arXiv subjects

R. Bufalo

Publications and source records attributed to R. Bufalo.

At least 19 recordsLinked to original sources

arXiv2026

Causality Violating Solutions in Curvature-Squared Gravity

In this paper, we consider some causality violating solutions in the curvature-squared gravity in order to examine whether closed timelike curves (CTCs) are allowed in these models. These aspects are studied in terms of the Gödel, Gödel-type and axially symmetric cosmological solutions. We observe that the Gödel and Gödel-type metrics are causal solutions of the model so that CTCs are now allowed and, surprisingly, every contribution involving the Weyl tensor is removed from the solutions. Hence, in order to study the effect (if any) of the Weyl tensor (an conformal symmetry) into CTCs a third metric is considered. In this case, we obtain contributions due to the Weyl tensor to the energy density and led to modifications of the weak energy condition.

gr-qc↗

arXiv2026

Thermal Bhabha scattering under the influence of non-hermiticity effects

In this paper, we investigate the Bhabha scattering process within the framework of non-Hermitian QED at finite temperature. In this theory, the hermiticity condition, typically required in quantum field theory to ensure the reality of physical observables, is replaced by the condition of unbroken $PT$-symmetry which favors the introduction of an axial mass and a vector-axial gauge coupling. Using the Thermofield Dynamics formalism, we derive and comprehensively analyze the thermal differential cross section for the Bhabha scattering. Furthermore, we explore the high-energy limit of the scattering amplitude and establish constraints upon the axial coupling constant, offering valuable insights into the system's behavior under extreme conditions.

hep-ph↗

arXiv2025

Gödel-type universes in unimodular gravity

In this paper, the Gödel-type universes are examined within the framework of unimodular gravity. Since the existence of Gödel solutions is intrinsically related to the presence of a cosmological constant in general relativity, one can naturally wonder how the acausal structure of the Gödel solutions behaves in different cosmological scenarios. One of the simplest, but important, frameworks within this context is the unimodular gravity, in which the cosmological constant emerges as an integration constant rather than a coupling constant. Hence, the validity of Gödel-type solutions is scrutinized within the unimodular approach, examining whether this theory can address the known issue of causality violation in Gödel universes. In detail, it is demonstrated that for certain gravitational sources, both causal and non-causal regions are permissible.

gr-qc↗

arXiv2025

Non-Hermitian electron-positron annihilation under thermal effects

In this paper we examine the thermal effects into the $e^{+}e^{-}\to \ell^{+}\ell^{-}$ scattering in a non-hermitian extension of QED. We compute the thermal contributions to this scattering cross-section within the Thermo Field Dynamics approach. In order to highlight the non-hermitian effects we have considered some limits of interest: i) zero-temperature limit and high-energy limit and ii) high-temperature regime. Since this type of scattering possesses accurate experimental data for the cross-section (for muon and tau at the final state) it can be used to set stringent bounds upon the non-hermitian parameters.

hep-ph↗

arXiv2024

On the photon mass generation in Rarita-Schwinger QED

This work examines the dynamical mass generation for the photon in Rarita-Schwinger QED. We focus our attention on the cases of $ω=2,3$ dimensional spacetime. In these frameworks, it is well known that in the usual QED, the photon field (dynamically) acquires a gauge invariant mass (the Schwinger and Chern-Simons mass, respectively). We wish to scrutinize this phenomenon in terms of the Rarita-Schwinger fields. The presence of higher-derivative terms is shown as the leading contributions to the $1$PI function $\langle AA \rangle$ at one-loop order. We study the pole structure of the photon's complete propagator to unveil the main effects of the Rarita-Schwinger fields on the photon's mass. In addition, we present some remarks about the renormalizability of this model (in different dimensions) due to the presence of higher-derivative corrections at one-loop.

hep-th↗

arXiv2024

On the gauge invariance of the higher-derivative Yang-Mills-Chern-Simons action

In this work, we study the perturbative generation of the gauge invariant effective action for the non-Abelian gauge field in a $(2+1)$-dimensional spacetime. We present a detailed analysis of the two, three and four-point functions in order to determine the non-Abelian Chern-Simons terms (parity odd) and Yang-Mills terms (parity even). Moreover, these terms are supplemented by the higher-derivative corrections which resulted in the Alekseev-Arbuzov-Baikov effective action (parity even) plus the higher-derivative (HD) corrections to the Chern-Simons terms (parity odd). In order to highlight some features about the perturbative generation of the effective action, we present a discussion based on the dimensional analysis, which allows us to establish the general structure of the permissible terms to guarantee the gauge invariance of the higher-derivative parts.

hep-th↗

arXiv2023

Thermal Pair Production from Photon-Photon Collision: Breit-Wheeler Process at Finite Temperature

In this paper we examine the pair production through the Breit-Wheeler process $γ~γ\to e^+ e^-$ in a thermal background. We compute the thermal contribution to the Breit-Wheeler differential cross section within the thermofield dynamics formalism. We evaluate in details the cross section for this process, which possess a surprisingly simple expression valid for any temperature $β$, from which we discuss some physically relevant aspects. We also consider the high temperature regime of the cross section in order to have a better understanding about its thermal behavior.

hep-ph↗

arXiv2023

Structure Formation in Non-local Bouncing Models

In this study, we investigate the growth of structures within the Deser-Woodard nonlocal theory and extend it to various bouncing cosmology scenarios. Our findings show that the observable structure growth rate, $fσ_8$, in a vacuum-dominated universe is finite within the redshift range of $0<z<2$, contrary to previous literature. Although $fσ_8$ exhibits no divergences, we observe a slight difference between the evolution of the $Λ$CDM and the non-local DW II models. Regarding structure formation in bouncing cosmologies, we evaluate the evolution of $fσ_8$ near the bouncing point. Among the different bouncing cases we explore, the oscillatory bounce and pre-inflationary asymmetrical bounce demonstrate a physical profile where the growth rate begins as a small perturbation in the early epoch and increases with inflation, which can be regarded as the seeds of large-scale structures. These findings are significant because they shed light on the growth of seed fluctuations into cosmic structures resulting from non-local effects.

gr-qc↗

arXiv2023

Induced CP-violation in the Euler-Heisenberg Lagrangian

In this paper, we examine the behaviour of the Euler-Heisenberg effective action in the presence of a novel axial coupling among the gauge field and the fermionic matter. This axial coupling is responsible to induce a CP-violating term in the extended form of the Euler-Heisenberg effective action, which is generated naturally through the analysis of the box diagram. However, this anomalous model is not a viable extension of QED, and we explicitly show that the induced CP-violating term in the Euler-Heisenberg effective Lagrangian is obtained only by adding an axial coupling to the ordinary QED Lagrangian. In order to perform our analysis, we use a parametrization of the vector and axial coupling constants, $g_{v}$ and $g_{a}$, in terms of a new coupling $β$. Interestingly, this parametrization allows us to explore a hidden symmetry under the change of $g_{v}\leftrightarrow g_{a}$ in some diagrams. This symmetry is explicitly observed in the analysis of the box diagram, where we determine the $λ_i$ coefficients of $\cal{L}_{\rm ext.}^{\rm \small EH}=λ_{1}\cal{F}^{2}+λ_{2}\cal{G}^{2}+λ_{3}\cal{F}\cal{G}$, specially the coefficient $λ_3$ related with the CP-violating term due to the axial coupling. As a phenomenological application of the results, we compute the relevant cross section for the light by light scattering through the extended Euler-Heisenberg effective action.

hep-th↗

arXiv2022

Induced non-Abelian Chern-Simons effective action in very special relativity

In this paper, we study the one-loop induced effective action for a non-abelian gauge field in the very special relativity (VSR) framework in a $(2+1)$-dimensional spacetime. We show that there are new graphs contributing to the amplitudes of gluon $n$-point functions, e.g. $\langle GG\rangle$, $\langle GGG \rangle$ and $\langle GGGG \rangle$, originating from the non-local couplings generated in VSR. Using the one-loop analysis of the relevant graphs, we evaluate the VSR corrections to the induced kinetic term of the non-abelian gauge field action in three dimensions. Next, by applying the Ward identity, we determine the general tensorial structure for the amplitude of $\langle GG\rangle$ in VSR, which is valid at any order of the loop analysis. Moreover, we discuss the leading VSR corrections to the non-abelian Chern-Simons and Yang-Mills action through the analysis of $\langle GG\rangle$, $\langle GGG \rangle$ and $\langle GGGG \rangle$. In order to highlight the VSR effects, we present the VSR modification of non-abelian Chern-Simons effective action and then obtain the equation of motion for the non-abelian gauge field. As a result, we observe that the well-known pure gauge solution does not hold in the presence of VSR effects.

hep-th↗

arXiv2022

Vacuum Cherenkov radiation at finite temperature

In this paper we examine the thermal effects of the vacuum Cherenkov radiation in a Lorentz- and CPT-violating electrodynamics. We compute the thermal contribution to the Cherenkov radiation rate within the Thermofield Dynamics approach. Since the model under consideration possess a consistent canonical quantization and also fulfils the physical constraints in order to this vacuum process to happen, it is a perfect candidate to implement the study at finite temperature. We evaluate in details the instantaneous rate of energy loss for a charge, and show that the radiation rate is significantly modified at very high temperatures. Intriguingly, we further observe that when the temperature goes to infinity the radiation rate goes to zero even if the process is kinematically allowed.

hep-th↗

arXiv2022

Non-local gravity in bouncing cosmology scenarios

In this work, we analyzed the improved Deser-Woodard non-local gravity over the background of five different bouncing cosmologies, whose premise is avoid the initial singular state of the universe. We developed the numerical solution for the non-local distortion function, which encompass the modifications to the Einstein-Hilbert action, using the reconstruction procedure and we have found that they have a viable cosmological solution. Afterwards, we discussed the physical aspects and outcomes of the evolution of the distortion function throughout the bouncing point for these models, specifically: the symmetric bounce, oscillatory bounce, the matter bounce, finite time singularity model, and the pre-inflationary asymmetrical bounce.

gr-qc↗

arXiv2022

Impossibility of obtaining a CP-violating Euler-Heisenberg effective theory from a viable modification of QED

In this paper, we examine the CP-violating term of the Euler-Heisenberg action. We focus in the aspects related with the generation of such a term from a QED-like model in terms of the effective action approach. In particular, we show that the generation of the CP-violating term is closely related with both of vector and axial fermionic bilinears. Although, these anomalous models are not a "viable" extension of QED, we argue that the CP-violating term in the photon sector is obtained only from this class of models, and not from any fundamental field theory.

hep-ph↗

arXiv2022

Path integral analysis of the axial anomaly in Very Special Relativity

In this note, we revise the problem of the axial anomaly in the framework of very special relativity following Fujikawa's path integral approach. We show nonperturbatively that no VSR contribution is present in the path integral measure in $(3+1)$-dimensional spacetime. Furthermore, we extend our results to $(1+1)$ dimensions, as well as to the two-dimesional curved spacetime.

hep-th↗

arXiv2021

A stochastic least action principle applied in the description of black swan events

In this paper we present a formulation of the stochastic least action principle (SAP) to encompass random movements with black swan events (of non dissipative systems) in terms of heavy tailed distributions. The black swan events are rare and drastic events, such as earthquakes and financial crisis. It has been observed that the Tsallis entropy suits well in the description of the black swan events rather than the Shannon-Boltzman-Gibbs entropy, which is intrinsically related to the fact that black swan events of physical systems are proportional to non-local correlations. As a consequence, we could assess the validity of the path probability distribution obtained using the non-additive Tsallis entropy.

cond-mat.stat-mech↗

arXiv2021

Vacuum Radiation in $z=2$ Lifshitz QED

We discuss in this paper the vacuum Cherenkov radiation in the $z=2$ Lifshitz electrodynamics. The improved ultraviolet behavior, in terms of higher spatial derivatives, and the renormalizable couplings, due to the time-space anisotropic scaling, present in the Lifshitz setting are extremely important in fulfilling the physical constraints in order to this vacuum process to happen. We evaluate in details the instantaneous rate of energy loss for a charge, and also analyze the emission of very soft photons in this framework.

hep-th↗

arXiv2021

Amorphous solids as fuzzy crystals: A Debye-like theory of low-temperature specific heat

We construct a quantum mechanical model of perfectly isotropic amorphous solids as fuzzy crystals and establish an analytical theory of vibrations for glasses at low temperature. Our theoretical framework relies on the basic principle that the disorder in a glass is similar to the disorder in a classical fluid, while the latter is mathematically encoded by noncommutative coordinates in the Lagrange formulation of fluid mechanics. We find that the density of states in the acoustic branches flattens significantly, leading naturally to a boson peak in the specific heat as a manifestation of a van Hove singularity. The model is valid in the same range as Debye's theory, namely up to circa 10% of the Debye temperature. Within this range, we find an excellent agreement between the theoretical predictions and the experimental data for two typical glasses, a-GeO$_2$ and Ba$_{8}$Ga$_{16}$Sn$_{30}$-clathrate. Our model supports the conceptual understanding of the nature of the boson peak in glasses as a manifestation of the liquid-like disorder. At the same time, it provides a novel, mathematically simple framework for a unitary treatment of thermodynamical and transport properties of amorphous materials and a solid background for inserting further elements of complexity specific to particular glasses.

cond-mat.dis-nn↗

arXiv2021

Adler-Bell-Jackiw anomaly in VSR electrodynamics

In this paper, we examine the problem of anomalies of the fermionic currents in the context of the very special relativity (VSR). We consider the VSR contributions to the triangle amplitude $\left\langle J_{5}^λ J^μ J^ν \right\rangle $, which allows the evaluation of the vector and axial Ward identities. Actually, we observe that the VSR nonlocal effects respect the vector Ward identity, and it also contributes in a very interesting and unique way for the Adler-Bell-Jackiw anomaly.

hep-th↗