We show that new physics models without new flavor violating interactions can explain the recent anomalies in the $b\to s\ell^+\ell^-$ transitions. The $b\to s\ell^+\ell^-$ arises from a $Z'$ penguin which automatically predicts the $V-A$ structure for the quark currents in the effective operators. This framework can be realized either in a renormalizable $U(1)'$ setup or be due to new strongly interacting dynamics. The di-muon resonance searches at the LHC are becoming sensitive to this scenario since the $Z'$ is relatively light, and could well be discovered in future searches by ATLAS and CMS.
We study the flavor universality of the lepton couplings with the W and Z bosons. Currently, it is constrained by the electroweak precision observables. We focus on the leptonic decay of the pion. It is shown that the decay is expected to have a comparable or better sensitivity to the lepton-flavor universality in the near future. Furthermore, it provides a complementary information in determining the Wlnu couplings without assuming new physics models. This result is applied to a model of the extra lepton.
Models of minimal lepton flavor violation where the seesaw scale is higher than the relevant flavor scale predict that all lepton flavor violation is proportional to the charged lepton Yukawa matrix. If extra vector-like leptons are within the reach of the LHC, it will be possible to test the resulting predictions in ATLAS/CMS.
We discuss the impact of fermion mass matrices on some lepton number violating processes, namely baryogenesis via leptogenesis and neutrinoless double beta decay, and on some lepton flavor violating processes, namely radiative lepton decays in supersymmetric seesaw models.
We analyze the possibility of universality violation in diagonal leptonic decays of the $Z$ boson, in the context of interfamily "see-saw" models. In a minimal extension of the Standard Model with right-handed neutrino fields, we find that universality-breaking effects increase quadratically with the heavy Majorana neutrino mass and may be observed in the running $LEP$ experiments.
The Standard Model requires the three known leptonic families to have identical couplings to the gauge bosons. The present experimental tests on lepton universality are reviewed, both for the charged and neutral current sectors. Our knowledge about the Lorentz structure of the $l^-\toν_l l'^-\barν_{l'}$ transition amplitudes is also discussed.
We investigate the physics potential of NuSOnG experiment to probe new physics contributions to Z-neutrino-neutrino couplings in muon-neutrino electron elastic and neutral-current deep-inelastic scattering processes. We employ an effective Lagrangian approach and do not a priori assume universality of the coupling of neutrinos to Z. We obtain 95% C.L. limits on possible universality violating couplings.
We extend the notion of Minimal Flavor Violation to the lepton sector. We introduce a symmetry principle which allows us to express lepton flavor violation in the charged lepton sector in terms of neutrino masses and mixing angles. We explore the dependence of the rates for flavor changing radiative charged lepton decays (ell(i) -> ell(j) + gamma) and mu-to-e conversion in nuclei on the scales for total lepton number violation, lepton flavor violation and the neutrino masses and mixing angles. Measurable rates are obtained when the scale for total lepton number violation is much larger than th…
The supersymmetric one-loop contribution to the quark-lepton universality violation in the low-energy charged current weak interactions is studied. It is shown that the recent experimental data with the 1-$σ$ deviation from the universality may be explained by the existence of the light sleptons ($<220$GeV) and relatively light chargino and neutralinos ($<600$GeV) with significant gaugino components. The sign of the universality violation is briefly discussed.
In this talk, I will present the status of lepton flavor physics in composite Higgs models with partial compositeness in the light of recent data in the lepton sector. I will consider anarchic flavor setups, scenarios with flavor symmetries, and minimal incarnations of the see-saw mechanism that naturally predict non-negligible lepton compositeness. The focus will be on lepton flavor violating processes, dipole moments, and on probes of lepton flavor universality, all providing stringent tests of partial compositeness. The expected size of effects in the different approaches to lepton flavor a…
We show that the observed anomalies in the lepton sector can be explained in extensions of the Standard Model that are natural and, therefore, resolve the Higgs sector hierarchy problem. The scale of new physics is around the TeV and Technicolor-like theories are ideal candidate models.
We show that in supersymmetric models with explicit flavor lepton number violation due to soft supersymmetry breaking terms there must be flavor lepton number violation in slepton decays. We propose to look for flavor lepton number violation in righthanded selectron and smuon decays. For selectron and smuon lighter than 80 Gev flavor lepton number violation in slepton decays could be discovered at LEP2 provided the mixing betwen selectron and smuon is not small. We also estimate NLC discovery potential of the lepton flavor number violation in slepton decays.
The observation of charged lepton flavor violation is a clear sign of physics beyond the Standard Model (SM). In this work, we investigate the sensitivity of future lepton colliders to charged lepton flavor violation via on-shell production of bileptons, and compare their sensitivity with current constraints and future sensitivities of low-energy experiments. Bileptons couple to two charged leptons with possibly different flavors and are obtained by expanding the general SM gauge invariant Lagrangians with or without lepton number conservation. We find that future lepton colliders will provide…
In this paper, we consider the Standard Model (SM) with one family of vector-like (VL) leptons, which couple to all three families of the SM leptons. We study the constraints on this model coming from the heavy charged lepton mass bound, electroweak precision data, the muon anomalous magnetic moment, lepton flavor violation, Higgs decay constraints and a recently measured lepton non-universality observable, $R_{K^{*0}}$. We find that the strongest constraints are coming from the muon $g-2, \ R_{μμ}=Γ(h\toμμ)/Γ(h\toμμ)_\text{SM}, \ R_{γγ}$ and $\text{BR}(μ\to eγ)$. Although VL leptons couple to…
Lepton Flavor Universality Violation (LFUV) in $b\rightarrow s l^+ l^-$ processes is investigated in the context of a 331 model. It is shown that in order to explain the experimentally observed deviations from the Standard Model in these FCNC transitions, a non-minimal version of the model has to be considered. We investigate the ability of this model in accomodating the model-independant scenarios currently favored by global fits.
We analyse the violations of lepton-flavour universality in the ratios B(P -> l nu)/B(P -> l' nu) using a general effective theory approach, discussing various flavour-symmetry breaking patterns of physics beyond the SM. We find that in models with Minimal Lepton Flavour Violation the effects are too small to be observed in the next generations of experiments in all relevant meson systems (P = Pi, K, B). In a Grand Unified framework with a minimal breaking of the flavour symmetry, the effects remain small in Pi and K decays while large violations of lepton-flavour universality are possible in …
Motivated by the prospects for an improved test of lepton universality in K -> l νdecays by the NA62 experiment at CERN, we study predictions for the possible lepton non-universality in K -> l νdecays in supersymmetric models. Violations of μ-e universality in this process may originate from mixing effects in the right-handed slepton sector, providing a unique window into this aspect of supersymmetric flavour physics in the large-\tanβregion. Minimal unification scenarios with universal soft supersymmetry-breaking terms at the GUT scale would predict negligible violation of lepton universality…
High precision electroweak tests, such as deviations from the Standard Model expectations of the Lepton Flavor Universality breaking in K->lnu (with l= e or mu), represent a powerful tool to test the Standard Model and, hence, to constrain or obtain indirect hints of New Physics beyond it. We explore such a possibility within Supersymmetric theories. Interestingly enough, a process that in itself does not need lepton flavor violation to occur, i.e. the violation of mu-e non-universality in K_l2, proves to be quite effective in constraining not only relevant regions of SUSY models where lepton …
Present measurements of b->c tau nu and b->u tau nu transitions differ from the standard model predictions of lepton flavor universality by almost 4sigma. We examine new physics interpretations of this anomaly. An effective field theory analysis shows that minimal flavor violating models are not preferred as an explanation, but are also not yet excluded. Allowing for general flavor violation, right-right vector and right-left scalar quark currents are identified as viable candidates. We discuss explicit examples of two Higgs doublet models, leptoquarks as well as quark and lepton compositeness…
We study rare four-body decays of the Z-boson involving at least one neutrino and one charged lepton. Large destructive interferences make these decays very sensitive to the Z couplings to neutrinos. As the identified charged leptons can determine the neutrino flavors, these decays probe the universality of the Z couplings to neutrinos. The rare four-body processes could be accurately measured at future lepton colliders, leading to percent level precision.
We show that in supersymmetric models with explicit flavor lepton number violation due to soft supersymmetry breaking mass terms there could be detectable flavor lepton number violation in slepton decays. We estimate LHC discovery potential of the lepton flavor number violation in slepton decays.
Now the bounds on the lepton-flavor violating (LFV) decay modes of tau lepton are being improved by the Belle experiment in the KEK B factory. In this paper the LFV decay of tau lepton is discussed in the supersymmetric seesaw model, using the current neutrino oscillation data.
I discuss different theories of leptonic flavor and their capability of describing the features of the lepton sector, namely charged lepton masses, neutrino masses, lepton mixing angles and leptonic (low and high energy) CP phases. In particular, I show examples of theories with an abelian flavor symmetry G_f, with a non-abelian G_f as well as theories with non-abelian G_f and CP.
We present a proposal for detecting new physics at a B-factory running at the $Υ(3S)$ resonance by testing lepton universality to the few percent level in the leptonic decays of the $Υ(1S)$ and $Υ(2S)$ resonances tagged by the dipion in the chain decay: $Υ(3S) \to pi^+π^-Υ(1S,2S)$; $Υ(1S,2S) \to \ell^+\ell^-$, $\ell=e,μ,τ$.
We propose a new mechanism to explain neutrino masses with lepton number conservation, in which the Dirac neutrino masses are generated at the two-loop level involving a dark matter candidate. In this model, branching ratios of lepton flavor violating decays of the Higgs boson can be much larger than those of lepton flavor violating decays of charged leptons. If lepton flavor violating decays of the Higgs boson are observed at future collider experiments without detecting lepton flavor violating decays of charged leptons, most of the models previously proposed for tiny neutrino masses are excl…
Supersymmetric theories supplemented by an underlying flavor-symmetry $\mathcal{G}_f$ provide a rich playground for model building aimed at explaining the flavor structure of the Standard Model. In the case where supersymmetry breaking is mediated by gravity, the soft-breaking Lagrangian typically exhibits large tree-level flavor violating effects, even if it stems from an ultraviolet flavor-conserving origin. Building on previous work, we continue our phenomenological analysis of these models with a particular emphasis on leptonic flavor observables. We consider three representative models wh…
We study the supersymmetric model with the A4 lepton flavor symmetry, in particular soft supersymmetry breaking terms, which are predicted from the A4 lepton flavor symmetry. We evaluate soft slepton masses and A-terms within the framework of supergravity theory. Constraints due to experiments of flavor changing neutral current processes are examined.
We study the radiative lepton flavor violating l_i -> l_jγdecays in the two Higgs doublet model, respecting the Randall Sundrum scenario and estimate the contributions of the KK modes of left (right) handed charged lepton doublets (singlets) on the branching ratios. We observe that the branching ratios are sensitive to the contributions of the charged lepton KK modes.
Large mixing parameters of neutrino mass eigenstates in the neutrino flavor eigenstates are determined, and interpreted as closely related to the neutrino and charged lepton mass-ratio patterns. The three known seemingly different charged lepton and quasi-degenerate neutrino deviation-from-extreme hierarchies are shown to be three particular manifestations of one unifying quadratic hierarchy-rule in lepton flavor physics.
In high energy exclusive processes involving leptons, QED corrections can be sensitive to infrared scales like the lepton mass and the soft photon energy cut, resulting in large logarithms that need to be resummed to all order in $α$. When considering the ratio of the exclusive processes between two different lepton flavors, the ratio $R$ can be expressed in terms of factorized functions in the decoupled leptonic sectors. While some of the functional terms cancel, there remain the large logarithms due to the lepton mass difference and the energy cut. This factorization process can be universal…
We analyze four-lepton contact interactions that lead to lepton flavor violating processes, with violation of individual family lepton number but total lepton number conserved. In an effective Lagrangian framework, the assumption of gauge invariance leads to relations among branching ratios and cross sections of lepton flavor violating processes. In this paper, we work out how to use these relations systematically. We also study the consequences of loop-induced processes.
We systematically analyze the correlations between the various leptonic and hadronic flavor violating processes arising in SUSY Grand Unified Theories. Using the GUT-symmetric relations between the soft SUSY breaking parameters, we assess the impact of hadronic and leptonic flavor observables on the SUSY sources of flavor violation.
Due to the smallness of the lepton Yukawa couplings, higher-dimensional operators can give a significant contribution to the lepton masses. In this case, the lepton mass matrix and the matrix of lepton-Higgs couplings are misaligned leading to lepton flavor violation (LFV) mediated by the Standard Model Higgs boson. We derive model-independent bounds on the Higgs flavor violating couplings and quantify LFV in decays of leptons and electric dipole moments for a class of lepton-Higgs operators contributing to lepton masses. We find significant Higgs-mediated LFV effects at both 1-loop and 2-loop…
In a generic supersymmetric extension of the Standard Model, there will be lepton flavor violation at a neutral gaugino vertex due to misalignment between the lepton Yukawa couplings and the slepton soft masses. Sleptons produced at the LHC through the cascade decays of squarks and gluinos can give a sizable number of events with 4 leptons. This channel could give a clean signature of supersymmetric lepton flavor violation under conditions which are identified.
The study for lepton flavor violation combined with the neutrino oscillation may provide more information about the lepton flavor structure of the grand unified theory. In this paper, we study two lepton flavor violation processes, $τ\to μγ$ and $Z\to τμ$, in the context of supersymmetric SO(10) grand unified models. We find the two processes are both of phenomenological interest. In particular the latter may be important in some supersymmetric parameter space where the former is suppressed. Thus, Z-dacay may offer another chance for looking for lepton flavor violation.
We discuss the current status of lepton flavour universality observables in semileptonic charged and neutral current $B$ meson decays. We recapitulate the current experimental situation. Assuming the current tensions with the SM are of beyond the Standard Model origin we present their implications for the effective theory couplings. Then we discuss general leptoquark models before finally focussing on the model with two scalar leptoquarks ($R_2$ and $S_3$) which have couplings connected by the underlying Grand Unified Theory framework. We present the viability of the model with regard to lepto…
We investigate whether the $2.6σ$ deviation from lepton flavour universality in $B^+\to K^+\ell^+\ell^-$ decays recently observed at the LHCb experiment can be explained in minimal composite Higgs models. We show that a visible departure from universality is indeed possible if left-handed muons have a sizable degree of compositeness. Constraints from $Z$-pole observables are avoided by a custodial protection of the muon coupling. The deficit in the invisible $Z$ width at LEP is explained in the same region of parameters.
We suggest the use of a universality-breaking observable based on lepton asymmetries as derived from the left-right asymmetry and the $τ$ polarization at the $Z$ peak, which can efficiently constrain the parameter space of unified theories. The new observable is complementary to the leptonic partial width differences and it depends critically on the chirality of a possible non-universal $Z$-boson coupling to like-flavour leptons. The LEP/SLC potential of probing universality violation is discussed in representative low-energy extensions of the Standard Model (SM) that could be derived by super…
A properly defined and suitably broken $U(2)$ flavour symmetry leads to successful quantitative relations between quark mass ratios and CKM angles. At the same time the intrinsic distinction introduced by $U(2)$ between the third and the first two families of quarks and leptons may support anomalies in charged and neutral current semi-leptonic $B$-decays of the kind tentatively observed in current flavour experiments. We show how this is possible by the exchange of the $(3,1)_{2/3}$ vector leptoquark in two $U(2)$-models with significantly different values of Lepton Flavour Universality violat…
The field of lepton flavor violation will live an era of unprecedented developments in the near future, with dedicated experiments in different fronts. The observation of a flavor violating process involving charged leptons would be a clear evidence of physics beyond the Standard Model, thus motivating the great effort in this direction. Furthermore, in case a positive signal is found, a proper theoretical understanding of the lepton flavor anatomy of a given model would become necessary. Here I briefly review the current situation, emphasizing the most relevant theoretical and phenomenologica…
The observation of lepton flavor violation indicates new physics beyond the Standard Model. Lepton colliders are ideal facilities to probe charged lepton flavor violation (CLFV) signals induced by new physics at high energy. In this work we perform a comprehensive study of the sensitivity of future lepton colliders to charged lepton flavor violation. We consider the most general renormalizable Lagrangian coupling two leptons to new bosonic particles, involving both $ΔL=0$ and $ΔL=2$ interactions. The CLFV processes are introduced by the exchange of off-shell new particles at tree-level. We fin…
We discuss charged lepton flavor violation in supersymmetric models with extended leptonic sectors at low energies. Contrary to the usual high-scale realizations of the seesaw mechanism, these non-minimal supersymmetric models have new superfields and/or interactions at the supersymmetric scale. As a consequence of this, the resulting lepton flavor violating phenomenology may be very different from that of minimal models.
Supposing only Lorentz and the gauge invariances of the Lagrangian, we derive energy and angular distributions for $τ^\pm \to μ^\pm μ^\pm μ^\mp$ lepton flavor violating decay process. Using these results, we discuss methods to determine the parameters associated with the lepton flavor violating interactions.
We study the lepton flavor violating $μ^- \to e^{-}2γ$ decay induced by the mediation of heavy leptons such as the excited and fourth generation leptons. We have examined the effects of these leptons in the $μ^- \to e^{-}2γ$ decay. We have found that branching ratio of this decay is very sensitive to excited and fourth generation leptons. Furthermore, we have obtained the bounds on model parameters by using the current experimental limit of this decay.
We analyze the one loop corrections to leptonic W and Z decays in an R-parity violating extension to the Minimal Supersymmetric Standard Model (MSSM). We find that lepton universality violation in the Z line-shape variables alone would strengthen the bounds on the magnitudes of the lambda' couplings, but a global fit on all data leaves the bounds virtually unchanged at |lambda'_{33k}| < 0.42 and |lambda'_{23k}| < 0.50 at the 2 sigma level. Bounds from W decays are less stringent: |lambda'_{33k}| < 2.4 at 2 sigma, as a consequence of the weaker Fermilab experimental bounds on lepton universalit…
We study a model with partial quark-lepton universality that can naturally arise in grand unified theories. We find that constraints on the model can be reduced to a single condition on the Dirac CP phase $δ$ in the neutrino sector. Using our current knowledge of the CKM and PMNS mixing matrices, we predict $-32.4^\circ < δ< 32.0^\circ$ at $2σ$.
We predict the branching ratios of the lepton flavor violating Higgs decays H^0->e^{\pm} μ^{\pm}, H^0-> e^{\pm} τ^{\pm} and H^0->μ^{\pm} τ^{\pm} in the case that the lepton flavor violation is carried by the scalar unparticle mediation. We observe that their branching ratios are strongly sensitive to the unparticle scaling dimension and they can reach to the values of the order of $10^{-4}$, for the heavy lepton flavor case and for the small values of the scaling dimension.
We consider the $ν$MSM which is an extension of the Standard Model by three right-handed neutrinos with masses below the electroweak scale, in which the origins of neutrino masses, dark matter, and baryon asymmetry of the universe are simultaneously explained. Among three heavy neutral leptons, $N_2$ and $N_3$, which are responsible to the seesaw mechanism of active neutrino masses and the baryogenesis via flavor oscillation, can induce sizable contributions to various lepton universality in decays of charged mesons. It is then investigated the possible deviations of the universality in the $ν…
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.
Lepton flavor violation at linear collider experiments is discussed. We show that detectable lepton flavor violation could occur through scalar lepton pair production and decay in the supersymmetric SU(5) grand unified theory in spite of the stringent present experimental constraints by rare process searches. Possible cross sections about 40fb for an e+e- collider and 280fb for an e-e- collider are illustrated.
Lepton flavor violating decays $Z \to μτ$ and $J/Ψ, Υ\to μτ$ are considered. It is shown that these decays can reach sizeable magnitudes if some specific lepton-flavor violating 4-fermion operators are generated by low scale quantum gravity effects, or by some other new physics at a TeV scale.
The impact of possible sources of lepton-flavor mixing on $K \to πν\barν$ decays is analysed. At the one-loop level lepton-flavor mixing originated from non-diagonal lepton mass matrices cannot generate a CP-conserving $K_L \to π^0ν\barν$ amplitude. The rates of these modes are sensitive to leptonic flavor violation when there are at least two different leptonic mixing matrices. New interactions that violate both quark and lepton universalities could enhance the CP-conserving component of $Γ(K_L \to π^0ν\barν)$ and have a substantial impact. Explicit examples of these effects in the context of…
The following proceedings contain a theory perspective on the flavor changing neutral current anomalies reported by LHCb on the ratios $B\to K^{(*)} μμ/B\to K^{(*)} ee$
We investigate the potentials of high energy lepton-proton colliders to detect a flavor changing $l-l^{\prime}-Z$ or $l-l^{\prime}-γ$ couplings from the processes $ep\toμ(τ)X$ and $μp\to τX$. For the $l-l^{\prime}-Z$ coupling we consider string inspired $E_6$ model whereas $l-l^{\prime}-γ$ coupling is analyzed in the framework of anomalous magnetic moment type interaction (AMMI) model.
