Lepton Flavor Violating Higgs Boson Research Articles

Lepton Flavor Violating Higgs Boson Decays in Supersymmetric High Scale Seesaw Models

Within the MSSM, we have evaluated the decay rates for the lepton flavour violating Higgs boson decays (LFVHD) $h \rightarrow l_i l_j$ where $l_{i,j}$ are charged leptons and $i\neq j$. This has been done in a model independent (MI) way as well as in supersymmetric high scale seesaw models, in particular Type I see-saw model. Lepton flavour violation (LFV) is generated by non-diagonal entries in the mass matrix of the sleptons. In a first step we use the model independent approach where LFV (off-diagonal entries in the mass matrix) is introduced by hand while respecting the direct search constraints from the charged lepton flavor violating (cLFV) processes. In the second step we use high scale see-saw models where LFV is generated via renormalization group equations (RGE) from the grand unification scale (GUT) down to electroweak scale. cLFV decays are the most restrictive ones and exclude a large part of the parameter space for the MI as well as the high scale see-saw scenarios. Due to very strict constraints from cLFV, it is difficult to find large corrections to LFVHD. This applies in particular to $h \rightarrow \tau \mu$ where hints of an excess have been observed. If this signal is confirmed, it could not be explained with the models under investigation.

Effective lepton flavor violating Hℓiℓj vertex from right-handed neutrinos within the mass insertion approximation

In this work we present a new computation of the lepton flavor violating Higgs boson decays that are generated radiatively to one-loop from heavy right-handed neutrinos. We work within the context of the inverse seesaw model with three $\nu_R$ and three extra singlets $X$, but the results could be generalized to other low scale seesaw models. The novelty of our computation is that it uses a completely different method by means of the mass insertion approximation which works with the electroweak interaction states instead of the usual 9 physical neutrino mass eigenstates of the inverse seesaw model. This method also allows us to write the analytical results explicitly in terms of the most relevant model parameters, that are the neutrino Yukawa coupling matrix $Y_\nu$ and the right-handed mass matrix $M_R$, which is very convenient for a phenomenological analysis. This $Y_\nu$ matrix, being generically nondiagonal in flavor space, is the only responsible for the induced charged lepton flavor violating processes of our interest. We perform the calculation of the decay amplitude up to order ${\cal O}(Y_\nu^2+Y_\nu^4)$. We also study numerically the goodness of the mass insertion approximation results. In the last part we present the computation of the relevant one-loop effective vertex $H\ell_i\ell_j$ for the lepton flavor violating Higgs decay which is derived from a large $M_R$ mass expansion of the form factors. We believe that our simple formula found for this effective vertex can be of interest for other researchers who wish to estimate the $H \to \ell_i \bar \ell_j$ rates in a fast way in terms of their own preferred input values for the relevant model parameters $Y_\nu$ and $M_R$.

Lepton flavor violating Higgs boson decay at the ILC

We study the possible reach of the proposed International Linear Collider (ILC) in exploring the lepton flavour violating (LFV) Higgs boson decay . Two generic types of models are investigated, both involving an extended scalar sector. For the first type, the rest of the scalars are heavy and beyond the reach of ILC, but the LFV decay occurs through a tiny admixture of the Standard Model (SM) doublet with heavy degrees of freedom. In the second class, which is more constrained from the existing data, the SM Higgs boson does not have any LFV decay but there are other scalars degenerate with it that give rise to the LFV signal. We show that the ILC can pin down the branching fraction of , and hence the effective LFV Yukawa coupling, to a very small value, due to the fact that there are signal channels with unlike flavour leptons but no missing energy. It turns out that the low-energy options of the ILC, namely, or 500 GeV, are better for investigating such channels, and the option of beam polarization helps too. At least an order of magnitude improvement is envisaged over the existing limits, and the effective LFV Yukawa coupling can be probed at the level of 10−4.

τ- andμ-physics in a general two Higgs doublet model withμ−τflavor violation

Motivated by the recent CMS excess in a flavor violating Higgs decay $h \rightarrow \mu \tau$ as well as the anomaly of muon anomalous magnetic moment (muon g-2), we consider a scenario where both the excess in $h \rightarrow \mu \tau$ and the anomaly of muon g-2 are explained by the $\mu-\tau$ flavor violation in a general two Higgs doublet model. We study various processes involving $\mu$ and $\tau$, and then discuss the typical predictions and constraints in this scenario. Especially, we find that the prediction of $\tau \rightarrow \mu \gamma$ can be within the reach of the Belle II experiment. We also show that the lepton non-universality between $\tau \rightarrow \mu \nu \bar{\nu}$ and $\tau \rightarrow e \nu \bar{\nu}$ can be sizable, and hence the analysis of the current Belle data and the future experimental improvement would have an impact on this model. Besides, processes such as $\tau \rightarrow \mu l^+ l^-~(l=e,~\mu)$, $\tau \rightarrow \mu \eta$, $\mu \rightarrow e \gamma$, $\mu \rightarrow 3e$, and muon EDM can be accessible, depending on the unknown Yukawa couplings. On the other hand, the processes like $\tau \rightarrow e \gamma$ and $\tau \rightarrow e l^+ l^-~ (l=e,~\mu)$ could not be sizable to observe because of the current strong constraints on the $e-\mu$ and $e-\tau$ flavor violations. Then we also conclude that contrary to $h \rightarrow \mu \tau$ decay mode, the lepton flavor violating Higgs boson decay modes $h\rightarrow e \mu$ and $h\rightarrow e\tau$ are strongly suppressed, and hence it will be difficult to observe these modes at the LHC experiment.

Enhancement of the lepton flavor violating Higgs boson decay rates from SUSY loops in the inverse seesaw model

In this article we study the full one-loop SUSY contributions to the lepton flavor violating Higgs decay $h \to \tau \bar \mu $, within the context of the supersymmetric inverse sesaw model. We assume that both the right-handed neutrino masses, $M_R$, and their supersymmetric partner masses, $m_{\tilde \nu_R}$, are not far from the interesting ${\cal O}({\rm TeV})$ energy scale, and we work with scenarios with large neutrino Yukawa couplings that transmit large lepton flavor violating effects. By exploring the behavior with the most relevant parameters, mainly $M_R$, $m_{\tilde \nu_R}$ and the trilinear sneutrino coupling $A_\nu$, we will look for regions of the parameter space where the enhancement of $\mathrm{BR}(h\rightarrow \tau \bar \mu)$ is large enough to reach values at the percent level, which could explain the excess recently reported by CMS and ATLAS at the CERN Large Hadron Collider.

Analysis of the h, H, A → τμ decays induced from SUSY loops within the Mass Insertion Approximation

In this paper we study the lepton favor violating decay channels of the neutral Higgs bosons of the Minimal Supersymmetric Standard Model into a lepton and an anti-lepton of different flavor. We work in the context of the most general flavor mixing scenario in the slepton sector, in contrast to the minimal flavor violation assumption more frequently used. Our analytic computation is a one-loop diagrammatic one, but in contrast to the full one-loop computation which is usually referred to the physical slepton mass basis, we use here instead the Mass Insertion Approximation (MIA) which uses the electroweak interaction slepton basis and treats perturbatively the mass insertions changing slepton flavor. By performing an expansion in powers of the external momenta in the relevant form factors, we will be able to separate explicitly in the analytic results the leading non-decoupling (constant at asymptotically large sparticle masses) and the next to leading decoupling contributions (decreasing with the sparticle masses). Our final aim is to provide a set of simple analytic formulas for the form factors and the associated effective vertices, that we think may be very useful for future phenomenological studies of the lepton flavor violating Higgs boson decays, and for their comparison with data. The accuracy of the numerical results obtained with the MIA are also analyzed and discussed here in comparison with the full one-loop results. Our most optimistic numerical estimates for the three neutral Higgs boson decays channels into $\tau$ and $\mu$ leptons, searching for their maximum rates that are allowed by present constraints from $\tau \to \mu \gamma$ data and beyond Standard Model Higgs boson searches at the LHC, are also included.

Imprints of massive inverse seesaw model neutrinos in lepton flavor violating Higgs boson decays

In this paper we consider a Higgs boson with mass and other properties compatible with those of the recently discovered Higgs particle at the LHC, and explore the possibility of new Higgs leptonic decays, beyond the standard model, with the singular feature of being lepton flavor violating (LFV). We study these LFV Higgs decays, $H \to l_k\bar l_m$, within the context of the inverse seesaw model (ISS) and consider the most generic case where three additional pairs of massive right-handed singlet neutrinos are added to the standard model particle content. We require in addition that the input parameters of this ISS model are compatible with the present neutrino data and other constraints, like perturbativity of the neutrino Yukawa couplings. We present a full one-loop computation of the BR($H \to l_k\bar l_m$) rates for the three possible channels, $l_k\bar l_m=\mu \bar \tau,\, e \bar \tau,\, e \bar \mu$, and analyze in full detail the predictions as functions of the various relevant ISS parameters. We study in parallel the correlated one-loop predictions for the radiative decays, $l_m \to l_k \gamma$, within this same ISS context, and require full compatibility of our predictions with the present experimental bounds for the three radiative decays, $\mu \to e \gamma$, $\tau \to \mu \gamma$, and $\tau \to e \gamma$. After exploring the ISS parameter space we conclude on the maximum allowed LFV Higgs decay rates within the ISS.

Explaining the CMS Higgs flavor-violating decay excess

Direct searches for lepton flavor violating Higgs boson decays in the $\tau \mu$ channel have been recently reported by the CMS collaboration. The results display a slight excess of signal events with a significance of 2.5$\sigma$, which translates into a branching ratio of about $1$%. By interpreting these findings as a hint for beyond the standard model physics, we show that the Type-III 2HDM is capable of reproducing such signal while at the same time satisfying vacuum stability, perturbativity, electroweak precision data, measured Higgs standard decay modes and low-energy lepton flavor violating constraints. We have found that the allowed signal strength ranges for the $b\bar b$, $WW^*$ and $ZZ^*$ standard channels shrink as soon as BR$(h\to\tau\mu)\sim 1$% is enforced. Thus, we point out that if the excess persists, improved measurements of these channels may be used to test our Type-III 2HDM scenario.

Lepton flavour violating Higgs boson decays, [formula omitted] and [formula omitted] in the constrained [formula omitted] with large [formula omitted

Realistic predictions are made for the rates of lepton flavour violating Higgs boson decays, τ → μ γ , μ → e γ , B s 0 → μ μ , B s 0 → τ μ and τ → 3 μ , via a top–down analysis of the minimal supersymmetric Standard Model (MSSM) constrained by SU ( 5 ) unification with right-handed neutrinos and large tan β . The third family neutrino Yukawa coupling is chosen to be of order 1, in this way our model bares a significant resemblance to supersymmetric SO ( 10 ) . In this framework the large PMNS mixings result in potentially large lepton flavour violation. Our analysis predicts τ → μ γ and μ → e γ rates in the region (10 −8–10 −6) and (10 −15–10 −14), respectively. We also show that the rates for lepton flavour violating Higgs decays can be as large as 10 −7. The non-decoupling nature of H 0 → τ μ is observed which leads to its decay rate becoming comparable to that for τ → μ γ for large values of m 0 and M 1 / 2 . We also find that the present bound on B s → μ μ is an important constraint on the rate of lepton flavour violating Higgs decays. The recently measured B s – B ¯ s mixing parameter Δ M s is also investigated.

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.