Ivo de Medeiros Varzielas’s research while affiliated with Instituto Superior Técnico and other places

A bstract We extend the littlest modular seesaw to a Grand Unified scenario based on SU (5) endowed with three modular S 4 symmetries. We leverage symmetry protected zeroes in the leptonic and down quark sectors to suppress deviations to the littlest modular seesaw predictions, but not contributions to the quark mixing. The model is supplemented by two weighton fields, such that the hierarchical nature of the charged-lepton masses, as well as the quark masses and mixing, stem from the content and symmetries of the model, rather than a hierarchical nature of the Yukawa coefficients.

A bstract We analyse the possibility of describing quark masses, mixing and CP violation in ${S}_4^{\prime }$ S 4 ′ modular flavour models without flavons. We focus on the case where the closeness of the modulus to the point of residual ${\mathbb{Z}}_3^{ST}$ ℤ 3 ST symmetry (the cusp) plays a role in generating quark mass hierarchies and discuss the role modular form normalisations play in such constructions. We find that fitting quark data requires explicit CP breaking, unless a second modulus is introduced.

We analyse the possibility of describing quark masses, mixing and CP violation in $S'_4$ modular flavour models without flavons. We focus on the case where the closeness of the modulus to the point of residual $\mathbb{Z}^{ST}_3$ symmetry (the cusp) plays a role in generating quark mass hierarchies and discuss the role modular form normalisations play in such constructions. We find that fitting quark data requires explicit CP breaking, unless a second modulus is introduced.

We revisit the phenomenological predictions of the Universal Texture Zero (UTZ) model of flavour originally presented in [1], and update them in light of both improved experimental constraints and numerical analysis techniques. In particular, we have developed an in-house Markov Chain Monte Carlo (MCMC) algorithm to exhaustively explore the UTZ’s viable parameter space, considering both leading- and next-to-leading contributions in the model’s effective operator product expansion. We also extract – for the first time – reliable UTZ predictions for the (poorly constrained) leptonic CP-violating phases, and ratio observables that characterize neutrino masses probed by (e.g.) oscillation, $\beta$ β -decay, and cosmological processes. We therefore dramatically improve on the proof-in-principle phenomenological analysis originally presented in [1], and ultimately show that the UTZ remains a minimal, viable, and appealing theory of flavour. Our results also further demonstrate the potential of robustly examining multi-parameter flavour models with MCMC routines.

A bstract We perform a comprehensive analysis of a version of the 3-Higgs doublet model whose scalar potential is invariant under a global ∆(27) discrete symmetry and where the three scalar doublets are chosen to transform as a triplet under this discrete group. For each of the known tree-level minima we study the mass spectra and use the oblique parameters STU as well as perturbative unitarity to constrain the parameter space of the model. We then discuss phenomenological consequences of some leading order flavour mixing quark Yukawa couplings by considering the flavour violation process b → sγ . We show that perturbative unitarity significantly constrains parameters of the model while, conversely, the beyond the Standard Model contributions to the b → sγ decay are automatically tamed by the symmetry.

A bstract We present the first complete model of the Littlest Modular Seesaw, based on two right-handed neutrinos, within the framework of multiple modular symmetries, justifying the use of multiple moduli fields which take their values at 3 specific stabilizers of Γ 4 ≃ S 4 , including a new phenomenological possibility. Using a semi-analytical approach, we perform a χ ² analysis of each case and show that good agreement with neutrino oscillation data is obtained, including predictive relations between the leptonic mixing angles and the ratio of light neutrino masses, which non-trivially agree with the experimental values. It is noteworthy that in this very predictive setup, the models fit the global fits of the experimental data remarkably well, both with and without the Super-Kamiokande atmospheric data, for both models presented here. By extending the model to include a weighton and the double cover group ${\Gamma}_4^{\prime}\simeq {S}_4^{\prime }$ Γ 4 ′ ≃ S 4 ′ , we are able to also account for the hierarchy of the charged leptons using modular symmetries, without altering the neutrino predictions.

We study the minimization of the potentials of multi-Higgs models with symmetries that are softly broken. The powerful method of geometric minimization enables analytic minimization of the potentials of multi-Higgs models invariant under large symmetries. When these symmetries are softly broken, the method needs to be adapted. We propose a useful generalization that considers the effect of the soft-breaking terms to the quadratic part of the potential, by applying the procedure to restricted orbit spaces. We exemplify our novel methodology by finding and classifying the minima for an $S_4$ S 4 multi-Higgs model that is softly broken with specific terms.

We consider the framework of flavoured leptoquarks, in models with scalar or vector leptoquarks, and look for constraints to the parameter space of these models. Using primarily direct searches at the Large Hadron Collider (LHC) and precision processes that probe Lepton Flavour Violation (LFV), we present lower bounds for the masses of the leptoquarks in different flavoured scenarios. We classify the models according to the specific leptoquark, distinguished with respect to their couplings to charged leptons (lepton isolation, two-columned patterns), and in scenarios with specific hierarchies in the couplings (hierarchical, democratic and flipped).

We revisit the phenomenological predictions of the Universal Texture Zero (UTZ) model of flavour originally presented in arXiv:1710.01741, and update them in light of both improved experimental constraints and numerical analysis techniques. In particular, we have developed an in-house Markov Chain Monte Carlo (MCMC) algorithm to exhaustively explore the UTZ's viable parameter space, considering both leading- and next-to-leading contributions in the model's effective operator product expansion. We also extract -- for the first time -- reliable UTZ predictions for the (poorly constrained) leptonic CP-violating phases, and ratio observables that characterize neutrino masses probed by (e.g.) oscillation, $\beta$-decay, and cosmological processes. We therefore dramatically improve on the proof-in-principle phenomenological analysis originally presented in arXiv:1710.01741, and ultimately show that the UTZ remains a minimal, viable, and appealing theory of flavour. Our results also further demonstrate the potential of robustly examining multi-parameter flavour models with MCMC routines.