arXiv subjects

Héctor Novales-Sánchez

Publications and source records attributed to Héctor Novales-Sánchez.

13 recordsLinked to original sources

arXiv2026

Charged lepton flavor violating decays $Z\to \ell_α\ell_β$ in the inverse seesaw

After confirmation of the massiveness and mixing of neutrinos, by neutrino oscillation data, the origin of neutrino mass and the occurrence of charged-lepton-flavor non-conservation in nature have become two main objectives for the physics of elementary particles. Taking inspiration from these two matters, we address the $Z$-boson lepton decays $Z\to\ell_α\ell_β$, with $\ell_α\ne\ell_β$, thus violating charged-lepton flavor. We calculate the set of contributing one-loop Feynman diagrams characterized by virtual neutral leptons, both light and heavy, emerged from the inverse seesaw mechanism for the generation of neutrino mass. We obtain general analytical results, which decouple in the limit as $Λ\to\infty$, with $Λ$ the high-energy scale involved in the inverse seesaw mechanism. Our quantitative analyses consider two scenarios of heavy-neutral-lepton mass: (1) the mass spectrum is degenerate; and (2) a more complete framework which includes both degenerate and non-degenerate mass spectra. The quantitative estimations we perform in both scenarios take into account upper bounds on non-unitarity deviations in the lepton mixing matrix and perturbativity of Yukawa couplings. We find that, even though current constraints on $Z\to\ell_α\ell_β$, by the ATLAS Collaboration, remain far from inverse-seesaw contributions, improved sensitivity from in-plans machines, such as the Future Circular Collider and the Circular Electron Positron Collider, shall be able to probe this mass-generating mechanism through the decay $Z\toτe$, whose contributions could be as large as $10^{-7}$.

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arXiv2026

New physics in the $ZZh$ vertex: One-loop contributions from a radiative seesaw model

Precision Higgs physics offers a sensitive window into physics beyond the Standard Model. In parallel, neutrino-oscillation experiments have established the existence of nonzero neutrino masses, thus implying the presence of new physics. Motivated by these facts, we investigate the one-loop contributions of light and heavy Majorana neutrinos to the $ZZh$ vertex within a variant of the type-I seesaw mechanism in which light-neutrino masses vanish at tree level and are then generated radiatively. We analyze the $CP$-conserving and $CP$-violating anomalous couplings which characterize the $ZZh$ vertex and study their phenomenological implications in two relevant kinematic scenarios at future lepton colliders: Higgsstrahlung production and Higgs production via neutral-current vector-boson fusion. We find that $CP$-conserving effects can reach magnitudes of order $10^{-3}$, potentially within projected future experimental sensitivities, whereas $CP$-violating contributions are strongly suppressed, lying well beyond such projections.

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arXiv2025

$WWh$ anomalous couplings at one loop from a seesaw variant of radiatively-induced neutrino masses

After the successful measurement of the Higgs-like particle at the Large Hadron Collider, the determination of whether it corresponds to either the minimal version of the Higgs scalar, provided by the Standard Model, or to some new physics, so far unknown, has become a main objective of the high-energy-physics community. In particular, the investigation of the couplings of this particle with the rest of the Standard-Model field content is a central task. To this aim, in the present paper we consider a variant of the seesaw neutrino-mass-generating mechanism in which the masses of light neutrinos are generated via radiative corrections of the neutrino 2-point function. In this framework, we calculate the contributions from virtual Majorana neutrinos to the $WWh$ vertex, at one loop. A set of anomalous couplings, some of them $CP$-even and the others $CP$-odd, emerge from this calculation. We perform numerical estimations of these anomalous couplings by taking into account the role of the $WWh$ vertex in (1) $Wh$ production from $pp$ collisions and (2) in Higgs production by vector-boson fusion, occurring in some lepton collider. Our estimations show that all the generated anomalous couplings are beyond current experimental sensitivity. However, we note that $CP$-conserving anomalous couplings could be as large as $\sim10^{-3}$, which would be near the reach of the high-luminosity phase of the Large Hadron Collider. On the other hand, $CP$-odd effects, amounting to $\sim10^{-4}$, turn out to be well below expectations for the Compact Linear Collider by about 2 orders of magnitude.

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arXiv2025

Effects of virtual Majorana neutrinos on charged Lepton Flavor Violation decays from a seesaw variant with radiatively induced light neutrino masses

Lepton flavor violating decays $\ell_α \to \ell_β γ$, being forbidden in the Standard Model framework, provide a sensitive probe for new physics. We study these processes in a seesaw variant in which small neutrino masses are generated radiatively. By analyzing the parameter space constrained by electroweak precision data, we investigate the correlation between these decays and non-unitary effects from TeV-scale heavy neutrinos. According to our results, $μ\to e γ$ is the most promising channel for new physics searches, with the bound $|η_{μe}| \lesssim 10^{-6}$ obtained for non-unitary effects in this radiative seesaw variant. Our estimations of $\mathcal{BR} \left( μ\to e γ\right)$, which depends on the mass of the heavy neutrinos, shows that both current and future experimental facilities might be sensitive to these effects.

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arXiv2025

Lepton-flavor changing decays and non-unitarity in the inverse seesaw mechanism

The pursuit for the genuine fundamental description, governing nature at some high-energy scale, must invariably consider the yet-unknown mechanism behind the generation of neutrino mass. Lepton-flavor violating decays $l_α\to l_β\,γ$, allowed in the presence of neutrino mass and mixing, provide a mean to look for physics beyond the Standard Model. In the present work we consider the inverse seesaw mechanism and then revisit the calculation of its contributions to the branching ratios of the aforementioned decay processes. Our analytic results are consistent, as they are gauge invariant, gauge independent, ultraviolet finite, and of decoupling nature. Among the decays $l_α\to l_βγ$, we find $μ\toγ\,e$ to be the most promising, in the light of current bounds by the MEG Collaboration. Deviations from unitarity in the mixing of light neutrinos are related to the branching ratios ${\rm Br}\big( l_α\toγ\,l_β\big)$ in a simple manner, which we address, then finding that the constraints on non-unitarity effects from MEG data will be improved by the upcoming MEG II update by a factor $\sim\frac{1}{3}$.

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arXiv2024

One-loop contributions to $WWZ$ from a seesaw variant with radiatively-induced light-neutrino masses

The experimental confirmation of neutrino mass and mixing disagrees with the picture of the Standard Model, and thus provides a good motivation to look for unknown physics. A seesaw variant in which neutrino masses are radiatively generated, with neutrinos characterized by Majorana fields and where a set of three hypothetical neutral heavy leptons are present, has been considered in the present work. We have calculated, estimated, and analyzed the contributions from virtual light and heavy neutrinos to the gauge vertex $WWZ$, in the context of a future electron-positron collider. We have found that contributions to both $CP$-even and $CP$-odd anomalous couplings are generated. Our estimations indicate that contributions as large as $~10^{-3}$ can be achieved in both cases. This is particularly interesting for the $CP$-odd effects, which are expected to appear in the framework of the Standard Model since the three-loop order, thus being quite suppressed. By considering the expected sensitivity of the International Linear Collider to this gauge vertex, we conclude that these new-physics effects would be within its reach for $e^+e^-$ collisions taking place at a center-of-mass energy of $800\,{\rm GeV}$.

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arXiv2023

Majorana neutrinos in the triple gauge boson coupling $ZZZ^*$

Confirmed by the measurement of neutrino oscillations, neutrino mass is recognized as a genuine manifestation of physics beyond the Standard Model, while its originating mechanism remains a mystery. Moreover, the proper field-theory description of neutrinos, whether they are Majorana or Dirac type, must be linked to such a mechanism. The present work addresses the calculation, estimation, and analysis of one-loop contributions from virtual Majorana neutrinos, light and heavy as well, to the neutral gauge boson coupling $ZZZ$, which participates in $Z$-boson pair production from $e^+e^-$ collisions. This task is carried out in the framework defined by a seesaw variant in which light neutrinos remain massless at tree level, then becoming massive radiatively. The $ZZZ^*$ coupling, with $Z^*$ an off-shell $Z$ boson, is defined by two form factors, namely, $f_4$, characterizing CP-odd effects, and $f_5$, which is CP-even. Constraints from the Large Hadron Collider on both these quantities are currently ${\cal O}(10^{-4})$. Our calculation yields CP-nonpreserving contributions to $ZZZ$, which are absent in the framework of the sole Standard Model. Our estimations show that the $f_4$ contribution might be as large as ${\cal O}(10^{-7})$ for heavy-neutrino masses $\sim1\,{\rm TeV}$. CP-even contributions $f_5$ are also generated, which are, in general, larger than their CP-odd counterparts. We estimate them to be as large as ${\cal O}(10^{-4})$ at a center-of-mass energy of $500\,{\rm GeV}$, in $e^+e^-$ collisions.

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arXiv2023

Lorentz violation in electromagnetic moments of fermions

Lorentz-violating Yukawa couplings lying within the renormalizable part of the SME generate Lorentz-invariant one-loop contributions to electromagnetic moments of fermions. These proceedings provide a discussion on such contributions and present bounds on SME coefficients from experimental data on electromagnetic moments. Constraints as restrictive as $\sim10^{-14}$ are found.

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arXiv2023

New physics in $WWγ$ at one loop via Majorana neutrinos

Current experimental data guarantees the presence of physics beyond the Standard Model in the neutrino sector. The responsible physical description might show itself through virtual effects on low-energy observables. In particular, massive neutrinos are able to produce contributions to the triple gauge coupling $WWγ$. The present paper deals with the calculation, estimation and analysis of one-loop contributions from Majorana neutrinos to the Lorentz-covariant $WWγ$ parametrization. Our calculations show that CP-odd effects vanish exactly, whereas CP-even contributions, $Δκ$ and $ΔQ$, remain. According to our estimations, the effects from heavy neutrinos with masses in the range of hundreds of GeVs dominate over those from light neutrinos. This investigation shows that contributions from heavy Majorana neutrinos to the anomaly $Δκ$ could be as large as $\sim{\cal O}(10^{-3})$, one order of magnitude below the Standard-Model contribution. We find that the International Linear Collider, sensitive to triple gauge couplings participating in $WW$ production, might measure these effects in electron-positron collisions at a center-of-mass energy of $800\,{\rm GeV}$, as long as heavy-neutrino masses are $\gtrsim300\,{\rm GeV}$ and below $\sim1500\,{\rm GeV}$.

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arXiv2022

Lorentz violation in nucleon electromagnetic moments

Yukawa couplings in the Lorentz- and $CPT$-violating Standard Model Extension (SME) induce quantum contributions to the electromagnetic moments of quarks, which are calculated in the present paper and then used to define contributions to electromagnetic moments of nucleons. High-sensitivity measurements of the proton and neutron electromagnetic moments then yield constrains on SME coefficients, reaching bounds as restrictive as $10^{-12}$. This approach notably grants access to SME effects from the second and third quark families.

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arXiv2020

Bounds on Lorentz-violating Yukawa couplings via lepton electromagnetic moments

The effective-Lagrangian description of Lorentz-invariance violation provided by the so-called Standard-Model Extension covers all the sectors of the Standard Model, allowing for model-independent studies of high-energy phenomena that might leave traces at relatively-low energies. In this context, the quantification of the large set of parameters characterizing Lorentz-violating effects is well motivated. In the present work, effects from the Lorentz-nonconserving Yukawa sector on the electromagnetic moments of charged leptons are calculated, estimated, and discussed. Following a perturbative approach, explicit expressions of leading contributions are derived and upper bounds on Lorentz violation are estimated from current data on electromagnetic moments. Scenarios regarding the coefficients of Lorentz violation are considered. In a scenario of two-point insertions preserving lepton flavor, the bound on the electron electric dipole moment yields limits as stringent as $10^{-28}$, whereas muon and tau-lepton electromagnetic moments determine bounds as restrictive as $10^{-14}$ and $10^{-6}$, respectively. Another scenario, defined by the assumption that Lorentz-violating Yukawa couplings are Hermitian, leads to less stringent bounds, provided by the muon anomalous magnetic moment, which turn out to be as restrictive as $10^{-14}$.

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arXiv2020

Quark-flavor-changing Higgs decays from a universal extra dimension

Kaluza-Klein fields characterizing, from a four-dimensional viewpoint, the presence of compact universal extra dimensions would alter low-energy observables through effects determined by some compactification scale, $R^{-1}$, since the one-loop level, thus being particularly relevant for physical phenomena forbidden at tree level by the Standard Model. The present paper explores, for the case of one universal extra dimension, such new-physics contributions to Higgs decays $h^{(0)}\to q^{(0)}_αq^{(0)}_β$, into pairs of quarks with different flavors, a sort of decay process which, in the Standard Model, strictly occurs at the loop level. Finite results, decoupling as $R^{-1}\to \infty$, are calculated. Approximate short expressions, valid for large compactification scales, are provided. We estimate that Kaluza-Klein contributions lie below predictions from the Standard Model, being about 2 to 3 orders of magnitude smaller for compactification scales within $1.4\,{\rm TeV}<R^{-1}<10\,{\rm TeV}$.

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arXiv2019

Effects of universal extra dimensions on top-quark electromagnetic interactions

Universal extra dimensions, presumably observable at some high-energy scale, would modify low-energy observables, being particularly relevant for physical processes forbidden at tree level by the Standard Model. We address the Kaluza-Klein contributions from the 5-dimensional Standard Model to the anomalous magnetic moment and to branching ratios of electromagnetic decays of the top quark. In accordance with present bounds on the compactification scale, contributions to both quantities are found to be at least 3 orders of magnitude below Standard-Model predictions.

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