arXiv subjects

J. I. Silva-Marcos

Publications and source records attributed to J. I. Silva-Marcos.

At least 19 recordsLinked to original sources

arXiv2026

Feasible Deviations from Unitarity with Vector-Like Quark Singlets

We deduce pertinent relations between the elements of the CKM matrix, and find that not all of these are totally compatible with experiment and/or the assumption of the SM $3 \times 3$ unitarity. We identify relations between moduli and complex phases of the CKM-elements which may signal deviations from unitary (DUs). Recent analyses of $|V_{cs}|$ appear to be in tension with the SM assumption of CKM unitarity in the second row, similar to previous hints of DUs in the first row from previous $|V_{ud,s}|$ results. Here, we explore DUs induced in VLQ iso-singlet models, in particular the possibility of having significant DUs of the first and second rows of the CKM matrix. We perform a extensive analysis of $n\leq 5$ VLQ iso-singlet models and identify a useful set of parametrizations with the intention of coherently exploring parameter space. We assess the feasibility of each model and confront it with constraints from key flavor observables, with particular attention to the neutral kaon and $D^0$-meson sectors, especially $ε_K$ and $x_D$. We find that smaller $n\leq 2$ VLQ-models cannot accommodate the new $|V_{cs}|$ results in the DUs of the second CKM-row. In contrast, larger models extending both up- and down-type sectors, can indeed produce significant second-row DUs, e.g. in a model with 3 up- and 2 down-type VLQs, even reaching the order of the Cabibbo angle. To our knowledge, the phenomenology of extensions combining both up- and down-type VLQs has yet to be thoroughly studied. Finally, we find that VLQ-singlet models may induce very large enhancements in the rephasing invariant phase $β_K$, entering in the Cabibbo sector.

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arXiv2025

Vector-like quark doublets, weak-basis invariants and CP violation

We study Standard Model extensions with isodoublet vector-like quarks with standard charges. Their presence induces right-handed charged and neutral currents. We identify minimal sets of independent parameters characterizing these extensions, describe useful weak bases, and provide parameterizations for all quark mixing. We analyze the intricacies of CP violation in such scenarios, finding a complete set of CP-odd invariants for the single doublet case. Crucially, we uncover a connection between weak-basis invariants and effective rephasing invariants involving only standard quarks. These results allow us to explore the phenomenology of doublet vector-like quarks through a rephasing-invariant analysis, with an emphasis on CP violation, including the potential role of these fields in explaining the Cabibbo angle anomalies.

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arXiv2024

Reducing Complex Phases and other Subtleties of CP Violation

When extending the SM, e.g. with vector-like quarks (VLQ), one inevitably encounters more than one CP violating complex phases. In this work, we develop a method useful in reducing the number of complex phases in scenarios with one and two VLQs. We show that these VLQ-models can be described with just one or two physical complex phases respectively and present an explicit alternative parametrization for the quark mixing matrix. Making use of this Reduction of Complex Phases in the case of the Bento-Branco-Parada (BBP) model, i.e. the simplest implementation of the Nelson-Barr solution to the strong CP problem, one also recovers the single phase formulation of BBP model. We comment on how this specific scenario can be distinguished from the general VLQ case with weak-basis invariants

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arXiv2024

Vector-like Singlet Quarks: a Roadmap

We review the theory and phenomenology of isosinglet vector-like quarks (VLQs). In recent years, interest in VLQs has been increasing, due to their contributions to new physics effects that can be tested in experiments at LHC and High-Luminosity LHC. The similarities of models with isosinglet VLQs and the seesaw framework in the leptonic sector are pointed out. The existence of VLQs leads to flavour-changing neutral currents at tree level and deviations from unitarity of the CKM matrix, introducing rich phenomenological implications. These new effects are naturally suppressed by the masses of the new quarks, that are constrained to be above the electroweak scale. In addition, striking new effects can be achieved with the inclusion of an extra complex scalar singlet. Such a minimal extension of the SM can give rise to new sources of CP violation with profound theoretical implications, allowing for a solution to the strong CP problem and a possible explanation for the baryon asymmetry of the Universe. We list and explain strong motivations to consider this class of models. We also briefly review how models with VLQs can be matched to the SM effective field theory (SMEFT). A detailed analysis of flavour observables that can be affected by the presence of VLQs is presented. Current bounds from collider searches of VLQs are summarized. We point out that the discovery of VLQs can be within the reach of present or future colliders being planned.

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arXiv2023

Leptonic CP violation from the seesaw

The seesaw extension of the SM explains the tiny neutrino masses and it is accompanied by many CP violating phases. We study the case where all leptonic CP violation arises from the soft breaking in the heavy Majorana mass matrix. Parameter counting reveals that one less parameter is needed to describe this case. This reduction leads to restrictions on the parameter space of heavy neutrinos. We analyze the minimal seesaw case in detail and find that mass degeneracy of heavy neutrinos is not possible for certain values of the CP phases.

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arXiv2023

CP-odd and CP-even Weak-Basis Invariants in the Presence of Vector-Like Quarks

We propose a minimal set of weak-basis invariants in an extension of the SM where one up-type isosinglet vector-like quark is introduced, which allows us to obtain all the physical content of the CKM matrix. We present CP-odd invariants of lower order in mass than the one in the SM, which may have important consequences for Baryogenesis. We study the extreme chiral limit, where the two lightest generations have vanishing mass, showing that in this extension, contrary to the SM, CP violation can be observed in collisions much above the electroweak scale.

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arXiv2023

Does Quark Mixing play a role in the Lepton Sector?

We suggest a simple relation between the quark and the lepton mixing within the framework of type-I seesaw mechanism. We show that within our ansatz the empirical King-Mohapatra-Smirnov relation, which suggests a connection between the CKM and PMNS mixing where $|V^\text{PMNS}_{13}|\approx \frac{1}{\sqrt{2}}\sin(θ_C) $, can be derived. This is possible within a restricted region of the Dirac and Majorana mass parameters.

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arXiv2023

Do the Small Numbers in the Quark Mixing arise from New Physics?

We put forward the conjecture that the small numbers in the $V_\text{CKM}$ matrix, are generated by physics beyond the Standard Model. We identify as small numbers $V_{ub}$ and the strength of CP violation, measured by $|\text{Im}Q|$, where $Q$ stands for a rephasing invariant quartet of $V_\text{CKM}$. We illustrate how the conjecture can be realised in the context of an extension of the Standard Model where an up-type vector-like quark is introduced leading to a realistic spectrum of quark masses and an effective $V_\text{CKM}$ in agreement with experiment.

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arXiv2022

Decays of the Heavy Top and New Insights on $ε_K$ in a one-VLQ Minimal Solution to the CKM Unitarity Problem

We propose a minimal extension of the Standard Model where an up-type vector-like quark, denoted $T$, is introduced and provides a simple solution to the CKM unitarity problem. We adopt the Botella-Chau parametrization in order to extract the $4\times 3$ quark mixing matrix which contains the three angles of the $3\times 3$ CKM matrix plus three new angles denoted $θ_{14}$, $θ_{24}$, $θ_{34}$. It is assumed that the mixing of $T$ with standard quarks is dominated by $θ_{14}$. Imposing a recently derived, and much more restrictive, upper-bound on the New Physics contributions to $ε_{K}$, we find, in the limit of exact $θ_{14}$ dominance where the other extra angles vanish, that $ε_{K}^{\text{NP}}$ is too large. However, if one relaxes the exact $θ_{14}$ dominance limit, there exists a parameter region, where one may obtain $ε_{K}^{\text{NP}}$ in agreement with experiment while maintaining the novel pattern of $T$ decays with the heavy quark decaying predominantly to the light quarks $d$ and $u$. We also find a reduction in the decay rate of $K_L\rightarrow π^o ν\barν$.

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arXiv2021

Addressing the CKM Unitarity Problem with a Vector-like Up Quark

We point out that hints of deviations from unitarity in the first row of the CKM matrix may be explained by the presence of a single vector-like top. We study how the stringent experimental constraints arising from CP Violation in the kaon sector and from meson mixing such as $D^0$-$\overline{D}^0$, $K^0$-$\overline{K}^0$ and $B^0_{d,s}$-$\overline{B}^0_{d,s}$ can be satisfied in the proposed framework. In order for the deviations from unitarity to be of the required size while keeping the theory perturbative, the new top quark should have a mass $m_T \lesssim 7$ TeV which could be probed in upcoming experiments at the energy frontier.

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arXiv2020

Type-I Seesaw with eV-Scale Neutrinos

We consider seesaw type-I models including at least one (mostly-)sterile neutrino with mass at the eV scale. Three distinct situations are found, where the presence of light extra neutrinos is naturally justified by an approximately conserved lepton number symmetry. To analyse these scenarios consistently, it is crucial to employ an exact parametrisation of the full mixing matrix. We provide additional exact results, including generalised versions of the seesaw relation and of the Casas-Ibarra parametrisation, valid for every scale of seesaw. We find that the existence of a light sterile neutrino imposes an upper bound on the lightest neutrino mass. We further assess the impact of light sterile states on short- and long-baseline neutrino oscillation experiments, emphasise future detection prospects, and address CP Violation in this framework via the analysis of CP asymmetries and construction of weak basis invariants. The proposed models can accommodate enough active-sterile mixing to play a role in the explanation of short-baseline anomalies.

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arXiv2018

Solving a Fine-Tuning Problem of the Standard Model through the Introduction of Vector-Like Quarks

We emphasise that even in the extreme chiral limit where only the top and bottom quarks acquire mass, quark mixing is physically meaningful. This implies that the natural value of $|V_{13}|^2 + |V_{23}|^2$ is of order one, which is to be compared to its experimental value of order $10^{-3}$ . We show how this fine-tuning problem of the Standard Model can be solved through an extension of the Standard Model where vector-like quarks and a complex singlet are introduced, together with a flavour symmetry. The mixings of the light quarks are generated through the mixing of the vector-like quarks with the standard quarks.

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arXiv2017

Can One have Significant Deviations from Leptonic $3\times 3$ Unitarity in the Framework of Type I Seesaw Mechanism?

We address the question of deviations from $3\times 3$ unitarity of the leptonic mixing matrix showing that, contrary to conventional wisdom, one may have significant deviations from unitarity in the framework of type I seesaw mechanism. In order for this scenario to be feasible, at least one of the heavy neutrinos must have a mass at the TeV scale, while the other two may have much larger masses. We present specific examples where deviations from $3\times 3$ unitarity are sufficiently small to conform to all the present stringent experimental bounds but are sufficiently large to have the potential for being detectable at the next round of experiments.

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arXiv2017

Exploring the Quark Flavour Puzzle within the 3 Higgs Model

We extend the standard model with two extra Higgs doublets. Making use of a symmetry principle, we present flavour symmetries based on cycle groups $Z_N$ that oblige each Higgs doublet to contribute to the mass of only one generation, thus accommodating the quark mass hierarchy. We systematically search for all charge configurations that naturally lead to the alignment in flavour space of the quark sectors, resulting in a CKM matrix near to the identity, determined by the quark mass hierarchy, and with the correct overall phenomenological features. The minimal realisation is by the group $Z_7$. We show that only a limited number of solutions exist, and that any accidental global symmetry that may occur together with the discrete symmetry is necessarily anomalous. A phenomenological study of each class of solutions concerning predictions to the flavour changing neutral current (FCNC) phenomena is also performed: for some solutions, it is possible to obtain realistic quark masses and mixing, while the flavour violating neutral Higgs are sufficient lighter to be accessible at LHC.

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arXiv2017

Leptonic Invariants, Neutrino Mass-Ordering and the Octant of $θ_{23}$

We point out that leptonic weak-basis invariants are an important tool for the study of the properties of lepton flavour models. In particular, we show that appropriately chosen invariants can give a clear indication of whether a particular lepton flavour model favours normal or inverted hierarchy for neutrino masses and what is the octant of $θ_{23}$. These invariants can be evaluated in any conveniently chosen weak-basis and can also be expressed in terms of neutrino masses, charged lepton masses, mixing angles and CP violation phases.

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arXiv2016

Vector-like Quarks at the Origin of Light Quark Masses and Mixing

We show how a novel fine-tuning problem present in the Standard Model can be solved through the introduction of a single flavour symmetry G, together with three $Q = - 1/3$ quarks, three $Q = 2/3$ quarks, as well as a complex singlet scalar. The symmetry G is extended to the additional fields and it is an exact symmetry of the Lagrangian, only spontaneously broken by the vacuum. Specific examples are given and a phenomenological analysis of the main features of the model is presented. It is shown that even for vector-like quarks with masses accessible at the LHC, one can have realistic quark masses and mixing, while respecting the strict constraints on process arising from flavour changing neutral currents (FCNC). The vector-like quark decay channels are also described.

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arXiv2016

What if the Masses of the First Two Quark Families are not Generated by the Standard Higgs?

We point out that, in the context of the SM, $|V^2_{13}| + | V^2_{23}|$ is expected to be large, of order one. The fact that $|V^2_{13}| + |V^2_{23}| \approx 1.6 \times 10^{-3}$ motivates the introduction of a symmetry S which leads to $V_{CKM} ={1\>\!\!\!\mathrm{I}} $, with only the third generation of quarks acquiring mass. We consider two scenarios for generating the mass of the first two quark generations and full quark mixing. One consists of the introduction of a second Higgs doublet which is neutral under S. The second scenario consists of assuming New Physics at a high energy scale , contributing to the masses of light quark generations, in an effective field theory approach. This last scenario leads to couplings of the Higgs particle to $s\overline s$ and $c \overline c$ which are significantly enhanced with respect to those of the SM. In both schemes, one has scalar-mediated flavour- changing neutral currents which are naturally suppressed. Flavour violating top decays are predicted in the second scenario at the level $ \mbox{Br} (t \rightarrow h c ) \geq 5\times 10^{-5}$.

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arXiv2015

Leptonic CP Violation from a New Perspective

We study leptonic CP violation from a new perspective. For Majorana neutrinos, a new parametrization for leptonic mixing of the form $V=O_{23} O_{12} K_{a}^{i}\cdot O$ reveals interesting aspects that are less clear in the standard parametrization. We identify several important scenario-cases with mixing angles in agreement with experiment and leading to large leptonic CP violation. If neutrinos happen to be quasi-degenerate, this new parametrization might be very useful, e.g., in reducing the number of relevant parameters of models.

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