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M. J. Herrero

Publications and source records attributed to M. J. Herrero.

70 records · Page 4Linked to original sources
arXiv2005

Lepton flavor violating Higgs boson decays from massive seesaw neutrinos

Lepton flavor violating Higgs boson decays are studied within the context of seesaw models with Majorana massive neutrinos. Two models are considered: The SM-seesaw, with the Standard Model Particle content plus three right handed neutrinos, and the MSSM-seesaw, with the Minimal Supersymmetric Standard Model particle content plus three right handed neutrinos and their supersymmetric partners. The widths for these decays are derived from a full one-loop diagrammatic computation in both models, and they are analyzed numerically in terms of the seesaw parameters, namely, the Dirac and Majorana mass matrices. Several possible scenarios for these mass matrices that are compatible with neutrino data are considered. In the SM-seesaw case, very small branching ratios are found for all studied scenarios. These ratios are explained as a consequence of the decoupling behaviour of the heavy right handed neutrinos. In contrast, in the MSSM-seesaw case, sizeable branching ratios are found for some of the leptonic flavor violating decays of the MSSM neutral Higgs bosons and for some choices of the seesaw matrices and MSSM parameters. The relevance of the two competing sources of lepton flavor changing interactions in the MSSM-seesaw case is also discussed. The non-decoupling behaviour of the supersymmetric particles contributing in the loop-diagrams is finally shown.

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arXiv2004

Lepton flavor violating Higgs boson decays in the MSSM-seesaw

Lepton flavor violating Higgs boson decays (LFVHD) are studied in the context of the Minimal Supersymmetric Standard Model (MSSM) enlarged with three right handed neutrinos and their supersymmetric partners, and with the neutrino masses being generated by the seesaw mechanism. We compute the partial widths for these decays to one-loop order and analyze numerically the corresponding branching ratios in terms of the MSSM and seesaw parameters. We analyze in parallel the lepton flavor changing $ł_j\to l_i γ$ decays and explore the maximum predicted rates for LFVHD, mainly for $H^0,A^0 \to τ{\bar μ}$ decays, by requiring compatibility with neutrino and $BR(ł_j\to l_i γ)$ data. We find LFVHD ratios of up to $10^{-5}$ in some regions of the MSSM-seesaw parameter space.

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arXiv2004

SUSY-electroweak one-loop contributions to Flavour-Changing Higgs-Boson Decays

The SUSY-EW one-loop quantum contributions to flavour-changing MSSM Higgs-boson decays into $b \bar s$ and $s \bar b$ are computed and discussed. We use the full diagrammatic approach that is valid for all $\tan β$ values and do not rely on the mass-insertion approximation for the characteristic flavour-changing parameter. We analyze in full detail the dependence of these flavour-changing partial widths on all the relevant MSSM parameters and also study the non-decoupling behaviour of these widths with the SUSY mass parameters. We find that these contributions are sizable as compared to the SM ones, and together with the SUSY-QCD contributions they can be very efficient as an indirect method in the future search for Supersymmetry.

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arXiv2001

Restriction on SUSY Masses from $Z\to hadrons$

We remark that the precision of recent determinations of $α_s(M^2_Z)$ is such that one can get bounds on supersymmetric partner masses (squark and gluino) by requiring consistency of determinations of $α_s$ at "low" energies, where those particles do not contribute, and on the $Z$ peak. For approximately degenerate squarks and gluinos with mass $\widetilde{m}$, we find the bound $\widetilde{m}\geq 173$ GeV, at the $2σ$ level. At the $2.5σ$ level, $\widetilde{m}\geq 121$ GeV.

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arXiv2001

Decoupling Properties of MSSM particles in Higgs and Top Decays

We study the supersymmetric (SUSY) QCD radiative corrections, at the one-loop level, to $h^0$, $H^{\pm}$ and t quark decays, in the context of the Minimal Supersymmetric Standard Model (MSSM) and in the decoupling limit. The decoupling behaviour of the various MSSM sectors is analyzed in some special cases, where some or all of the SUSY mass parameters become large as compared to the electroweak scale. We show that in the decoupling limit of both large SUSY mass parameters and large CP-odd Higgs mass, the $Γ(h^0\to b \bar b)$ decay width approaches its Standard Model value at one loop, with the onset of decoupling being delayed for large $\tanβ$ values. However, this decoupling does not occur if just the SUSY mass parameters are taken large. A similar interesting non-decoupling behaviour, also enhanced by $\tanβ$, is found in the SUSY-QCD corrections to the $Γ(H^+\to t \bar b)$ decay width at one loop. In contrast, the SUSY-QCD corrections in the $Γ(t\to W^+ b)$ decay width do decouple and this decoupling is fast.

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arXiv2000

LHC sensitivity to the resonance spectrum of a minimal strongly interacting electroweak symmetry breaking sector

We present a unified analysis of the two main production processes of vector boson pairs at the LHC, VV-fusion and qqbar annihilation, in a minimal strongly interacting electroweak symmetry breaking sector. Using a unitarized electroweak chiral Lagrangian formalism and modeling the final V_L V_L strong rescattering effects by a form factor, we describe qqbar annihilation processes in terms of the two chiral parameters that govern elastic V_L V_L scattering. Depending on the values of these two chiral parameters, the unitarized amplitudes may present resonant enhancements in different angular momentum-isospin channels. Scanning this two parameter space, we generate the general resonance spectrum of a minimal strongly interacting electroweak symmetry breaking sector and determine the regions that can be probed at the LHC.

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arXiv1999

The SM as the Quantum Low Energy Effective Theory of the MSSM

In the framework of the Minimal Supersymmetric Standard Model we compute the one-loop effective action for the electroweak bosons obtained after integrating out the different sleptons, squarks, neutralinos and charginos, and present the result in terms of the physical sparticle masses. In addition we study the asymptotic behaviour of the two, three and four point Green's functions with external electroweak bosons in the limit where the physical sparticle masses are very large in comparison with the electroweak scale. We find that in this limit all the effects produced by the supersymmetric particles can be, either absorbed in the Standard Model parameters and gauge bosons wave functions, or else they are supressed by inverse powers of the supersymmetric particle masses. This work, therefore, completes the proof of decoupling of the heavy supersymmetric particles from the standard ones in the electroweak bosons effective action and in the sense of the Appelquist-Carazzone Theorem that we started in a previous work. From the point of view of effective field theories this work can be seen as a (partial) proof that the SM can be obtained indeed from the MSSM as the quantum low energy effective theory of this latter when the SUSY spectra is much heavier than the electroweak scale.

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arXiv1998

The Standard Model

These lectures provide an introduction to the basic aspects of the Standard Model, $SU(3)_{C} \times SU(2)_{L} \times U(1)_{Y}$.

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arXiv1998

Decoupling of supersymmetric particles

The possibility of a heavy supersymmetric spectrum at the Minimal Supersymmetric Standard Model is considered and the decoupling from the low energy electroweak scale is analyzed in detail. The formal proof of decoupling of supersymmetric particles from low energy physics is stated in terms of the effective action for the particles of the Standard Model that results by integrating out all the sparticles in the limit where their masses are larger than the electroweak scale. The computation of the effective action for the standard electroweak gauge bosons W^{+-}, Z and γis performed by integrating out all the squarks, sleptons, charginos and neutralinos to one-loop. The Higgs sector is not considered in this paper. The large sparticle masses limit is also analyzed in detail. Explicit analytical formulae for the two-point functions of the electroweak gauge bosons to be valid in that limit are presented. Finally, the decoupling of sparticles in the S, T and U parameters is studied analitically. A discussion on how the decoupling takes place in terms of both the physical sparticle masses and the non-physical mass parameters as the μ-parameter and the soft-breaking parameters is included.

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arXiv1998

Do heavy sfermions decouple from low energy Standard Model?

We explore analytically how does the Standard Model emerge as the quantum low energy effective theory of the Minimal Supersymmetric Standard Model (MSSM) in the decoupling limit where the sparticles are much heavier than the electroweak scale. In this work we integrate the sfermions to one-loop and compute their contributions to the effective action for standard electroweak gauge bosons. A proof of decoupling of sfermions is performed by analyzing the resulting effective action in the asymptotic limit $m_{\tilde f} \gg m_z$. A discussion on how the decoupling takes place in terms of both the sparticle physical masses and the non-physical MSSM parameters is included.

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arXiv1997

Decoupling of Supersymmetric Particles in the MSSM

A heavy supersymmetric spectrum at the Minimal Supersymmetric Standard Model is considered and the decoupling from the low energy electroweak scale is analyzed. A formal and partial proof of decoupling of supersymmetric particles in the limit where their masses are larger than the electroweak scale is performed by integrating out all the sparticles to one loop and by evaluating the effective action for the standard electroweak gauge bosons $W^{\pm}, Z$ and $γ$. The Higgs sector is not considered here. Analytical results for the two-point functions of the electroweak gauge bosons and the $S, T$ and $U$ parameters, to be valid in that limit, are also presented. A discussion on how the decoupling takes place in terms of both the physical sparticle masses and the non-physical mass parameters as the $μ$-parameter and the soft-breaking parameters is included.

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arXiv1995

Learning about the strongly interacting symmetry breaking sector at LHC

In the present work we study the predictions for $WZ$ and $ZZ$ production at LHC with the Electroweak Chiral Lagrangian (EChL) approach. Our analysis will be focused on the less favored case from the experimental point of view, in which the predictions for the gauge bosons scattering amplitudes are considered in the low energy range where, by construction of the low energy approach, they reveal no resonant behavior. The study includes the complete set of amplitudes for all the polarization states of the initial and/or final gauge bosons and makes no use of the Equivalence Theorem. We express the results in terms of the range of values of the chiral parameters that will be accessible at LHC.

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arXiv1994

The electroweak chiral Lagrangian as an effective field theory of the standard model with a heavy Higgs

Using effective field theory methods, we integrate out the standard model Higgs boson to one loop and represent its non-decoupling effects by a set of gauge invariant effective operators of the electroweak chiral Lagrangian. We briefly discuss the relation between the renormalization of both the standard model and the effective theory, which is crucial for a correct understanding and use of the electroweak chiral Lagrangian. Some examples have been chosen to show the applicability of this effective Lagrangian approach in the calculation of low energy observables in electroweak theory.

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arXiv1994

The electroweak chiral parameters for a heavy Higgs in the Standard Model

We study the electroweak interactions within the standard electroweak theory in the case where the Higgs particle is heavy, namely, $M_H \leq 1\;TeV$. By integrating out the Higgs boson to one loop we find the complete effective lagrangian, called electroweak chiral lagrangian, that is $SU(2)_L\times U(1)_Y$ invariant and contains the whole set of operators up to dimension four. The values of the chiral parameters representing the non-decoupling effects of the heavy Higgs are presented. Some examples have been chosen to show the applicability of this effective lagrangian approach to compute phenomenologically relevant quantities that either are being meassured at present or will be meassured in future experiments.

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arXiv1993

The Electroweak Chiral Lagrangian for the Standard Model with a Heavy Higgs

The most general chiral Lagrangian for electroweak interactions with the complete set of $SU(2)_L\times U(1)_Y$ invariant operators up to dimension four is considered. The two-point and three-point functions with external gauge fields are derived from this effective chiral Lagrangian to one-loop order in a generic $R_ξ$-gauge. The same set of Green's functions are paralelly studied in the renormalizable standard model to one-loop order, in a $R_ξ$-gauge and in the large Higgs mass limit. An appropriate set of matching conditions connecting the Green's functions of the two theories allows us to derive, systematically, the values of the chiral Lagrangian coefficients corresponding to the large Higgs mass limit of the standard model. These chiral parameters represent the non-decoupling effects of a heavy Higgs particle and incorporate both the leading logarithmic dependence on $\mh$ and the next to leading constant contributions. Some phenomenological implications are also discussed.

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