crucial, particularly in magnetized and dense environments where conventional regularization procedures may introduce unphysical artifacts. We show that MFIR consistently isolates divergent vacuum contributions from finite magnetic-field-dependent terms, while MSS extends this separation to the medium sector, ensuring that only vacuum quantities are regularized. Within this unified framework, we analyze the thermodynamics of cold and dense quark matter, including color-superconducting phases, and demonstrate that the superconducting gap remains finite at large chemical potentials, even in the presence of strong magnetic fields. In contrast to results obtained with traditional regularization schemes, we find no evidence for a transition to a normal phase at zero temperature, highlighting the importance of a proper separation between vacuum and medium contributions. These results eliminate spurious oscillations and other nonphysical artifacts, leading to a more robust and physically consistent description of strongly interacting matter under extreme conditions relevant to compact stars and heavy-ion collisions.

hep-ph↗

arXiv2025

Two-flavor chirally imbalanced quark matter beyond large $N_c$

We investigate a chirally imbalanced medium in the context of the two-flavor Nambu--Jona-Lasinio model using both the large-$N_c$ (LN) and beyond large-$N_c$ (BLN) approximations. To incorporate BLN effects, we consider the optimized perturbation theory (OPT) to the first nontrivial order, which includes two-loop (exchange) contributions. This procedure allows us to explicitly explore how finite $N_c$ corrections affect the thermodynamics as well as the phase diagram of chirally imbalanced quark matter. We then compare the results obtained with a sharp three-dimensional cutoff -- generically referred to as the traditional regularization scheme -- and with an alternative procedure called the medium separation scheme (MSS). In the first case, we observe that the pseudocritical temperature decreases as the chiral chemical potential increases, an effect dubbed inverse chiral catalysis. On the other hand, when considering the MSS regularization, which properly isolates the medium contributions from the vacuum, we find the opposite result. We show that the results obtained with MSS are consistent with well-established LQCD data in both the LN and BLN approximations. Finally, we suggest that to cope with the high-density limit, the standard OPT interpolation prescription must be modified with the inclusion of an extra variational parameter.

hep-ph↗

arXiv2025

Chiral and deconfinement transitions in spin-polarized quark matter

We investigate the influence of spin polarization in strongly interacting matter by introducing a finite spin potential, $μ_Σ$, which effectively controls the spin density of the system without requiring rotation or specific boundary conditions. Inspired by recent lattice QCD simulations that incorporated such a potential, we implement this approach within an effective QCD framework. Our results show that increasing spin polarization leads to a simultaneous decrease in both the chiral and deconfinement restoration temperatures. The resulting phase structure is qualitatively consistent with lattice findings, and notably, we observe the emergence of a first-order chiral phase transition at low temperature. These results suggest that spin-polarized environments can significantly impact the QCD phase diagram and offer a controlled route for studying spin effects in hot and dense matter.

hep-ph↗

arXiv2025

Thermomagnetic effects on light pseudo-scalar meson masses within the SU(3) Nambu-Jona--Lasinio model

We calculate the screening masses of pseudoscalar mesons in a hot and strongly magnetized medium within the framework of the SU(3) Nambu-Jona--Lasinio model, using a magnetic field-independent regularization scheme. Inverse magnetic catalysis (IMC) is implemented through the use of a magnetic field-dependent coupling $G(B)$, fitted to reproduce lattice quantum chromodynamics (QCD) results for the pseudocritical chiral transition temperature $T_c^B$. For the external homogeneous magnetic field considered, neutral screening masses separate in two types: perpendicular and parallel to the direction of the field, while for charged mesons only parallel energies can be defined for each Landau level. We obtain $m_{\mathrm{scr},\perp} > m_{\mathrm{scr},\parallel}$, as expected from causality. Thermally, all screening energies are almost constant until some critical temperature, whose behavior is correlated with $T_c^B$. They rapidly increase around this value, keeping a steady enhancement afterward due to thermal excitation. Magnetically, neutral parallel masses are enhanced (suppressed) at high temperatures when considering $G(B)$ ($G$). Perpendicular ones display a non-monotonic magnetic behavior for $G$ (due to increasing $T_c^B$) when $T \lesssim 500$~MeV, but become magnetically enhanced when $T \gtrsim 500$~MeV. For $G(B)$ they always increase with $B$. Charged parallel energies are always magnetically enhanced, for both couplings. In the high-temperature limit, we show that both neutral and charged screening energies converge to $2πT$. At $B=0$ the model overestimates the remaining quark interaction in this regime. At $B\neq 0$ we find that, when IMC is accounted for, the interaction is suppressed as $B$ increases, a fact that appears to be at odds with currently available lattice QCD results.

hep-ph↗

arXiv2025

Chiral vortical catalysis constrained by LQCD simulations

Evidences of vortical effects have been recently found by experiments in heavy ion collisions, instigating new insights into the phase diagram of quantum chromodynamics (QCD). Considering the effect of rotations, lattice QCD data shows that the temperatures for deconfinement and chiral symmetry restoration should increase with real angular velocity, and the dominant effects are related to gluonic degrees of freedom. These findings could be essential for quark models in rotating systems that lack gluonic interactions, which predicts the decreasing of the chiral temperature transition with the angular velocity. To address this issue properly, in this work we apply the two-flavor Nambu--Jona-Lasinio model to explore the phase diagram in a rotating rigid cylinder with constant angular velocity in the mean field approximation. To circumvent the absence of gluons, we propose the application of an effective coupling dependent of the angular velocity, fitted to match the pseudocritical temperature of chiral phase transition in the model through lattice QCD data. Our results indicate that the running coupling induces the enhancement of the chiral condensate as a function of angular velocity, strengthening the breaking of chiral symmetry, an effect previously dubbed as chiral vortical catalysis. For the chiral susceptibility we observe stronger fluctuations around the transition temperature when we consider the running coupling. The phase diagram is affected by these findings shifting the critical end point (CEP) to higher temperatures and chemical potentials.

hep-ph↗

arXiv2025

Charged scalars at finite electric field and temperature in the optimized perturbation theory

We study the symmetry breaking and restoration behavior of a self-interacting charged scalar field theory under the influence of a constant electric field and finite temperature. Our study is performed in the context of the optimized perturbation theory. The dependence of the effective potential with constant electric fields is established by means of the bosonic propagators in the Schwinger proper-time method. Explicit analytical expressions for the electric and thermal contributions are found. Our results show a very weak decreasing behavior of the vacuum expectation value as a function of the electric field, which is strengthened by the temperature effect. A first-order phase transition that occurs at zero/weak electric fields changes to a second-order phase transition under strong electric fields. The critical temperature for the phase transition exhibited a very weak dependence on the electric field. Additionally, we computed the vacuum persistence probability rate for the interacting theory, finding a peak at the critical point. The maximum value of this rate at the critical point is found to be independent of the coupling constant but depended solely on the magnitude of the electric field.

hep-ph↗

arXiv2024

Artificial first-order phase transition in a magnetized Nambu--Jona-Lasinio model with a quark anomalous magnetic moment

Recently, first-order phase transitions have been predicted as an effect of the inclusion of quark anomalous magnetic moment (AMM) in the hot and magnetized Nambu--Jona-Lasinio model (NJL). These transitions appear in the chiral condensate for different combinations of AMM and magnetic fields and could lead to inverse magnetic catalysis. However, in this work, we show that the predicted first-order phase transitions are related to regularization-dependent issues. To show this, we explore, in the context of the vacuum magnetic regularization (VMR) scheme, two different scenarios: when mass-dependent (MD) and mass-independent (MI) terms are present in the subtraction of the divergences. In the MD case, as we increase the AMM value, it is observed the appearance of a nonmassive minimum in the thermodynamical potential, which induces a first-order phase transition from the massive minimum. We argue that the MD terms must be avoided in order to satisfy the predictions of Lattice QCD, and we propose a MI solution that is valid in the limit which the magnetic fields are smaller than the squared of vacuum effective quark mass.

hep-ph↗

arXiv2023

Causality violation and the speed of sound of hot and dense quark matter in the Nambu--Jona-Lasinio model

The Nambu--Jona-Lasinio model is widely used to study strong-interaction phenomena in vacuum and quark matter. Since the model is nonrenormalizable, one needs to work within a specific regularization scheme to obtain finite results. Here we show that a commonly used cutoff regularization scheme leads to unphysical results, such as superluminal speed of sound and wrong high-temperature behavior of the specific heat and other thermodynamical quantities. Such a troublesome feature of the cutoff regularization invalidates the model for temperature and baryon density values relevant to the phenomenology of heavy-ion collisions and compact stars. We show that the source of the problems stems from cutting off momentum modes in finite integrals depending on thermal distribution functions in the grand canonical potential. The problems go away when taking into account the full momentum range of those integrals. Explicit examples are worked out in the SU(2)-flavor version of the model.

hep-ph↗

arXiv2023

Quark matter under strong electric fields in the Linear Sigma Model coupled with quarks

In this work we study the influence of external electric field and temperature on the chiral phase transition of Quantum Chromodynamics. We use the two-flavor Linear Sigma Model coupled with quarks (LSMq) in a thermal and electrized medium to evaluate the effective quark mass and the Schwinger pair production. To this end, we apply one-loop correction to the fermionic sector of the model and the simple tree-level approximation in the mesonic contributions. The electric fields strengthen the partial restoration of the chiral symmetry when applied with finite temperature in a crossover transition. The expected decrease of the pseudocritical temperature as a function of the electric field is observed until electric fields reach $eE\approx 13.5 m_π^2$. For stronger electric fields, the effect is the opposite, which is in a very good agreement with previous results obtained with four-point non-renormalizable models, showing that this effect is independent of renormalizability issues. We also show the thermal and electric effects on the behavior of the Schwinger pair production.

hep-ph↗

arXiv2022

Magnetized pole-mass of neutral $ρ$ meson within full RPA evaluation

In this work we calculate the pole-mass of the $ρ^0$ meson with different spin projections $s_z=0,\pm 1$ in the context of the magnetized two-flavor Nambu--Jona--Lasinio model. Making use of the mean field approximation to obtain the effective quark mass as a function of the magnetic field, we apply the random phase approximation (RPA) to the vector channel in order to calculate the polarization function for each spin component. We adopt the magnetic field independent regularization (MFIR) in our evaluations as a method of separating divergences and the Pauli-Villars regularization for the vacuum contributions. The $ρ^0$ meson mass with spin projection $s_z=\pm 1$ is always catalysed with the magnitude of the magnetic field, showing good agreement with Lattice QCD results. The mass projection $s_z=0$ has a non-monotonic behavior, decreasing until the minimum at $eB\lesssim 0.15$ GeV$^2$, which is in contrast with available LQCD data.

hep-ph↗

arXiv2021

Nambu--Jona-Lasinio $SU(3)$ model constrained by lattice QCD: thermomagnetic effects in the magnetization

We use a three-flavor Nambu--Jona-Lasinio model to study the thermodynamics of strange quark matter under a strong magnetic field. The model Lagrangian features flavor SU(3) four-quark interactions and six-quark interactions that break the UA(1) symmetry. We incorporate thermomagnetic effects in the four-quark coupling. The model predicts magnetic catalysis at low temperatures and inverse magnetic catalysis at temperatures close to the pseudocritical temperature of the QCD transition, in agreement with lattice QCD results. We compute the pressure at the mean field level and obtain the magnetization of quark matter. We adopt the recently proposed vacuum magnetic regularization (VMR) scheme, in that divergent quark mass independent contributions are not subtracted, thereby avoiding unphysical results for the magnetization. We devote special attention to the renormalized magnetization, a projected quantity that allows for direct comparisons with lattice QCD simulations. Our results are in very good agreement with lattice data indicating a paramagnetic behavior for quark matter.

hep-ph↗

arXiv2021

Effects of the quark anomalous magnetic moment in the chiral symmetry restoration: magnetic catalysis and inverse magnetic catalysis

In this work, we consider the effect of a constant anomalous magnetic moment (AMM) of quarks in the SU(2) Nambu--Jona-Lasinio model in the mean field approximation. To this end, we use the Schwinger {\it ansatz}, which represents a linear magnetic field term in the Lagrangian. A regularization method inspired in the vacuum magnetic regularization (VMR) is adopted to avoid ultraviolet divergences. Our results indicate a smooth decrease of the pseudocritical temperature and quark condensates for magnetic fields $B \leq 0.1$ GeV$^2$ when a sizable AMM is considered. We found only a small window for Inverse Magnetic Catalysis (IMC), in contradiction with NJL predictions made in the literature. For a low value of AMM, we observe for all ranges of magnetic fields considered that the pseudocritical temperature increases with the magnetic field, indicating only Magnetic Catalysis (MC). In our approach, for nonvanishing quark AMM, the chiral symmetry restoration happens always as a smooth crossover and never turns into a first order phase transition.

hep-ph↗

arXiv2021

Regularizing thermo and magnetic contributions within nonrenormalizable theories

The importance of implementing a proper regularization procedure in order to treat thermo and magnetic contributions within nonrenormalizable theories is investigated. Our study suggests that potential divergences should be isolated into the vacuum and purely magnetic contributions and then regularized while the convergent thermomagnetic contributions should be integrated over the full momentum range. This prescription is illustrated by applying the proper time formalism to the two flavor Polyakov--Nambu--Jona-Lasinio model, whose magnetic field dependent coupling has been recently determined. Observables such as the pressure, magnetization, speed of sound squared, and specific heat evaluated within our scheme are compared with results furnished by other three possible prescriptions. We show that these quantities display a thermomagnetic behavior which is physically more consistent when our scheme is adopted. In particular, we demonstrate that naively regulating the (entangled) vacuum, magnetic and thermomagnetic contributions leads to physically inconsistent results especially at the high temperature domain.

hep-ph↗

arXiv2020

Deconfinement and chiral phase transitions in quark matter with a strong electric field

The deconfinement and chiral phase transitions are studied in the context of the electrized quark matter at finite temperature in the two-flavor Polyakov-Nambu--Jona-Lasinio model. Using the mean field approximation and an electric field independet regularization we show that the effect of temperature and/or electric fields is to partially restore the chiral symmetry. The deconfinement phase transition is slightly affected by the magnitude of the electric field. To this end we show how the effective quark masses and the expectation value of the Polyakov Loop are affected by the electric fields at finite temperatures. As a very interesting result, the pseudocritical temperatures for chiral symmetry restoration and deconfinement decrease as we increase the magnitude of the electric fields, however, both start to increase after some critical value of the electric field.

hep-ph↗

arXiv2019

Neutral meson properties in hot and magnetized quark matter: a new magnetic field independent regularization scheme applied to NJL-type model

A magnetic field independent regularization scheme (zMFIR) based on the Hurwitz-Riemann zeta function is introduced. The new technique is applied to the regularization of the mean-field thermodynamic potential and mass gap equation within the SU(2) Nambu-Jona-Lasinio model in a hot and magnetized medium. The equivalence of the new and the standard MFIR scheme is demonstrated. The neutral meson pole mass is calculated in a hot and magnetized medium and the advantages of using the new regularization scheme are shown.

hep-ph↗