arXiv subjects

F. R. Joaquim

Publications and source records attributed to F. R. Joaquim.

At least 19 recordsLinked to original sources

arXiv2026

Sterile neutrino Dark Matter in the minimal Dirac Seesaw

We study sterile neutrino dark matter in a minimal Type-I Dirac seesaw framework where the states responsible for generating Dirac neutrino masses at tree level can be viable dark matter candidates. A $\mathcal{Z}_6$ symmetry, spontaneously broken to a residual $\mathcal{Z}_3$ by the vacuum expectation value of a singlet scalar, forbids Majorana mass operators and ensures neutrino Diracness. The lightest sterile neutrino is produced non-thermally via freeze-in from decays of Standard Model particles and an additional scalar state. We show that the presence of an additional right-handed mixing angle, $θ_R$, opens up viable regions of parameter space where the observed dark matter relic abundance can be reproduced while maintaining cosmological stability. This mainly stems from the absence of X-ray astrophysical constraints in our scenario. We further find that the freeze-in production of right-handed neutrinos yields a negligible contribution to $ΔN_{\rm eff}$, consistent with current cosmological bounds.

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arXiv2026

Flavor specific chiral $U(1)_X$ framework for explaining the ATOMKI anomaly

Recent anomalies in nuclear transitions observed by the ATOMKI Collaboration suggest the existence of a new boson with a mass of $\sim 17$ MeV. A theoretically consistent interpretation requires a framework that not only matches the kinematics but also reproduces the observed decay rates while satisfying stringent experimental constraints. Among various possibilities, an axial-vector or mixed vector--axial-vector mediator $Z'$ emerges as the most viable candidate. However, getting such couplings for a light $Z'$ gauge boson is a highly nontrivial task. In this work, we construct a gauged chiral, flavor specific $U(1)_X$ extensions of the Standard Model where the associated $Z'$ boson acts as the $17$ MeV particle. By employing a two Higgs doublet framework, we generate the necessary nonvanishing axial-vector couplings while ensuring gauge anomaly cancellation and consistent fermion mass generation. Focusing on the $^8\mathrm{Be}$ and $^4\mathrm{He}$ signals, we show that in this model the viable parameter space to resolve the ATOMKI anomalies is also consistent with a diverse set of experimental constraints, including atomic parity violation, beam dump experiments, meson decays, and neutrino nucleus and neutrino electron scatterings. Our results demonstrate that this framework offers a theoretically sound and phenomenologically robust solution to the ATOMKI anomaly.

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arXiv2025

Minimal U(1) two-Higgs-doublet models for quark and lepton flavour

In the context of the 2HDM, and assuming that neutrinos acquire masses via the Weinberg operator, we perform a systematic analysis to determine the minimal quark and lepton flavour patterns, compatible with masses, mixing and CP violation data, realisable by Abelian symmetries. We determine four minimal models for quarks, where the number of independent parameters matches the number of observables. For the lepton sector, three minimal predictive models are identified. Namely, we find scenarios with a preference for the upper/lower octant of the $θ_{23}$ atmospheric mixing angle, that exhibit lower bounds on the lightest neutrino masses currently probed by cosmology and testable at future neutrinoless double beta decay experiments, even for a normally-ordered neutrino masses. We investigate the phenomenology of each model taking into account all relevant theoretical, electroweak precision observables, scalar sector constraints, as well as stringent quark flavour processes such as $\overline{B} \rightarrow X_s γ$, $B_s \rightarrow μ^- μ^+$ and meson oscillations, and the charged lepton flavour-violating decays $e_α^{-} \rightarrow e_β^{-} e_γ^{+} e_δ^{-}$ and $e_α\rightarrow e_βγ$. We show that, in some cases, Abelian flavour symmetries provide a natural framework to suppress flavour-changing neutral couplings and lead to scenarios featuring heavy neutral/charged scalar masses below the TeV scale within the reach of current experiments.

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arXiv2025

Flavored Peccei-Quinn symmetries in the minimal $ν$DFSZ model

We consider a Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) axion model extended with two right-handed neutrino fields to realize the minimal type-I seesaw. In this $ν$DFSZ scheme we systematically determine the simplest quark and lepton flavor patterns compatible with masses, mixing and charge-parity violation data, realized by flavored U(1) Peccei-Quinn (PQ) symmetries. We discuss axion dark matter production in pre and post-inflationary cosmology in this context, and predictions for the axion couplings to photons and fermions. In particular, helioscopes and haloscopes are able to probe our models via their distinct axion-to-photon couplings, while in the quark sector the most stringent constraints on axion-fermion couplings are set by $K^+ \rightarrow π^+ + a$. Flavor-violating constraints in the lepton sector are not as relevant as those stemming from star cooling that restrict the diagonal $ee$ and $μμ$ axion couplings to charged leptons. We also obtain axion mass bounds for the most interesting models and discuss how minimal flavored PQ symmetries provide a natural framework to suppress flavor-violating couplings.

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arXiv2025

Axion framework with color-mediated Dirac neutrino masses

We propose a KSVZ-type axion framework in which vector-like quarks (VLQ) and colored scalars generate Dirac neutrino masses radiatively. The global Peccei-Quinn symmetry (under which the exotic fermions are charged) addresses the strong CP problem and ensures the Dirac nature of neutrinos. The axion also accounts for the observed cosmological dark matter. We systematically explore all viable VLQ representations. Depending on the specific scenario, the framework predicts distinct axion-to-photon couplings, testable through haloscope and helioscope experiments, as well as potentially significant flavor-violating quark-axion interactions.

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arXiv2024

Gradient Boosting MUST taggers for highly-boosted jets

The MUST (Mass Unspecific Supervised Tagging) method has proven to be successful in implementing generic jet taggers capable of discriminating various signals over a wide range of jet masses. We implement the MUST concept by using eXtreme Gradient Boosting (XGBoost) classifiers instead of neural networks (NNs) as previously done. We build both fully-generic and specific multi-pronged taggers, to identify 2, 3, and/or 4-pronged signals from SM QCD background. We show that XGBoost-based taggers are not only easier to optimize and much faster than those based in NNs, but also show quite similar performance, even when testing with signals not used in training. Therefore, they provide a quite efficient alternative machine-learning implementation for generic jet taggers.

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arXiv2024

Axion Paradigm with Color-Mediated Neutrino Masses

We propose a generalized Kim-Shifman-Vainshtein-Zakharov-type axion framework in which colored fermions and scalars act as two-loop Majorana neutrino-mass mediators. The global Peccei-Quinn symmetry under which exotic fermions are charged solves the strong CP problem. Within our general proposal, various setups can be distinguished by probing the axion-to-photon coupling at helioscopes and haloscopes. We also comment on axion dark-matter production in the early Universe.

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arXiv2023

A closer look at the $U(1)_{B-L}$ explanation of the ATOMKI nuclear anomalies

We revisit the gauged $U(1)_{B-L}$ explanation of the ATOMKI nuclear anomalies, in which the new gauge boson is the hypothetical $X(17)$ particle. It is known that the vanilla $B-L$ scenario is unable to account for appropriate couplings, namely the suppression of the couplings of $X(17)$ to neutrinos, which motivates adding vector-like leptons. The simplest case, in which the new fields have $B-L$ charges equal to $1$, is highly disfavoured since it requires large mixing with the Standard Model fields. One solution recently put forward is to consider large $B-L$ charges to counterbalance small mixing. We show that, in this scenario, and after taking into account several phenomenological constraints, the dominant contribution to the muon anomalous magnetic moment $(g-2)_μ$ is expected to be extremely large and with a negative sign, being thus excluded by experiment.

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arXiv2023

Dark-sector seeded solution to the strong CP problem

We propose a novel realization of the Nelson-Barr mechanism "seeded'' by a dark sector containing scalars and vector-like quarks. Charge-parity (CP) and a $\mathcal{Z}_8$ symmetry are spontaneously broken by the complex vacuum expectation value of a singlet scalar, leaving a residual $\mathcal{Z}_2$ symmetry that stabilizes dark matter (DM). A complex Cabibbo-Kobayashi-Maskawa matrix arises via one-loop corrections to the quark mass matrix mediated by the dark sector. In contrast with other proposals where non-zero contributions to the strong CP phase arise at the one-loop level, in our case this only occurs at two loops, enhancing naturalness. Our scenario also provides a viable weakly interacting massive particle scalar DM candidate.

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arXiv2023

Confronting the 95 GeV excesses within the UN2HDM

We consider the small excesses around 95 GeV found in several searches for a new scalar in $γγ$, $ττ$ and $b \bar b$ final states. Instead of trying to accommodate them all, as is usually done in the literature, in the context of a given Standard Model~(SM) extension, we investigate whether it would be possible that one or two of these excesses correspond to an actual new scalar, while the remaining ones are merely statistical fluctuations. To this end, we use as benchmark model the UN2HDM, a SM extension with one scalar doublet, one scalar singlet, and an extra $\text{U}(1)'$ symmetry, which has been previously studied in the context of multiboson cascade decays. We show that most of the possibilities where the excesses in one or two of these channels disappear in the future can be accommodated by type-I or type-III UN2HDMs.

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arXiv2023

Dark linear seesaw mechanism

We propose a minimal model where a dark sector, odd under a $\mathcal{Z}_2$ discrete symmetry, is the seed of lepton number violation in the neutrino sector at the loop level, in the context of the linear seesaw mechanism. Neutrino mass suppression stems from a naturally small scalar potential coupling which breaks the lepton number symmetry softly. The fact that we consider (dark) Dirac vector-like neutral leptons, prevents the appearance of other mass terms that could contribute to neutrino masses via alternative mechanisms. We study the dark-matter phenomenology of the model, focusing on the case in which the stable particle is the lightest neutral scalar arising from the dark scalar sector. Prospects for testing our framework with the results of current and future lepton flavour violation searches are also discussed.

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arXiv2023

Hybrid scoto/seesaw: flavour and dark matter

We propose a model based on the interplay between the type-II seesaw and scotogenic neutrino mass generation mechanisms. The setup features a $\mathbb{Z}_8$ discrete flavour symmetry which is broken down to a residual $\mathbb{Z}_2$ responsible for stabilising dark matter. A singlet scalar field is introduced to implement spontaneous CP violation. The effective neutrino mass matrix two-texture zero structure leads to sharp neutrino sector predictions. We analyse the constraints imposed on the model by current and future charged lepton flavour violation experiments. This framework provides two viable dark matter candidates, scalar or fermion. We investigate the scalar dark matter scenario considering relic density, direct-detection and collider constraints.

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arXiv2022

Multiboson signals in the UN2HDM

We address multiboson production from a heavy $Z'$ resonance in the context of the UN2HDM, a standard model extension with an additional $\text{U}(1)'$ symmetry and an enlarged scalar sector with an extra doublet and a singlet. After taking into account theoretical and experimental constraints on the model, it turns out this type of signals - mostly uncovered by current searches - could be sizeable. We focus on three benchmark scenarios, each of them predicting up to 4000 multiboson events with the LHC Run 2 collected luminosity. Anomaly-detection methods could uncover those signals, if present in data.

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arXiv2022

Texture-zero patterns of lepton mass matrices from modular symmetry

Texture zeros in fermion mass matrices have been widely considered in tackling the Standard Model flavour puzzle. In this work, we perform a systematic analysis of texture zeros in lepton mass matrices in the framework of $Γ_{3}'\cong T'$ modular symmetry. Assuming that the lepton fields transform as irreducible representations of $T'$, we obtain all possible texture-zero patterns for both charged-lepton and neutrino mass matrices which can be achieved from $T'$ modular symmetry. We provide representative models for the phenomenologically-viable textures which can accommodate the experimental data. The predictions for lepton mixing angles, CP-violating phases, light neutrino masses and effective neutrino mass relevant for neutrinoless doble beta decay, are discussed. We find that the minimal viable lepton model depends on only $7$ real free parameters including the modulus $τ$ (the corresponding charged-lepton mass matrix contains $4$ vanishing entries, and the neutrino mass matrix has $1$ texture zero). Finally, we study in detail three benchmark models, one for each neutrino mass generation mechanism considered (Dirac, Majorana via Weinberg operator and Majorana via minimal type-I seesaw mechanism).

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arXiv2022

Scalar-singlet assisted leptogenesis with CP violation from the vacuum

In the vanilla type-I seesaw leptogenesis scenario, CP violation required to generate the lepton asymmetries in the heavy Majorana neutrino decays stem from complex Dirac-type Yukawa couplings. In this paper we explore the case in which that CP violation originates from the vacuum expectation value of a complex scalar singlet at a very high scale. This non-trivial CP-violating phase can be successfully communicated to the low-energy neutrino sector via the heavy neutrino portal. The new scalar-singlet degrees of freedom generate new contributions to the CP asymmetries relevant for leptogenesis not only at the one-loop level but also through tree-level three-body decays. These are computed here for an arbitrary number of heavy neutrinos, Higgs doublets and scalar singlets. We also take into account the new decays and scattering processes that enter the unflavoured Boltzmann equations governing the heavy-neutrino particle densities and the $(B-L)$-asymmetry evolution. Having established the framework of interest, we present a simple model with two RH neutrinos, two Higgs doublets and a complex scalar singlet, supplemented with a $\mathcal{Z}_8$ flavour symmetry. This symmetry minimises the number of free parameters without compromising the possibility of spontaneous CP violation and compatibility with neutrino data. In fact, the only viable $\mathcal{Z}_8$ charge assignment shows a preference for a non-trivial spontaneous CP-violating phase, which in turn leads to a non-vanishing CP asymmetry due to the direct link between high- and low-energy CP violation. An interesting feature of this simple setup is that the usual wave and vertex type-I seesaw contributions to the CP asymmetry vanish due to the $\mathcal{Z}_8$ symmetry. Thus, the observed baryon-to-photon ratio can be explained thanks to the new couplings among the heavy neutrinos and the new scalar degrees of freedom.

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arXiv2022

Flavour and dark matter in a scoto/type-II seesaw model

The neutrino mass and dark matter (DM) problems are addressed in a Standard Model extension where the type-II seesaw and scotogenic mechanisms coexist. The model features a flavour $\mathcal{Z}_8$ discrete symmetry which is broken down to a $\mathcal{Z}_2$, stabilising the (scalar or fermion) DM particle. Spontaneous CP violation is implemented through the complex vacuum expectation value of a singlet scalar field, inducing observable CP-violating effects in the lepton sector. The structure of the effective neutrino mass matrix leads to constraints on the low-energy neutrino observables, namely the atmospheric neutrino mixing angle $θ_{23}$, the Dirac CP-violating phase $δ$ and the absolute neutrino mass scale $m_{\rm lightest}$. In particular, in most cases, the model selects one $θ_{23}$ octant with $δ\simeq 3π/2$. Moreover, the obtained lower bounds on $m_{\rm lightest}$ are typically in the range probed by cosmology. We also analyse the constraints imposed on the model by current experimental limits on charged lepton flavour violating (cLFV) processes, as well as future projected sensitivities. It is shown that the Higgs triplet and scotogenic contributions to cLFV never overlap and that the interplay among Yukawa couplings, dark charged scalar masses and mixing leads to a wide parameter-space region compatible with current experimental bounds. We investigate the scalar and fermion DM parameter space of our model by considering relic density, direct-detection (DD) and collider constraints. For scalar DM the mass interval $68 \ \text{GeV} \lesssim m_{\text{DM}} \lesssim 90 \ \text{GeV}$ is viable and will be probed by future DD searches. In the fermion DM case, correct relic density is always obtained for $m_{\text{DM}} \gtrsim 45$ GeV thanks to dark fermion-scalar coannihilation channels.

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arXiv2022

Quantum gravity phenomenology at the dawn of the multi-messenger era -- A review

The exploration of the universe has recently entered a new era thanks to the multi-messenger paradigm, characterized by a continuous increase in the quantity and quality of experimental data that is obtained by the detection of the various cosmic messengers (photons, neutrinos, cosmic rays and gravitational waves) from numerous origins. They give us information about their sources in the universe and the properties of the intergalactic medium. Moreover, multi-messenger astronomy opens up the possibility to search for phenomenological signatures of quantum gravity. On the one hand, the most energetic events allow us to test our physical theories at energy regimes which are not directly accessible in accelerators; on the other hand, tiny effects in the propagation of very high energy particles could be amplified by cosmological distances. After decades of merely theoretical investigations, the possibility of obtaining phenomenological indications of Planck-scale effects is a revolutionary step in the quest for a quantum theory of gravity, but it requires cooperation between different communities of physicists (both theoretical and experimental). This review is aimed at promoting this cooperation by giving a state-of-the art account of the interdisciplinary expertise that is needed in the effective search of quantum gravity footprints in the production, propagation and detection of cosmic messengers.

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arXiv2022

The hidden side of scalar-triplet models with spontaneous CP violation

Scalar triplet extensions of the Standard Model provide an interesting playground for the explanation of neutrino mass suppression through the type-II seesaw mechanism. Propelled by the possible connections with leptonic CP violation, we explore under which conditions spontaneous CP violation can arise in models with extra scalar triplets. The minimal model satisfying such conditions requires adding two such triplets to the SM field content. For this model, the scalar mass spectrum in both the CP-conserving and spontaneous CP-violating scenarios is studied. In the former case, a decoupling limit for the new scalars can be achieved, while this is not the case when CP is spontaneously broken. In particular, we show that the existence of two light neutral scalars with masses below a few tenths of GeVs is unavoidable in the CP-violating case. Using matrix theory theorems, we derive upper bounds for the masses of those light scalars and briefly examine whether they can still be experimentally viable. Other interesting features of the scalar mass spectrum are discussed as, e.g., the existence of relations among the charged and neutral scalar masses.

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