arXiv subjects

R. Gaitán

Publications and source records attributed to R. Gaitán.

16 recordsLinked to original sources

arXiv2026

Searching for the LFV $γγeμ$ interaction at future $e^- e^+$ colliders

We investigate the Lepton Flavor Violating (LFV) process $e^{+}e^{-}\to e^{+}e^{-}eμ$ ($e=e^-,\,e^+,\,μ=μ^-,\,μ^+ $) at future circular colliders, probing the effective $γγeμ$ interaction through photon fusion. Within an Effective Field Theory (EFT) framework compatible with theoretical and experimental constraints, we identify regions in the parameter space of the effective couplings where the signal could be observed at the Circular Electron-Positron Collider (CEPC) at $\sqrt{s}=240\ \text{GeV}$ and the Future Circular Collider (FCC-ee) at $\sqrt{s}=240$ and $350\ \text{GeV}$. Our analysis leverages distinctive kinematic distributions -- particularly the transverse momentum of the scattered electron $p_{T}^{(e^{-}\ \text{scattered})}$ and the central muon $p_{T}^μ$ -- to achieve efficient signal-background separation. By employing a neural network classifier, we enhance the sensitivity beyond traditional cut-based methods, demonstrating the discovery potential of these facilities for LFV searches in the clean environment of $e^{+}e^{-}$ collisions.

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arXiv2025

Lepton Flavor Violating Higgs Decays in a Minimal Doublet Left-Right Symmetric Model with an Inverse Seesaw

In this study, we analyse the lepton flavor violation (LFV) decays within the framework of the Doublet Left-Right Symmetric model (DLRSM), based on the gauge group SU\left(2\right)_{L}\otimes SU\left(2\right)_{R}\otimes U\left(1\right)_{B-L}. The model features an extended gauge and scalar sector, including a bidoublet and two doublets which induce new charged currents interactions. Spontaneous Symmetry Breaking (SSB) occurs in two stages, introducing a new scale associated with the vacuum expectation value (VEV) of the right-handed doublet v_{R} assumed to lie above the electroweak scale. Neutrino masses are generated via the inverse seesaw mechanism, allowing sizeable mixing between active and sterile neutrinos. We diagonalize the full neutrino mass matrix and express the mixing in terms of physical parameters. We compute the branching ratios for LFV Higgs decays as functions of the heavy neutrino mass scale. Our numerical analysis incorporates current experimental bounds and projected sensitivities, highlighting viable regions of parameter space where LFV signals could be observed at future colliders.

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arXiv2025

Non-diagonal contributions to $ZγV^\ast$ vertex, polarizations and bounds on $Z\overline{t}q$ couplings

We study the flavor-changing neutral current contributions to the trilinear neutral gauge boson couplings $ZγV^\ast$ ($V=Z$, $γ$) at the one-loop level. Only the $CP$-conserving form factor $h_3^Z$ is induced, with the relevant contributions arising from top quark couplings. To our numerical analysis, constraints on the $Z\overline{t}q$ ($q=c$, $u$) couplings are obtained from current LHC data, leading to bounds of $|g_{\text{V}}^{tu}|\text{,}|g_{\text{A}}^{tu}|\leqslant 0.007$ and $|g_{\text{V}}^{tc}|\text{,}|g_{\text{A}}^{tc}|\leqslant 0.0095$ for the vector and axial couplings. The new contributions to the $h_3^Z$ form factor are of order $10^{-7}$. Furthermore, we analyze the polarized partial decay widths of the $V^\ast\rightarrow Zγ$ process, and a new type of left-right asymmetry is also discussed. We find that in the presence of new physics, the polarized $Γ(V^\ast\rightarrow Zγ)$ exhibits significant deviations from the SM prediction, resulting in a non-zero asymmetry.

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arXiv2025

$CP$ violation in the $HZZ$ vertex and left-right asymmetries

We investigate new contributions to the $HZZ$ vertex from the Flavor Changing Neutral Current (FCNC) involving the Higgs and $Z$ bosons. Our calculations reveal that the form factors $h_2^V$ and $h_3^V$ ($V=H$, $Z$) can be induced through these couplings, and we present our results in terms of the Passarino-Veltman scalar functions. Using the current limits on $H\overline{t}c$ and $Z\overline{t}c$ couplings, we determine that the new contributions to the $CP$-conserving form factor $h_2^V$ are small compared to the Standard Model (SM) predictions. However, for the $CP$-violating form factor $h_3^V$, the contributions can reach values as high as $10^{-6}$, five orders of magnitude larger than in the SM. Furthermore, we examine how these results influence the left-right asymmetries in the processes $H^\ast\to ZZ$ and $Z^\ast\to ZH$. Our findings suggest that significant deviations from SM predictions may occur when considering FCNC contributions.

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arXiv2025

Two dark matter candidates in a doublet-triplet Higgs model

We study a Standard Model extension that provides a bicomponent dark matter scenario as well as a mechanism for the generation of left-handed neutrino masses. We extend the Standard Model scalar sector by adding an inert $SU(2)_L$ doublet with hypercharge $Y= 1/2$ and a triplet with hypercharge $Y=0$. These scalars provide dark matter candidates in two dark sectors stabilised by discrete symmetries. We consider the contribution of both candidates to the total relic abundance in order to recover the desert regions in their standard alone cases. In addition, we add an active scalar $SU(2)_L$ triplet with hypercharge $Y=1$ in order to generate light neutrino masses. We analyse the results of dark matter phenomenology for the model and the neutrino mass generation through the type-II seesaw mechanism.

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arXiv2025

Light quark contributions to Higgs decays

The literature establishes that the light fermions contributions to the decays $H\to Zγ$ and $H\toγγ$ are negligible since their coupling with the Higgs is proportional to $m_f$. In the present letter, we show that although such a conclusion is true for leptons, the light quark contributions are zero when we consider their non-perturbative effects.

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arXiv2025

$H\rightarrow Zγ$ decay and $CP$ violation

This study examines the impact of $CP$-violation on the signal strength $μ^{Zγ}$, which was reported as $2.2\pm 0.7$ by the LHC. We obtain constraints on the real and absorptive parts of the $CP$-violating form factor $h_3^{Zγ}$ and find that they are less than 1.15 GeV. Additionally, we revisit the leading order Standard Model contributions to the $H\rightarrow Zγ$ decay and calculate contributions to $h_3^{Zγ}$ from FCNC complex couplings mediated by the $Z$ and $H$ bosons. By using the current bounds on such couplings, we find that the FCNC contribution to $h_3^{Zγ}$ with top and charm quarks in the loop is of order $10^{-5}$ GeV. While in a model with new quarks that preserves the SM predictions on Higgs decays, the $CP$-violating form factor $h_3^{Zγ}$ can be of order $10^{-1}$ GeV and could explain the excess on the signal strength $μ^{Zγ}$.

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arXiv2024

Lepton Flavor-Violating Higgs Decays Mediated by Ultralight Gauge Boson

We present an analysis of the lepton-flavor violating decay of the Higgs boson mediated by an ultralight gauge boson, $χ$. Our analysis matches a model generating the lepton flavor-violating interaction $\bar{\ell}_i\ell_jχ$ at tree level with an effective field theory, safeguarding a physical massless $χ$-boson limit of the observables. By utilizing the upper bounds on $H\to\ell_i \bar\ell_j$ decays from CMS and ATLAS Collaborations, we establish an indirect upper limit on the nonstandard decay $H\to\ell_i \bar\ell_jχ$. The analysis encompasses various observables such as the lepton energy spectrum, Dalitz plot distribution, and Lepton Charge and Forward-Backward Asymmetries.

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arXiv2024

Polarized and unpolarized off-shell $H^\ast\to ZZ\rightarrow 4\ell$ decay above the $2m_Z$ threshold

An analysis of the off-shell $H^\ast\rightarrow ZZ \rightarrow \overline{\ell}_1\ell_1\overline{\ell}_2\ell_2$ decay width is presented for both unpolarized and polarized $Z$ gauge bosons in the scenario with the most general $H^*ZZ$ vertex function, which is given in terms of two $CP$-even ($\hat b_Z$ and $\hat c_Z$) and one $CP$-odd ($\tilde b_Z$) anomalous couplings. The SM contributions to the $H^*ZZ$ coupling up to the one-loop level are also included. Explicit analytic results for the unpolarized and polarized $H^\ast\rightarrow ZZ \rightarrow \overline{\ell}_1\ell_1\overline{\ell}_2\ell_2$ square amplitudes and the four-body phase space are presented, out of which several observable quantities can be obtained straightforwardly. As far as the numerical analysis is concerned, a cross-check is performed via \texttt{MadGraph5\_aMC@NLO}, where our model was implemented with the aid of FeynRules. We then consider the most stringent bounds on anomalous complex $H^*ZZ$ couplings and analyze the effects of the polarizations of the $Z$ gauge bosons through the polarized $H^\ast\rightarrow ZZ \rightarrow \overline{\ell}_1\ell_1\overline{\ell}_2\ell_2$ decay width as well as left-right and forward-backward asymmetries, which are found to be sensitive to new-physics effects. Particular focus is put on the effects of the absorptive parts of the anomalous $H^*ZZ$ couplings, which have been largely overlooked up to now in LHC analyses. It is found that the studied observable quantities, particularly the left-right asymmetries, can be helpful to look for effects of $CP$-violation in the $H^*ZZ$ coupling and set bounds on the absorptive parts. For completeness, we also analyze the case of unpolarized $Z$ gauge bosons.

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arXiv2022

Single top production via FCNC couplings at an $e^{-}e^{+}$ collider with center-of-mass energy $\sqrt{s}=240$ GeV

Aspects of top quark physics can be studied at a future Circular Electron-Positron Collider (CEPC). In particular, Flavor-Changing Neutral Current interactions (FCNC) of the top quark can be studied even with a center-of-mass energy from $\sqrt{s}\gtrsim m_t$ to $\sqrt{s}=240$ GeV, through the production a single top quark in association with a charm quark, which can arise in several scenarios of new physics. We explore here this signal within the Left-Right Mirror Model, which is a Left-Right model that includes mirror fermions; this model is motivated as a possible solution to the strong CP problem and by its rich phenomenology. We find that for particular scenarios of the model parameters, which are in agreement with the most up-to-date experimental constraints, the signal coming from the process $e^+e^-\to V \to tc$ ($V=Z,\,Z^{\prime}$) provide a detectable signal and thus it can give evidence for FCNC interactions; in these scenarios the mass of a new neutral gauge boson can be as high as $m_{Z'}=7$ TeV.

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arXiv2020

Flavor-changing decay $h\to τμ$ at super hadron colliders

We study the flavor-changing decay $h\to τμ$ with $τ=τ^-+τ^+$ and $μ=μ^-+μ^+$ of a Higgs boson at future hadron colliders, namely: a) High Luminosity Large Hadron Collider, b) High Energy Large Hadron Collider and c) Future hadron-hadron Circular Collider. The theoretical framework adopted is the Two-Higgs-Doublet Model type III. The free model parameters involved in the calculation are constrained through Higgs boson data, Lepton Flavor Violating processes and the muon anomalous magnetic dipole moment; later they are used to analyze the branching ratio of the decay $h\toτμ$ and to evaluate the $gg\to h$ production cross section. We find that at the Large Hadron Collider is not possible to claim for evidence of the decay $h\toτμ$ achieving a signal significance about of $1.46σ$ by considering its final integrated luminosity, 300 fb$^{-1}$. More promising results arise at the High Luminosity Large Hadron Collider in which a prediction of 4.6$σ$ when an integrated luminosity of 3 ab$^{-1}$ and $\tanβ=8$ are achieved. Meanwhile, at the High Energy Large Hadron Collider (Future hadron-hadron Circular Collider) a potential discovery could be claimed with a signal significance around $5.04σ$ ($5.43σ$) for an integrated luminosity of 3 ab$^{-1}$ and $\tanβ=8$ (5 ab$^{-1}$ and $\tanβ=4$).

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arXiv2017

Probing flavor parameters in the scalar sector and new bounds for the fermion sector

In this paper we study the Flavor-Changing mediated by Higgs boson within Two-Higgs Doublet Model-III context. We explore the parameter space and, considering recent results, find new limits for the model parameters. We also obtain the total Higgs decay width and the branching ratios for different channels taking limits for the $t\to cV$ and $b\to sγ$ processes. Considering different constraints, we estimate the branching ratio for the $h\to t^*_{}c$ in the model as well as the bounds for the $b \to sγ$, $h \to μτ$ and $h\to γZ.$ Considering the quark top decays to $W\bar{b},$ we obtained $Br(h \to Wc\bar{b})\sim10^{-3}$.

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arXiv2016

Brief description of the flavor-changing neutral scalar interactions at two-loop level

In this letter we show a general description about flavor-changing neutral currents (FCNC) mediated by scalars. The analysis is extended at two-loop level for the Two-Higgs Doublet Model type-III because others models have strong constraints on its parameters, even at high orders of the perturbation. For this letter we focus on the standard model, calculating the amplitude for the $h \to γγ$ process and discussing the results briefly.

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arXiv2013

Rare top decay and CP violation in THDM

We discuss the formalism of two Higgs doublet model type III with CP violation from CP-even CP-odd mixing in the neutral Higgs bosons. The flavor changing interactions among neutral Higgs bosons and fermions are presented at tree level in this type of model. These assumptions allow the study rare top decays mediated by neutral Higgs bosons, particularly we are interested in $t\rightarrow c l^+l^-$. For this process we estimated upper bounds of the branching ratios $\textrm{Br}(t\rightarrow c τ^+τ^-)$ of the order of $10^{-9}\sim 10^{-7}$ for a neutral Higgs boson mass of 125 GeV and $\tanβ=1$, 1.5, 2, 2.5. For the case of $t\rightarrow c τ^+τ^-$ the number of possible events is estimated from 1 to 10 events which could be observed in future experiments at LHC with a luminosity of 300 $\textrm{fb}^{-1}$ and 14 GeV for the energy of the center of mass. Also we estimate that the number of events for the process $t\rightarrow c l^+l^-$ in different scenarios is of order of $2500$.

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arXiv2013

Scalar-pseudoscalar interactions in neutrino-electron scattering

Many extensions to the Standard Model imply the existence of new charged scalar Higgs bosons. We study the contribution of a general scalar or pseudoscalar coupling for the neutrino-electron scattering. We take a phenomenological approach in order to obtain model independent limits to the couplings that arise in this picture. We illustrate the reach of the constraints by studying the particular case of the type III two Higgs doublet model, where we have found new constraints to some elements of the Yukawa couplings mixing matrix ($|Y_{ee}| \leq 1 \times 10^{-1}$ and $|Y_{eμ}| \leq 7\times10^{-2}$ at 90% CL).

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arXiv2006

Neutrino masses and mixing parameters in a left-right model with mirror fermions

In this work we consider a left-right model containing mirror fermions with gauge group SU(3)$_{C} \otimes SU(2)_{L} \otimes SU(2)_{R} \otimes U(1)_{Y^\prime}$. The model has several free parameters which here we have calculated by using the recent values for the squared-neutrino mass differences. Lower bound for the mirror vacuum expectation value helped us to obtain crude estimations for some of these parameters. Also we estimate the order of magnitude of the masses of the standard and mirror neutrinos.

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