L. Merlo | Universidad Autónoma de Madrid (original) (raw)
Papers by L. Merlo
The Higgs portal to scalar Dark Matter is considered in the context of non-linearly realised elec... more The Higgs portal to scalar Dark Matter is considered in the context of non-linearly realised electroweak symmetry breaking. We determine the dominant interactions of gauge bosons and the physical Higgs particle hhh to a scalar singlet dark matter candidate. Phenomenological consequences are also studied in detail, including the possibility of distinguishing this scenario from the standard Higgs portal in which the electroweak symmetry breaking is linearly realised. Two features of significant impact are: i) the connection between the electroweak scale vvv and the Higgs particle departs from the (v+h)(v+h)(v+h) functional dependence, as the Higgs field is not necessarily an exact electroweak doublet; ii) the presence of specific couplings that arise at different order in the non-linear and in the linear expansions. These facts deeply affect the dark matter relic abundance, as well as the expected signals in direct and indirect searches and collider phenomenology, where Dark Matter production rates are enhanced with respect to the standard portal.
Journal of High Energy Physics, 2016
Flavour physics is a priceless window on physics beyond the Standard Model. In particular, flavou... more Flavour physics is a priceless window on physics beyond the Standard Model. In particular, flavour violation in the lepton sector looks very promising, as high precision measurements are prospected in future experiments investigating on murightarrowe\mu\rightarrow emurightarrowe conversion in atomic nuclei: the predictions for this observable are analysed in the context of the type I Seesaw mechanism. Furthermore, new ideas to explain the Flavour Puzzle recently appeared, mainly based on a possible dynamical origin of the Yukawa couplings and on flavour symmetries. The focus of this proceeding will be set on the Minimal Flavour Violation ansatz and on the role of the neutrino Majorana character: when an O(2)NO(2)_{N}O(2)N flavour symmetry acts on the right-handed neutrino sector, the minimum of the scalar potential allows for large mixing angles -in contrast to the simplest quark case- and predicts a maximal Majorana phase. This leads to a strong correlation between neutrino mass hierarchy and mixing pattern.
With the discovery of a scalar resonance at ATLAS and CMS, the understanding of the electroweak s... more With the discovery of a scalar resonance at ATLAS and CMS, the understanding of the electroweak symmetry breaking origin seems a much closer goal. A strong dynamics at relatively low scales is still a good candidate. In this talk, the complete effective Lagrangian up to d < 6 will be presented, both for the gauge and the flavour sectors. Interesting features in the flavour phenomenology will be discussed.
![Research paper thumbnail of Bimaximal Neutrino Mixing and Weak Complementarity with S[sub 4] Discrete Symmetry](https://attachments.academia-assets.com/45989350/thumbnails/1.jpg)
](The neutrino oscillation data are well explained by the tri-bimaximal pattern. Recently a paper a... more The neutrino oscillation data are well explained by the tri-bimaximal pattern. Recently a paper appeared showing that also the bimaximal pattern could be a very good starting point in order to describe the lepton mixing. In this paper I review both the flavour structures and then I present an explicit model.
In composite Higgs models the Higgs is a pseudo-Goldstone boson of a high-energy strong dynamics.... more 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)SU(5)/SO(5)SU(5)/SO(5) Georgi-Kaplan model, the minimal custodial-preserving SO(5)/SO(4)SO(5)/SO(4)SO(5)/SO(4) model and the minimal SU(3)/(SU(2)timesU(1))SU(3)/(SU(2)\times U(1))SU(3)/(SU(2)timesU(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.
Journal of High Energy Physics, 2011
Assuming the Minimal Flavour Violation hypothesis, we derive the general scalar potential for fie... more Assuming the Minimal Flavour Violation hypothesis, we derive the general scalar potential for fields whose background values are the Yukawa couplings. We analyze the minimum of the potential and discuss the fine-tuning required to dynamically generate the mass hierarchies and the mixings between different quark generations. Two main cases are considered, corresponding to Yukawa interactions being effective operators of dimension five or six (or, equivalently, resulting from bi-fundamental and fundamental scalar fields, respectively). At the renormalizable and classical level, no mixing is naturally induced from dimension five Yukawa operators. On the contrary, from dimension six Yukawa operators one mixing angle and a strong mass hierarchy among the generations result.
Journal of High Energy Physics, 2011
We analyze the most natural formulations of the minimal lepton flavour violation hypothesis compa... more 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 × SU (3) ℓ+N (being e and ℓ 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 → 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.
Physics Letters B, 2013
We generalize the basis of CP-even chiral effective operators describing a dynamical Higgs sector... more We generalize the basis of CP-even chiral effective operators describing a dynamical Higgs sector, to the case in which the Higgs-like particle is light. Gauge and gauge-Higgs operators are considered up to four derivatives. This analysis completes the tool needed to explore at leading order the connection between linear realizations of the electroweak symmetry breaking mechanism -whose extreme case is the Standard Model -and non-linear realizations with a light Higgs-like particle present. It may also provide a model-independent guideline to explore which exotic gauge-Higgs couplings may be expected, and their relative strength to Higgsless observable amplitudes. With respect to fermions, the analysis is reduced by nature to the consideration of those flavour-conserving operators that can be written in terms of pure-gauge or gauge-Higgs ones via the equations of motion, but for the standard Yukawa-type couplings.
Physical Review D, 2013
The Higgs-fermion couplings are sensitive probes of possible new physics behind a stable light Hi... more The Higgs-fermion couplings are sensitive probes of possible new physics behind a stable light Higgs particle. It is then essential to identify the flavour pattern of those interactions. We consider the case in which a strong dynamics lies behind a light Higgs, and explore the implications within the Minimal Flavour Violation ansatz. The dominant effects on flavour-changing Higgs-fermion couplings stem in this context from operators with mass dimension ≤ 5, and we analyze all relevant chiral operators up to that order, including loop-corrections induced by 4-dimensional ones. Bounds on the operator coefficients are derived from a plethora of low-energy flavour transitions, providing a guideline on which flavour-changing Higgs interactions may be open to experimental scrutiny. In particular, the coefficient of a genuinely CP-odd operator is only softly constrained and therefore its impact is potentially interesting.
Nuclear Physics B, 2010
In models with flavour symmetries added to the gauge group of the Standard Model the CP-violating... more In models with flavour symmetries added to the gauge group of the Standard Model the CP-violating asymmetry necessary for leptogenesis may be related with low-energy parameters. A particular case of interest is when the flavour symmetry produces exact Tri-Bimaximal lepton mixing leading to a vanishing CP-violating asymmetry. In this paper we present a model-independent discussion that confirms this always occurs for unflavoured leptogenesis in type I see-saw scenarios, noting however that Tri-Bimaximal mixing does not imply a vanishing asymmetry in general scenarios where there is interplay between type I and other see-saws. We also consider a specific model where the exact Tri-Bimaximal mixing is lifted by corrections that can be parametrised by a small number of degrees of freedom and analyse in detail the existing link between low and high-energy parameters -focusing on how the deviations from Tri-Bimaximal are connected to the parameters governing leptogenesis.
Fortschritte der Physik, 2013
We review the application of non-Abelian discrete groups to Tri-Bimaximal (TB) neutrino mixing, w... more 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 θ13 favour versions of these models where a suitable mechanism leads to corrections to θ13 that can naturally be larger than those to θ12 and θ23. 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.
The pattern of deviations from Standard Model predictions and couplings is different for theories... more The pattern of deviations from Standard Model predictions and couplings is different for theories of new physics based on a non-linear realization of the SU(2)LtimesU(1)YSU(2)_L\times U(1)_YSU(2)LtimesU(1)Y gauge symmetry breaking and those assuming a linear realization. We clarify this issue in a model-independent way via its effective Lagrangian formulation in the presence of a light Higgs particle, up to first order in the expansions: dimension-six operators for the linear expansion and four derivatives for the non-linear one. Complete sets of pure gauge and gauge-Higgs operators are considered, implementing the renormalization procedure and deriving the Feynman rules for the non-linear expansion. We establish the theoretical relation and the differences in physics impact between the two expansions. Promising discriminating signals include the decorrelation in the non-linear case of signals correlated in the linear one: some pure gauge versus gauge-Higgs couplings and also between couplings with the same number of Higgs legs. Furthermore, anomalous signals expected at first order in the non-linear realization may appear only at higher orders of the linear one, and vice versa. We analyze in detail the impact of both type of discriminating signals on LHC physics.
The Higgs portal to scalar Dark Matter is considered in the context of non-linearly realised elec... more The Higgs portal to scalar Dark Matter is considered in the context of non-linearly realised electroweak symmetry breaking. We determine the dominant interactions of gauge bosons and the physical Higgs particle hhh to a scalar singlet dark matter candidate. Phenomenological consequences are also studied in detail, including the possibility of distinguishing this scenario from the standard Higgs portal in which the electroweak symmetry breaking is linearly realised. Two features of significant impact are: i) the connection between the electroweak scale vvv and the Higgs particle departs from the (v+h)(v+h)(v+h) functional dependence, as the Higgs field is not necessarily an exact electroweak doublet; ii) the presence of specific couplings that arise at different order in the non-linear and in the linear expansions. These facts deeply affect the dark matter relic abundance, as well as the expected signals in direct and indirect searches and collider phenomenology, where Dark Matter production rates are enhanced with respect to the standard portal.
Journal of High Energy Physics, 2016
Flavour physics is a priceless window on physics beyond the Standard Model. In particular, flavou... more Flavour physics is a priceless window on physics beyond the Standard Model. In particular, flavour violation in the lepton sector looks very promising, as high precision measurements are prospected in future experiments investigating on murightarrowe\mu\rightarrow emurightarrowe conversion in atomic nuclei: the predictions for this observable are analysed in the context of the type I Seesaw mechanism. Furthermore, new ideas to explain the Flavour Puzzle recently appeared, mainly based on a possible dynamical origin of the Yukawa couplings and on flavour symmetries. The focus of this proceeding will be set on the Minimal Flavour Violation ansatz and on the role of the neutrino Majorana character: when an O(2)NO(2)_{N}O(2)N flavour symmetry acts on the right-handed neutrino sector, the minimum of the scalar potential allows for large mixing angles -in contrast to the simplest quark case- and predicts a maximal Majorana phase. This leads to a strong correlation between neutrino mass hierarchy and mixing pattern.
With the discovery of a scalar resonance at ATLAS and CMS, the understanding of the electroweak s... more With the discovery of a scalar resonance at ATLAS and CMS, the understanding of the electroweak symmetry breaking origin seems a much closer goal. A strong dynamics at relatively low scales is still a good candidate. In this talk, the complete effective Lagrangian up to d < 6 will be presented, both for the gauge and the flavour sectors. Interesting features in the flavour phenomenology will be discussed.
The neutrino oscillation data are well explained by the tri-bimaximal pattern. Recently a paper a... more The neutrino oscillation data are well explained by the tri-bimaximal pattern. Recently a paper appeared showing that also the bimaximal pattern could be a very good starting point in order to describe the lepton mixing. In this paper I review both the flavour structures and then I present an explicit model.
In composite Higgs models the Higgs is a pseudo-Goldstone boson of a high-energy strong dynamics.... more 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)SU(5)/SO(5)SU(5)/SO(5) Georgi-Kaplan model, the minimal custodial-preserving SO(5)/SO(4)SO(5)/SO(4)SO(5)/SO(4) model and the minimal SU(3)/(SU(2)timesU(1))SU(3)/(SU(2)\times U(1))SU(3)/(SU(2)timesU(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.
Journal of High Energy Physics, 2011
Assuming the Minimal Flavour Violation hypothesis, we derive the general scalar potential for fie... more Assuming the Minimal Flavour Violation hypothesis, we derive the general scalar potential for fields whose background values are the Yukawa couplings. We analyze the minimum of the potential and discuss the fine-tuning required to dynamically generate the mass hierarchies and the mixings between different quark generations. Two main cases are considered, corresponding to Yukawa interactions being effective operators of dimension five or six (or, equivalently, resulting from bi-fundamental and fundamental scalar fields, respectively). At the renormalizable and classical level, no mixing is naturally induced from dimension five Yukawa operators. On the contrary, from dimension six Yukawa operators one mixing angle and a strong mass hierarchy among the generations result.
Journal of High Energy Physics, 2011
We analyze the most natural formulations of the minimal lepton flavour violation hypothesis compa... more 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 × SU (3) ℓ+N (being e and ℓ 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 → 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.
Physics Letters B, 2013
We generalize the basis of CP-even chiral effective operators describing a dynamical Higgs sector... more We generalize the basis of CP-even chiral effective operators describing a dynamical Higgs sector, to the case in which the Higgs-like particle is light. Gauge and gauge-Higgs operators are considered up to four derivatives. This analysis completes the tool needed to explore at leading order the connection between linear realizations of the electroweak symmetry breaking mechanism -whose extreme case is the Standard Model -and non-linear realizations with a light Higgs-like particle present. It may also provide a model-independent guideline to explore which exotic gauge-Higgs couplings may be expected, and their relative strength to Higgsless observable amplitudes. With respect to fermions, the analysis is reduced by nature to the consideration of those flavour-conserving operators that can be written in terms of pure-gauge or gauge-Higgs ones via the equations of motion, but for the standard Yukawa-type couplings.
Physical Review D, 2013
The Higgs-fermion couplings are sensitive probes of possible new physics behind a stable light Hi... more The Higgs-fermion couplings are sensitive probes of possible new physics behind a stable light Higgs particle. It is then essential to identify the flavour pattern of those interactions. We consider the case in which a strong dynamics lies behind a light Higgs, and explore the implications within the Minimal Flavour Violation ansatz. The dominant effects on flavour-changing Higgs-fermion couplings stem in this context from operators with mass dimension ≤ 5, and we analyze all relevant chiral operators up to that order, including loop-corrections induced by 4-dimensional ones. Bounds on the operator coefficients are derived from a plethora of low-energy flavour transitions, providing a guideline on which flavour-changing Higgs interactions may be open to experimental scrutiny. In particular, the coefficient of a genuinely CP-odd operator is only softly constrained and therefore its impact is potentially interesting.
Nuclear Physics B, 2010
In models with flavour symmetries added to the gauge group of the Standard Model the CP-violating... more In models with flavour symmetries added to the gauge group of the Standard Model the CP-violating asymmetry necessary for leptogenesis may be related with low-energy parameters. A particular case of interest is when the flavour symmetry produces exact Tri-Bimaximal lepton mixing leading to a vanishing CP-violating asymmetry. In this paper we present a model-independent discussion that confirms this always occurs for unflavoured leptogenesis in type I see-saw scenarios, noting however that Tri-Bimaximal mixing does not imply a vanishing asymmetry in general scenarios where there is interplay between type I and other see-saws. We also consider a specific model where the exact Tri-Bimaximal mixing is lifted by corrections that can be parametrised by a small number of degrees of freedom and analyse in detail the existing link between low and high-energy parameters -focusing on how the deviations from Tri-Bimaximal are connected to the parameters governing leptogenesis.
Fortschritte der Physik, 2013
We review the application of non-Abelian discrete groups to Tri-Bimaximal (TB) neutrino mixing, w... more 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 θ13 favour versions of these models where a suitable mechanism leads to corrections to θ13 that can naturally be larger than those to θ12 and θ23. 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.
The pattern of deviations from Standard Model predictions and couplings is different for theories... more The pattern of deviations from Standard Model predictions and couplings is different for theories of new physics based on a non-linear realization of the SU(2)LtimesU(1)YSU(2)_L\times U(1)_YSU(2)LtimesU(1)Y gauge symmetry breaking and those assuming a linear realization. We clarify this issue in a model-independent way via its effective Lagrangian formulation in the presence of a light Higgs particle, up to first order in the expansions: dimension-six operators for the linear expansion and four derivatives for the non-linear one. Complete sets of pure gauge and gauge-Higgs operators are considered, implementing the renormalization procedure and deriving the Feynman rules for the non-linear expansion. We establish the theoretical relation and the differences in physics impact between the two expansions. Promising discriminating signals include the decorrelation in the non-linear case of signals correlated in the linear one: some pure gauge versus gauge-Higgs couplings and also between couplings with the same number of Higgs legs. Furthermore, anomalous signals expected at first order in the non-linear realization may appear only at higher orders of the linear one, and vice versa. We analyze in detail the impact of both type of discriminating signals on LHC physics.