arXiv subjects

Luca Merlo

Publications and source records attributed to Luca Merlo.

At least 55 records · Page 3Linked to original sources

arXiv2014

Sigma Decomposition

In composite Higgs models the Higgs is a pseudo-Goldstone boson of a high-energy strong dynamics. We have constructed the effective chiral Lagrangian for a generic symmetric coset, restricting to CP-even bosonic operators up to four momenta which turn out to depend on seven parameters, aside from kinetic terms. Once the same sources of custodial symmetry breaking as in the Standard Model are considered, the total number of operators in the basis increases up to ten, again aside from kinetic terms. Under these assumptions, we have then particularised the discussion to three distinct frameworks: the original $SU(5)/SO(5)$ Georgi-Kaplan model, the minimal custodial-preserving $SO(5)/SO(4)$ model and the minimal $SU(3)/(SU(2)\times U(1))$ model, which intrinsically breaks custodial symmetry. The projection of the high-energy electroweak effective theory into the bosonic sector of the Standard Model is shown to match the low-energy chiral effective Lagrangian for a dynamical Higgs, and it uncovers strong relations between the operator coefficients. Finally, the relation with the bosonic basis of operators describing linear realisations of electroweak symmetry breaking is clarified.

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arXiv2014

Neutrino Masses and Mixings from Continuous Symmetries

Flavour symmetries are fundamental tools in the search for an explanation to the flavour puzzle: fermion mass hierarchies, the neutrino mass ordering, the differences between the mixing matrices in the quark and lepton sector, can all find an explanation in models where the fermion generations undergo specific geometric relations. An overview on the implementation of continuous symmetries in the flavour sector is presented here, focussing on the lepton sector.

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arXiv2014

A Bayesian comparison of U(1) lepton flavour models

Whether the anarchical ansatz or more symmetric structures best describe the neutrino parameters is a long standing question that underwent a revival of interest after the discovery of a non- vanishing reactor angle and the indication of a non-maximal atmospheric angle. In this letter, a Bayesian statistical approach is adopted in order to analyse and compare the two hypotheses within the context of U(1) flavour models. We study the constraints on individual model parameters and perform model comparison: the results elect constructions with built-in hierarchies among the matrix elements as preferred over the anarchical ones, with values of the evidence that depends slightly on whether the U(1) charges are also considered as free parameters or not, and on the priors used.

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arXiv2013

Tri-Bimaximal Neutrino Mixing and Discrete Flavour Symmetries

We review the application of non-Abelian discrete groups to Tri-Bimaximal (TB) neutrino mixing, which is supported by experiment as a possible good first approximation to the data. After summarizing the motivation and the formalism, we discuss specific models, mainly those based on A4 but also on other finite groups, and their phenomenological implications, including the extension to quarks. The recent measurements of θ_13 favour versions of these models where a suitable mechanism leads to corrections to θ_13 that can naturally be larger than those to θ_12 and θ_23. The virtues and the problems of TB mixing models are discussed, also in connection with lepton flavour violating processes, and the different approaches are compared.

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arXiv2013

Discrete Flavour Groups, θ_13 and Lepton Flavour Violation

Discrete flavour groups have been studied in connection with special patterns of neutrino mixing suggested by the data, such as Tri-Bimaximal mixing (groups A4, S4...) or Bi-Maximal mixing (group S4...) etc. We review the predictions for sin(θ_13) in a number of these models and confront them with the experimental measurements. We compare the performances of the different classes of models in this respect. We then consider, in a supersymmetric framework, the important implications of these flavour symmetries on lepton flavour violating processes, like μ-> e gamma and similar processes. We discuss how the existing limits constrain these models, once their parameters are adjusted so as to optimize the agreement with the measured values of the mixing angles. In the simplified CMSSM context, adopted here just for indicative purposes, the small tan(beta) range and heavy SUSY mass scales are favoured by lepton flavour violating processes, which makes it even more difficult to reproduce the reported muon g-2 discrepancy.

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arXiv2013

Repressing Anarchy in Neutrino Mass Textures

The recent results that $θ_{13}$ is relatively large, of the order of the previous upper bound, and the indications of a sizable deviation of $θ_{23}$ from the maximal value are in agreement with the predictions of Anarchy in the lepton sector. The quark and charged lepton hierarchies can then be reproduced in a SU(5) GUT context by attributing non-vanishing $U(1)_{FN}$ charges, different for each family, only to the SU(5) tenplet states. The fact that the observed mass hierarchies are stronger for up quarks than for down quarks and charged leptons supports this idea. As discussed in the past, in the flexible context of $SU(5)\otimes U(1)_{FN}$, different patterns of charges can be adopted going from Anarchy to various types of hierarchy. We revisit this approach by also considering new models and we compare all versions to the present data. As a result we confirm that, by relaxing the extreme ansatz of equal $U(1)_{FN}$ charges for all SU(5) pentaplets and singlets, better agreement with the data than for Anarchy is obtained without increasing the model complexity. We also present the distributions obtained in the different models for the Dirac CP-violating phase. Finally we discuss the relative merits of these simple models.

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arXiv2013

Dark matter within the minimal flavour violation ansatz

Minimal Flavour Violation hypothesis can provide an attractive framework for Dark Matter (DM). We consider scalar DM candidates carrying flavour quantum numbers and whose representation under the flavour group guarantees DM stability. They interact with the Standard Model fields through Higgs portal at renormalisable level and also to quarks through dimension-6 operators. We provide a systematic analysis of the viable parameter space for the DM fields, which are triplet of the flavour group, considering several DM-quark interactions. In this framework, we analyse in which cases the viable parameter space differs from Higgs portal models thanks to the underlying flavour structure. In contrast to minimal Higgs portal scenarios, we find that light DM in the GeV mass range as well as heavier candidates above Higgs resonance could be allowed by colliders, direct and indirect DM detection searches as well as flavour constraints. The large mass regime above the top mass could even be beyond the reach of future experiments such as Xenon 1T.

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arXiv2012

Constraining Flavour Symmetries At The EW Scale I: The A4 Higgs Potential

We consider an extension of the Standard Model in which the symmetry is enlarged by a global flavour factor A4 and the scalar sector accounts for three copies of the Standard Model Higgs, transforming as a triplet of A4. In this context, we study the most general scalar potential and its minima, performing for each of them a model independent analysis on the related phenomenology. We study the scalar spectrum, the new contributions to the oblique corrections, the decays of the Z and W, the new sources of flavour violation, which all are affected by the introduction of multiple Higgses transforming under A4. We find that this model independent approach discriminates the different minima allowed by the scalar potential.

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arXiv2012

Constraining Flavour Symmetries At The EW Scale II: The Fermion Processes

We study the set of models in which the Standard Model symmetry is extended with the flavour group A4 and there are three copies of the Standard Model Higgs that transform as a triplet under this group. In this setup, new channels for flavour violating processes can be studied once the A4 representations of the fermions in the theory are given. We show that it is of great importance to take these constraints into account as they can put severe constraints on the viability of flavour models.

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arXiv2012

Neutrino Mixings and the S4 Discrete Flavour Symmetry

Discrete non-Abelian Symmetries have been extensively used to reproduce the lepton mixings. In particular, the S4 group turned out to be suitable to describe predictive mixing patterns, such as the well-known Tri-Bimaximal and the Bimaximal schemes, which all represent possible first approximations of the experimental lepton mixing matrix. We review the main application of the S4 discrete group as a flavour symmetry, first dealing with the formalism and later with the phenomenological implications. In particular, we summarize the main features of flavour models based on S4, commenting on their ability in reproducing a reactor angle in agreement with the recent data and on their predictions for lepton flavour violating transitions.

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arXiv2012

Theoretical Models for Neutrino Masses

The recent measurements of the neutrino reactor angle require a re-examination of flavour models based on discrete groups. Indeed, when these models deal with the Tri-Bimaximal, the Bimaximal and the Golden Ratio mixing patterns, some tensions arise in order to accommodate the reactor angle. In particular, strong constraints come from lepton flavour violating processes, like mu -> e gamma. We present the analysis and the main results.

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arXiv2012

Phenomenology of a Gauged SU(3)^3 Flavour Model

We present an extensive analysis of Delta F=2 observables and of B -> X_s gamma in the framework of a specific Maximally Gauged Flavour (MGF) model of Grinstein et al. including all relevant contributions, in particular tree-level heavy gauge boson exchanges that are considered in the present paper for the first time. The model allows in principle for significant deviations from the Standard Model predictions for epsilon_K, Delta M_{B_{d,s}}, mixing induced CP-asymmetries S_{psi K_S} and S_{psi phi} and B -> X_s gamma decay. The tension between epsilon_K and S_{psi K_S} present in the SM can be removed by enhancing |epsilon_K| without modifying S_{psi K_S}. In this case, we find that in this model i) the results for S_{psi phi} and B -> X_s gamma turn out to be SM-like, ii) the exclusive determination of |Vub| is favoured and most importantly iii) the values of Delta M_{B_d} and Delta M_{B_s} being strongly correlated in this model with epsilon_K turn out to be much larger than the data for the central values of input parameters: Delta M_{B_d}~0.75/ps and Delta M_{B_s}~27/ps. Therefore, from the present perspective, the model suffers from a serious epsilon_K-Delta M_{B_{d,s}} tension. However, this tension can be softened considering theoretical and parametric uncertainties and in particular the decrease of the weak decay constants. On the other side, the model can be strongly constrained considering the theoretically cleaner ratios Delta M_{B_d}/Delta M_{B_s} and BR(B^+ -> tau^+ nu)/Delta M_{B_d} and we find that it is unable to remove simultaneously all the SM tensions on the data. Finally, we compare the pattern of flavour violation in MGF with selected extensions of the SM.

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arXiv2012

The Impact of Flavour Changing Neutral Gauge Bosons on B->X_s gamma

The branching ratio of the rare decay B->X_s gamma provides potentially strong constraints on models beyond the Standard Model. Considering a general scenario with new heavy neutral gauge bosons, present in particular in Z' and gauge flavour models, we point out two new contributions to the B->X_s gamma decay. The first one originates from one-loop diagrams mediated by gauge bosons and heavy exotic quarks with electric charge -1/3. The second contribution stems from the QCD mixing of neutral current-current operators generated by heavy neutral gauge bosons and the dipole operators responsible for the B->X_s gamma decay. The latter mixing is calculated here for the first time. We discuss general sum rules which have to be satisfied in any model of this type. We emphasise that the neutral gauge bosons in question could also significantly affect other fermion radiative decays as well as non-leptonic two-body B decays, epsilon'/epsilon, anomalous (g-2)_mu and electric dipole moments.

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arXiv2012

Proceedings of the first workshop on Flavor Symmetries and consequences in Accelerators and Cosmology (FLASY2011)

The main goals of the first "Workshop on FLAvor SYmmetries and consequences in accelerators and cosmology" (FLASY) was to summarize the theoretical status of flavor symmetries, bringing together young researchers in the field to stimulate discussions and new collaborations, with the aim of investigating possible new physics scenarios to be tested at the LHC, as well as in future neutrino, cosmology experiments and dark matter searches.

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arXiv2011

Flavour violation in a supersymmetric T' model

We describe the phenomenology of the flavour changing neutral current sector of a supersymmetric model, invariant under the T' discrete flavour group. This model has been proposed in Ref.[1] and describes realistic leptonic and hadronic masses and mixings and predicts the amount of flavour changing in terms of the small flavour breaking parameter u in [0.007, 0.05]. For small values of u, the model almost reduces to the MSUGRA framework, while sizable deviations from MSUGRA can be, instead, observed when larger values of u and tan(beta) are considered. We analyse in detail the T' BR(mu --> e gamma) prediction, concerning the leptonic sector, while for the hadronic sector we concentrate on b --> s gamma and neutral B meson mass differences. Moreover, for the first time a comparative study on leptonic and hadronic observables for the T' model is performed. The experimental data on FCNC observables severely constrain the model in the small m_0 region. Conversely for larger m_0, the T' model satisfies all the bounds.

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arXiv2011

A T' Flavour Model for Fermions and its Phenomenology

We present a supersymmetric flavour model based on the T' discrete group, which explains fermion masses and mixings. The flavour symmetry, acting in the supersymmetric sector, provides well defined sfermion mass matrices and the resulting supersymmetric spectrum accounts for sufficiently light particles that could be seen at LHC. Furthermore, several contributions to FCNC processes are present and they can be useful to test the model in the present and future experiments. We will review the main results for both leptons and quarks.

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arXiv2011

Minimal flavour violation extensions of the seesaw

We analyze the most natural formulations of the minimal lepton flavour violation hypothesis compatible with a type-I seesaw structure with three heavy singlet neutrinos N, and satisfying the requirement of being predictive, in the sense that all LFV effects can be expressed in terms of low energy observables. We find a new interesting realization based on the flavour group $SU(3)_e\times SU(3)_{\ell+N}$ (being $e$ and $\ell$ respectively the SU(2) singlet and doublet leptons). An intriguing feature of this realization is that, in the normal hierarchy scenario for neutrino masses, it allows for sizeable enhancements of $μ\to e$ transitions with respect to LFV processes involving the $τ$ lepton. We also discuss how the symmetries of the type-I seesaw allow for a strong suppression of the N mass scale with respect to the scale of lepton number breaking, without implying a similar suppression for possible mechanisms of N production

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arXiv2011

Bimaximal Neutrino Mixing with Discrete Flavour Symmetries

In view of the fact that the data on neutrino mixing are still compatible with a situation where Bimaximal mixing is valid in first approximation and it is then corrected by terms of order of the Cabibbo angle, we present examples where these properties are naturally realized. The models are supersymmetric in 4-dimensions and based on the discrete non-Abelian flavour symmetry S4.

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