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We study lepton flavor models with the $S_4$ flavor symmetry. We construct simple models with smaller numbers of flavon fields and free parameters, such that we have predictions among lepton masses and mixing angles. The model with a $S_4$ triplet flavon is not realistic, but we can construct realistic models with two triplet flavons, or one triplet and one doublet flavons.
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The LHC has recently reported a slight excess in the $hrightarrow tau mu$ channel. If this lepton flavor violating (LFV) decay is confirmed, an extension of the Standard Model (SM) will be required to explain it. In this paper we investigate two different possibilities to accommodate such a LFV process: the first scenario is based on flavor off-diagonal $A$-terms in the Minimal Supersymmetric Standard Model (MSSM), and the second is a model where the Higgs couples to new vectorlike fermions that couple to the SM leptons through a LFV four fermion interaction. In the supersymmetric model, we find that the sizes of the $A$-terms needed to accommodate the $hrightarrow taumu$ excess are in conflict with charge- and color-breaking vacuum constraints. In the second model, the excess can be successfully explained while satisfying all other flavor constrains, with order one couplings, vectorlike fermion masses as low as 15 TeV, and a UV scale higher than 35 TeV.
We consider the minimal supersymmetric standard model within a scenario of large $tanbeta$ and heavy squarks and gluinos, with masses of the heavy neutral Higgs bosons below the TeV scale. We allow for the presence of a large, model independent, source of lepton flavor violation (LFV) in the slepton mass matrix in the $tau-mu$ sector by the mass insertion approximation. We constrain the parameter space using the $tau$ LFV decays together with the $B$-mesons physics observables, the anomalous magnetic moment of the muon and the dark matter relic density. We further impose the exclusion limit on spin-independent neutralino-nucleon scattering from CDMS and the recent CDF limit from direct search of the heavy neutral Higgs at the TEVATRON. We re-examine the prospects for the detection of Higgs mediated LFV at LHC, at a photon collider and in LFV decays of the $tau$ such as $tautomueta$, $tautomugamma$. We find rates probably too small to be observed at future experiments if models have to accommodate for the relic density measured by WMAP and explain the $(g-2)_{mu}$ anomaly: better prospects are found if these two constraints are applied only as upper bounds. The spin-independent neutralino-nucleon cross section in the studied constrained parameter space is just below the present CDMS limit and the running XENON100 experiment will cover the region of the parameter space where the lightest neutralino has large gaugino-higgsino mixing.
We consider the minimal 3-3-1 model with three sterile neutrinos transforming as singlet under the $SU(3)_Lotimes U(1)_X$ symmetry. This model, with or without sterile neutrinos, predicts flavor violating interactions in both quark and lepton sectors, since all the charged fermions mass matrices can not be assumed diagonal in any case. Here we accommodate the lepton masses and the Pontecorvo-Maki-Nakawaga-Sakata matrix at the same time, and as consequence the Yukawa couplings and the unitary matrices which diagonalize the mass matrices are not free parameters anymore. We study some phenomenological consequences, i.e., $l_ito l_jl_k bar{l}_k$ and $l_ito l_jgamma$ which are induced by neutral and doubly charged particles present in the model. In particular we find that if the decay $muto eebar{e}$ is observed in the future, the only particle in the model that could explain this decay is the doubly charged vector bilepton.
We present the lepton flavor model with $Delta (54)$, which appears typically in heterotic string models on the $T^2/Z_3$ orbifold. Our model reproduces the tri-bimaximal mixing in the parameter region around degenerate neutrino masses or two massless neutrinos. We predict the deviation from the tri-bimaximal mixing by putting the experimental data of neutrino masses in the normal hierarchy of neutrino masses. The upper bound of $sin^2theta_{13}$ is 0.01. There is the strong correlation between $theta_{23}$ and $theta_{13}$. Unless $theta_{23}$ is deviated from the maximal mixing considerably, $theta_{13}$ remains to be tiny.
We present a variant of the warped extra dimension, Randall-Sundrum (RS), framework which is based on five dimensional (5D) minimal flavor violation (MFV), in which the only sources of flavor breaking are two 5D anarchic Yukawa matrices. The Yukawa matrices also control the bulk masses, which are responsible for the resulting flavor structure and mass hierarchy in the low energy theory. An interesting result of this set-up is that at low energies the theory flows to next to MFV model where flavor violation is dominantly coming from the third generation. Low energy flavor violation is further suppressed by a single parameter that dials the amount of violation in the up or down sector. There is therefore a sharp limit in which there is no flavor violation in the down type quark sector which, remarkably, is favored when we fit for the flavor parameters. This mechanism is used to eliminate the current RS flavor and CP problem even with a Kaluza-Klein scale as low as 2 TeV! Our construction also suggests that economic supersymmetric and non-supersymmetric, strong dynamic-based, flavor models may be built based on the same concepts.
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