arXiv subjects

Gustavo P. de Brito

Publications and source records attributed to Gustavo P. de Brito.

At least 19 recordsLinked to original sources

arXiv2026

Charting the different phases of Yang-Mills-Chern-Simons-Higgs theories

We explore Yang-Mills-Chern-Simons theories coupled to a Higgs-like field in the fundamental representation of $SU(2)$ quantized in linear covariant gauges in three Euclidean dimensions. We analyze the modifications in the analytic structure of the gluon propagator due to the elimination of infinitesimal Gribov copies. The interplay between the Higgs, the Chern-Simons and Gribov mass parameters is investigated. Two different phases are identified: a confining one, where all poles are complex, and a deconfined one, where would-be physical gluon excitation can appear. Unlike previous works, the Gribov parameter is consistently fixed by its gap equation as a function of the other mass parameters and the gauge coupling. This imposes a constraint in the parameter space and makes transparent how the competition of the mass parameters affects the relevance of the Gribov parameter for the characterization of the spectrum of the theory.

hep-th↗

arXiv2026

On the structure of interactions of mass dimension one fermions: a functional renormalization group perspective

In this paper, we provide the first systematic investigation of renormalization group properties of mass dimension one fermions described by ELKO spinors. By construction, ELKOs must be neutral under any Standard Model charge, therefore, providing a natural candidate for dark matter. We consider two versions of scalar-ELKO systems: the first characterized by a derivative Yukawa-like interaction, while the second involves ELKO four-fermion interactions as well as a scalar-ELKO portal. We also considered a system composed of ELKOs interacting with an Abelian gauge field via Pauli-like term. In all cases, we identified the minimal set of interactions that are required by a consistent renormalization group flow, and we discussed the possibility of constructing UV-complete trajectories based on asymptotic freedom. We used the functional renormalization group as a method of investigation.

hep-th↗

arXiv2025

Yukawa interactions in Quantum Gravity

We present the first complete next-to-leading-order analysis of a Yukawa system within the framework of asymptotically safe quantum gravity. Our results are obtained through a systematic resummation of higher-order operators, revealing two distinct resummation mechanisms -- one of which has not been explored previously. In addition, we introduce a novel approach to estimate systematic uncertainties by simulating the impact of neglected higher-order contributions. We demonstrate that quantum gravity fluctuations anti-screen Yukawa interactions, thereby resolving previously inconclusive leading-order results. This anti-screening mechanism enables the generation of finite interactions from an asymptotically free Yukawa fixed point. Consequently, our findings provide strong evidence that non-vanishing Yukawa couplings are compatible with asymptotically safe quantum gravity, which is a necessary requirement for the Standard Model to emerge from an asymptotically safe ultraviolet completion.

hep-th↗

arXiv2025

Quark Propagator at one-loop in the Refined Gribov-Zwanziger framework

The Refined Gribov-Zwanziger scenario is a local and renormalizable setup in which infinitesimal Gribov copies are eliminated and further non-perturbative effects are accounted for. The gluon propagator that arises from this framework fits lattice data very well in the Landau gauge. We investigate the coupling of quarks to this setting at one-loop order by computing the quark propagator. The fermionic sector is introduced by a minimal coupling and the non-perturbative effects are transmitted to the matter sector through gluonic loops which, in this case, carry information from the elimination of infinitesimal Gribov copies and the formation of condensates. We compare our findings with available lattice data both for the unquenched gluon propagator as well as for the quark propagator in the Landau gauge. Our results are comparable with those obtained in the Curci-Ferrari model at one-loop order. In particular, we are able to fit the unquenched gluon propagator and use the fixed parameters to predict the quark mass function and find good agreement with lattice data. However, the quark dressing function does not agree, even at a qualitative level, with lattice data in the infrared. This is agreement with the analogue computation in Curci-Ferrari model. Inspired by the developments in the Curci-Ferrari results, such a disagreement is likely to be cured by the inclusion of two-loops corrections. Finally, we compare the present minimal coupling with a non-perturbative matter coupling proposed in the Refined Gribov-Zwanziger literature.

hep-th↗

arXiv2025

Neutrino mass generation in asymptotically safe gravity

There exist several distinct phenomenological models to generate neutrino masses. We explore, which of these models can consistently be embedded in a quantum theory of gravity and matter. We proceed by invoking a minimal number of degrees of freedom beyond the Standard Model. Thus, we first investigate whether the Weinberg operator, a dimension-five-operator that generates neutrino masses without requiring degrees of freedom beyond the Standard Model, can arise in asymptotically safe quantum gravity. We find a negative answer with far-reaching consequences: new degrees of freedom beyond gravity and the Standard Model are necessary to give neutrinos a mass in the asymptotic-safety paradigm. Second, we explore whether the type-I Seesaw mechanism is viable and discover an upper bound on the Seesaw scale. The bound depends on the mass of the visible neutrino. We find a numerical value of $10^{14}\, \rm GeV$ for this bound when neglecting neutrino mixing for a visible mass of $10^{-10}\, \rm GeV$. Conversely, for the most ``natural" value of the Seesaw scale in a quantum-gravity setting, which is the Planck scale, we predict an upper bound for the neutrino mass of the visible neutrino of approximately $10^{-15}\, \rm GeV$. Third, we explore whether neutrinos could also be Pseudo-Dirac-neutrinos in asymptotic safety and find that this possibility can be accommodated.

hep-ph↗

arXiv2024

Infrared gluon propagator in the Refined Gribov-Zwanziger scenario at one-loop order in the Landau gauge

The Refined Gribov-Zwanziger (RGZ) action in the Landau gauge provides a local and renormalizable framework to account for the existence of infinitesimal Gribov copies in the path integral together with other relevant infrared effects such as the formation of condensates. The properties of the tree-level gluon propagator obtained in this setup has been thoroughly investigated over the past decade. It accommodates important properties seen in lattice simulations such as a finite value at vanishing momentum and positivity violation. Yet a comprehensive study about the stability of such properties against quantum corrections was lacking. In this work, we compute the gluon propagator in the RGZ scenario at one-loop order and implement an appropriate renormalization scheme in order to compare our findings with lattice data. Remarkably, the qualitative properties of the tree-level gluon propagator are preserved. In particular, the fits with lattice data show evidence for positivity violation and the existence of complex poles for SU(2) and SU(3) gauge groups. We comment on the results for the ghost propagator as well.

hep-th↗

arXiv2024

Ruling out models of vector dark matter in asymptotically safe quantum gravity

The nature of dark matter is a problem with too many potential solutions. We investigate whether a consistent embedding into quantum gravity can decimate the number of solutions to the dark-matter problem. Concretely, we focus on a hidden sector composed of a gauge field and a charged scalar, with gauge group U(1)$_{\textmd{D}}$ or SU(2)$_\textmd{D}$. The gauge field is the dark matter candidate, if the gauge symmetry is broken spontaneously. Phenomenological constraints on the couplings in this model arise from requiring that the correct dark matter relic density is produced via thermal freeze-out and that recent bounds from direct-detection experiments are respected. We find that the consistent embedding into asymptotically safe quantum gravity gives rise to additional constraints on the couplings at the Planck scale, from which we calculate corresponding constraints at low energy scales. We discover that phenomenological constraints cannot be satisfied simultaneously with theoretical constraints from asymptotically safe quantum gravity, ruling out these dark matter models.

hep-ph↗

arXiv2023

Light fermions in color: why the quark mass is not the Planck mass

We investigate whether quantum gravity fluctuations can break chiral symmetry for fermions that are charged under a $U(1)$ and an $SU(N_\text{c})$ gauge symmetry and thus closely resemble Standard-Model fermions. Unbroken chiral symmetry in the quantum-gravity regime is a necessary prerequisite to recover the Standard Model from a joint gravity-matter theory; if chiral symmetry is broken by quantum gravity, fermions cannot generically be much lighter than the Planck mass and the theory is ruled out. To answer this, we work in a Fierz-complete basis of four-fermion interactions and explore whether they are driven to criticality. We discover that the interplay of quantum gravity with the non-Abelian gauge theory results in chiral symmetry breaking, because gravitational and gauge field fluctuations act together to produce bound states. Chiral symmetry breaking is triggered by four-fermion channels that first appear when non-Abelian charges are introduced and that become critical if the non-Abelian symmetry is gauged. Extrapolating our result to the Standard Model fermions, we conjecture that the non-Abelian gauge coupling, Abelian gauge coupling and Newton coupling are all bounded from above, if Standard Model fermions are to remain much lighter than the Planck mass. In contrast, fermions that are charged under a global non-Abelian group can remain light for arbitrarily large values of the Newton coupling. We find that different chiral symmetries are emergent at low energies, depending on the strength of the gravitational coupling. This is an example of fixed points with different degrees of enhanced global symmetry trading stability in fixed-point collisions driven by gravitational fluctuations.

hep-th↗

arXiv2023

Quadratic gravity in analogy to quantum chromodynamics: Light fermions in its landscape

We investigate a non-perturbative approach to quantum gravity built in terms of analogies between quadratic gravity and quantum chromodynamics. This approach is based on a conjectured phase transition between quadratic gravity in the trans-Planckian regime and an effective field theory, with general relativity as the leading-order term, below the Planck scale. We point out that not all aspects of the analogy between quadratic gravity and quantum chromodynamics are desired. A possible mechanism of chiral symmetry breaking driven by quantum gravity fluctuations could make this setup incompatible with our observed Universe. Here, we put forward a first investigation of chiral symmetry breaking in the context of quadratic gravity. We find indication that gravity, despite being an attractive force, does not trigger chiral symmetry breaking in a non-perturbative regime. This result is based on a (functional) renormalization group analysis of four-fermion interactions coupled to quadratic gravity. We also comment on the particularities associated with the single-flavor case. In summary, the analogy between quadratic gravity and chromodynamics passes this first phenomenological viability test.

hep-th↗

arXiv2023

Refined Gribov-Zwanziger theory coupled to scalar fields in the Landau gauge

The Refined Gribov-Zwanziger (RGZ) action in the Landau gauge accounts for the existence of infinitesimal Gribov copies as well as the dynamical formation of condensates in the infrared of Euclidean Yang-Mills theories. We couple scalar fields to the RGZ action and compute the one-loop scalar propagator in the adjoint representation of the gauge group. We compare our findings with existing lattice data. The fate of BRST symmetry in this model is discussed, and we provide a comparison to a previous proposal for a non-minimal coupling between matter and the RGZ action. We find good agreement with the lattice data of the scalar propagator for the values of the mass parameters that fit the RGZ gluon propagator to the lattice. This suggests that the non-perturbative information carried by the gluon propagator in the RGZ framework provides a suitable mechanism to reproduce the behavior of correlation functions of colored matter fields in the infrared.

hep-th↗

arXiv2023

Fate of chiral symmetry in Riemann-Cartan geometry

We study the mechanism of chiral symmetry breaking for fermionic systems in a gravitational background with curvature and torsion. The analysis is based on a scale-dependent effective potential derived from a bosonized version of the Nambu-Jona-Lasino model in a Riemann-Cartan background. We have investigated the fate of chiral symmetry in two different regimes. First, to gain some intuition on the combined effect of curvature and torsion, we investigate the regime of weak curvature and torsion. However, this regime does not access the deep infrared limit, which is essential to answer questions related to the mechanism of gravitational catalysis in fermionic systems. Second, we look at the regime of vanishing curvature and homogeneous torsion. In this case, although we cannot probe the combined effects of curvature and torsion, we can access the deep infrared contributions of the background torsion to the mechanism of chiral symmetry breaking. Our main finding is that, in the scenario where only torsion is present, there is no indication of a mechanism of gravitational catalysis.

hep-th↗

arXiv2023

Towards a bound on the Higgs mass in causal set quantum gravity

In the Standard Model of particle physics, the mass of the Higgs particle can be linked to the scale at which the Standard Model breaks down due to a Landau pole/triviality problem: for a Higgs mass somewhat higher than the measured value, the Standard Model breaks down before the Planck scale. We take a first step towards investigating this relation in the context of causal set quantum gravity. We use a scalar-field propagator that carries the imprints of spacetime discreteness in a modified ultraviolet behavior that depends on a nonlocality scale. We investigate whether the modification can shift the scale of the Landau pole in a scalar field theory with quartic interaction. We discover that the modifications speed up the onset of the Landau pole considerably, so that the scale of new physics occurs roughly at the nonlocality scale. Our results call into question, whether a separation between the nonlocality scale and the discreteness scale, which is postulated within causal set quantum gravity, and which has been argued to give rise to phenomenological consequences, is in fact achievable. Methodologically, our paper is the first to apply continuum functional Renormalization Group techniques in the context of a causal-set inspired setting.

gr-qc↗

arXiv2023

On the weak-gravity bound for a shift-symmetric scalar field

The weak-gravity bound has been discovered in several asymptotically safe gravity-matter systems. It limits the strength of gravitational fluctuations that are compatible with an ultraviolet-complete matter sector, and results from the collision of two partial fixed points of the matter system as a function of the strength of the gravitational interactions. In this paper, we will investigate this mechanism in detail for a shift-symmetric scalar field. First, we will study the fixed point structure of the scalar system without gravity. We find indications that the Gaussian fixed point is the only viable fixed point, suggesting that a weak-gravity bound resulting from the collision of two partial fixed points is a truncation artefact. We will then couple the scalar system to gravity and perform different expansions to track the Gaussian fixed point as gravitational fluctuations become stronger. We also introduce a new notion of the weak-gravity bound that is based on the number of relevant operators.

hep-th↗

arXiv2022

Are there ALPs in the asymptotically safe landscape?

We investigate axion-like particles (ALPs) in the context of asymptotically safe gravity-matter systems. The ALP-photon interaction, which facilitates experimental searches for ALPs, is a dimension-5-operator. Quantum fluctuations of gravity lower its scaling dimension, and the ALP-photon coupling can become asymptotically free or even asymptotically safe. However, quantum fluctuations of gravity need to be strong to overcome the canonical scaling and this strong-gravity regime is in tension with the weak-gravity bound in asymptotic safety. Thus, we tentatively conclude that fundamental ALPs can likely not be accommodated in asymptotically safe gravity-matter systems. In turn, an experimental discovery of an ALP would thus shed valuable light on the quantum nature of gravity.

gr-qc↗

arXiv2022

Nonvanishing gravitational contribution to matter beta functions for vanishing dimensionful regulators

We explore the effect of quantum gravity on matter within a Renormalization Group framework. First, our results provide an explicit example of how misleading conclusions can be drawn by analyzing the gravitational contributions to beta functions, instead of analyzing universal quantities, such as critical exponents, that can be extracted from the beta functions. This could be key to explain differences between perturbative studies and Functional Renormalization Group studies. Second, we strengthen the evidence that asymptotically safe gravity could generate a predictive ultraviolet completion for matter theories with gauge interactions, even in the limit of vanishing dimensionful regulator function. We also find that the situation can be more subtle with higher-order, gravity-induced matter interactions.

hep-th↗

arXiv2022

The weak-gravity bound and the need for spin in asymptotically safe matter-gravity models

We discover a weak-gravity bound in scalar-gravity systems in the asymptotic-safety paradigm. The weak-gravity bound arises in these systems under the approximations we make, when gravitational fluctuations exceed a critical strength. Beyond this critical strength, gravitational fluctuations can generate complex fixed-point values in higher-order scalar interactions. Asymptotic safety can thus only be realized at sufficiently weak gravitational interactions. We find that within truncations of the matter-gravity dynamics, the fixed point lies beyond the critical strength, unless spinning matter, i.e., fermions and vectors, is also included in the model.

gr-qc↗

arXiv2021

Light charged fermions in quantum gravity

We study the impact of quantum gravity on a system of chiral fermions that are charged under an Abelian gauge group. Under the impact of quantum gravity, a finite value of the gauge coupling could be generated and in turn drive four-fermion interactions to criticality. We find indications that the gravity-gauge-fermion interplay protects the lightness of fermions for a large enough number of fermions. On the other hand, for a smaller number of fermions, chiral symmetry may be broken, which would be in tension with the observation of light fermions.

hep-th↗

arXiv2021

Can quantum fluctuations differentiate between standard and unimodular gravity?

We formally prove the existence of a quantization procedure that makes the path integral of a general diffeomorphism-invariant theory of gravity, with fixed total spacetime volume, equivalent to that of its unimodular version. This is achieved by means of a partial gauge fixing of diffeomorphisms together with a careful definition of the unimodular measure. The statement holds also in the presence of matter. As an explicit example, we consider scalar-tensor theories and compute the corresponding logarithmic divergences in both settings. In spite of significant differences in the coupling of the scalar field to gravity, the results are equivalent for all couplings, including non-minimal ones.

gr-qc↗