Lepton-flavour symmetry in the Standard Model is broken by small masses for charged leptons and neutrinos. Introducing neutrino masses via dimension-5 operators associated to lepton-number violation at a very high scale, the corresponding coupling matrix may still have entries of order 1, resembling the situation in the quark sector with large top Yukawa coupling. As we have shown recently, in such a situation one may introduce the coupling matrices between lepton and Higgs fields as non-linear representations of lepton-flavour symmetry within an effective-theory framework. This allows us to separate the effects related to the large mass difference observed in atmospheric neutrino oscillations from those related to the solar mass difference. We discuss the cases of normal or inverted hierarchical and almost degenerate neutrino spectrum, give some examples to illustrate minimal lepton-flavour violation in radiative and leptonic decays, and also provide a systematic definition of next-to-minimal lepton-flavour violation within the non-linear framework.
In this manuscript, based on arXiv:2209.00276, I present a model which can simultaneously explain and connect the flavour hierarchies of the Standard Model with flavour anomalies in $B$-physics. I will briefly introduce the model and highlight the main features, including a common origin of Yukawa couplings and vector leptoquark $U_{1}$ couplings to Standard Model fermions. A GIM-like mechanism allows for large leptoquark couplings which can explain the $B$-anomalies, while protecting from the most dangerous FCNCs that could be mediated by a heavy coloron and $Z'$. Finally, I will highlight so…
We incorporate clockwork mechanism into the Standard Model flavour sector and show that the observed pattern of fermion masses and mixing can be obtained without any unnaturally small or large parameter in the fundamental theory. By introducing $N_f$ pairs of vectorlike fermions, as clockwork gears, for each generation of the Standard Model fermions and setting up a characteristic clockwork potential, it is shown that the inter-generational mass hierarchies are determined by $N_f$. For a given type of fermions, strong or mild hierarchy in the masses and mixing parameters can be obtained by tak…
The tree level potential for a scalar multiplet of `Yukawa fields' $Y$ for one type of quarks admits the promising vacuum configuration $<Y> \propto {\rm diag}(0,0,1)$ that breaks spontaneously $SU(3)_L\times SU(3)_R$ flavour symmetry. We investigate whether the vanishing entries could be lifted to nonvanishing values by slightly perturbing the potential, thus providing a mechanism to generate the Yukawa hierarchies. For theories where at the lowest order the only massless states are Nambu-Goldstone bosons we find, as a general result, that the structure of the tree-level vacuum is perturbativ…
We propose a low-scale renormalizable trinification theory that successfully explains the flavor hierarchies and neutrino puzzle in the Standard Model (SM), as well as provides a dark matter candidate and also contains the necessary means for efficient leptogenesis. The proposed theory is based on the trinification $\SU{3}{C}\times \SU{3}{L}\times \SU{3}{R}$ gauge symmetry, which is supplemented with an additional flavor symmetry $\U{X}\times Z_{2}^{(1)} \times Z_{2}^{(2)}$. In the proposed model the top quark and the exotic fermions acquire tree-level masses, whereas the lighter SM charged fe…
We construct a simple model of fermion masses based on a spontaneously broken S3 X Z3 flavour group. At the leading order, in the neutrino sector S3 is broken down to a ν_μ-ν_τparity subgroup that enforces a maximal atmospheric mixing angle and a vanishing θ_{13}. In the charged lepton sector the ν_μ-ν_τparity is maximally broken and the resulting mass matrix is nearly diagonal. The charged lepton mass hierarchy is automatically reproduced by the S3 symmetry breaking parameter alone. A careful analysis shows that, after the inclusion of all relevant subleading effects, the model predicts θ_{23…
Combining Pati-Salam (PS) and flavour symmetries in a renormalisable setup, we devise a scenario which produces realistic masses for the charged leptons. Flavour-symmetry breaking scalar fields in the adjoint representations of the PS gauge group are responsible for generating different flavour structures for up- and down-type quarks as well as for leptons. The model is characterised by new heavy fermions which mix with the Standard Model quarks and leptons. In particular, the partners for the third fermion generation induce sizeable sources of flavour violation. Focusing on the charged-lepton…
We give a pedagogical review of flavour and CP violation in the lepton sector, with a particular emphasis on new physics - and in particular supersymmetric - contributions to flavour and CP violating observables involving leptons.
In supersymmetric extensions of the Standard Model, the Yukawa and/or mass terms of the heavy neutrinos can generate lepton flavour violating slepton mass terms. These new supersymmetric sources of lepton flavour violation may both enhance the rates of charged lepton flavour violating processes, l_a -> l_b y, and generate distinct final states, like l_b l_a + jets + missing E_T, at future colliders. First, we discuss the sensitivity of future e^+e^- colliders to the SLFV independently of the lepton flavour violating mechanism. Second, we study lepton flavour violating slepton pair production a…
In leptonic flavour models with discrete flavour symmetries, couplings between flavons and leptons can result in special flavour structures after they gain vacuum expectation values. At the same time, they can also contribute to the other lepton-flavour-violating processes. We study the flavon-induced LFV 3-body charged lepton decays and radiative decays and we take as example the $A_4$ discrete symmetry. In $A_4$ models, a $Z_3$ residual symmetry roughly holds in the charged lepton sector for the realisation of tri-bimaximal mixing at leading order. The only processes allowed by this symmetry…
We present a framework for lepton flavour models such that the first column of the lepton mixing matrix is (2,-1,-1)/sqrt(6). We show that the flavour symmetry group adequate for this purpose is S4. Our models are based on a vacuum alignment that can be obtained in a supersymmetric framework.
In supersymmetric models new sources of lepton flavour violation (LFV) may enhance the rates of charged lepton flavour violating processes, like mu to e gamma, and generate distinct final states at future colliders, e.g. tau mu + jets + missing energy. We discuss the role of chargino pair production for the LFV signals at e+e- colliders.
The sources of lepton flavour and lepton number violation in the left-right symmetric models are reviewed and the most sensitive processes to these violations are discussed. Both the non-SUSY and SUSY versions of the theories are considered. Present and future experimental constraints at low as well as at high energies are discussed. Some emphasis has been put on the doubly charged particle production at future colliders.
The Little Higgs model with T-parity (LHT) belongs to the non-minimal flavour violating model. This model has new sources of flavour and CP violation both in quark and leptonic sectors. These new sources of flavour violation originates by the interaction of Standard Model (SM) fermions with heavy gauge bosons and heavy (or mirror) fermions. In this work we will present the impact of the new flavour structure of T-parity models on flavour violations in leptonic sector.
We investigate the neutrino masses and mixing patten in a version of the $SU(3)_c\otimes SU(3)_L\otimes U(1)_X$ model with one extra exotic charged lepton per family as introduced by Ozer. It is shown that an extended scalar sector, together with a discrete $Z_2$ symmetry, is able to reproduce a consistent lepton mass spectrum without a hierarchy in the Yukawa coupling constants, the former as a carefull balance between one universal see-saw and two radiative mechanisms.
We calculate flavour dependent lepton asymmetries within the $E_6$ inspired Supersymmetric Standard Model. Our analysis reveals that the substantial lepton CP asymmetries can be induced even if $M_1\simeq 10^6 {GeV}$.
We briefly review flavour violation in the lepton sector: starting from neutrino oscillations and their implications, we consider several charged lepton flavour violating observables at high and low energies. We present new physics models and discuss the rôle of the latter in disentangling them. In particular, we show how the interplay of different observables allows to derive important information on the underlying mechanism of lepton flavour violation. As an example, we discuss the impact of a type-I SUSY seesaw concerning lepton flavour violation at low energies and at colliders (LHC and a …
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 …
We study lepton flavour violating decays of neutralinos and sleptons within the Minimal Supersymmetric Standard Model, assuming two and three generation mixings in the slepton sector. We take into account the most recent bounds on flavour violating rare lepton decays. Taking the SPS1a' scenario as an example, we show that some of the lepton flavour violating branching ratios of neutralinos and sleptons can be sizable (~ 5-10%). We study the impact of the lepton flavour violating neutralino and slepton decays on the di-lepton mass distributions measured at LHC. We find that they can result in n…
We explore the possibility of detecting lepton flavour violation, now a well established experimental fact, at the International Linear Collider. Using a model independent approach we conclude that, given all experimental constraints available, there is still room to detect lepton flavour violation at the ILC.
A mechanism to generate realistic fermion mass hierarchies based on supersymmetric gauged $U(1)_F$ symmetry in flat five-dimensional (5D) spacetime is proposed. The fifth dimension is compactified on $S^1/Z_2$ orbifold. The standard model fermions charged under the extra abelian symmetry along with their superpartners live in the 5D bulk. Bulk masses of fermions are generated by the vacuum expectation value of $N=2$ superpartner of $U(1)_F$ gauge field, and they are proportional to $U(1)_F$ charges of respective fermions. This decides localization of fermions in the extra dimension, which in t…
After a summary of the motivations for searching flavour violating decays and electric dipole moments in the leptonic sector, I briefly sketch the present status of the limits on lepton-slepton misalignement and discuss the potentialities of the planned experimental improvements. Candidates for theories beyond the Standard Model have to satisfy all these experimental constraints; I will discuss in particular how the supersymmetric SM extended with the see-saw faces the flavour problem.
I discuss theoretical and phenomenological aspects of flavour violation in charged lepton transitions. This includes minimal extensions of the Standard Model within effective-theory approaches, as well as an incomplete selection of concrete new physics models.
If observed, charged lepton flavour violation is a clear sign of new physics - beyond the Standard Model minimally extended to accommodate neutrino oscillation data. We briefly review several extensions of the Standard Model which could potentially give rise to observable signals, also emphasising the rôle of charged lepton flavour violation in probing such new physics models.
The flavour-changing neutral currents (FCNC) are derived at tree level if the electroweak gauge group depends on the fermion family, which are absent in the standard model. We study the lepton flavour violation (LFV) through the FCNC interactions in a nonuniversal gauge interaction model where the third generation fermions are subjected to the separate SU(2)$_L$ gauge group.
The flavour lepton symmetry is gauged for both the Standard Model flavor group and its extension with three degenerate right-handed neutrinos. As a consequence of anomaly cancellation, exotic leptons are introduced which induce a see-saw mechanism that generates the masses of the SM leptons. The model is compared with Minimal Flavour Violation, the difference between the low-energy effective operators resulting is identified, as well as the distinctive experimental signals, particularly promising in the $μ-τ$ sector.
We discuss flavour dependent lepton asymmetries in the Exceptional Supersymmetric Standard Model (E$_6$SSM). In the E$_6$SSM, the right-handed neutrinos do not participate in gauge interactions, and they decay into leptons and leptoquarks. Their Majorana nature allows violation of lepton number. New particles and interactions can result in substantial lepton asymmetries, even for scales as low as $10^6 {\rm GeV}$.
A variety of lepton flavour violating effects related to the recent discovery of neutrino oscillations and mixings is here systematically discussed in terms of an S_3-flavour permutational symmetry. After a brief review of some relevant results on lepton masses and mixings, that had been derived in the framework of a Minimal S_3-Invariant Extension of the Standard Model, we derive explicit analytical expressions for the matrices of the Yukawa couplings and compute the branching ratios of some selected flavour changing neutral current (FCNC) processes, as well as, the contribution of the exchan…
Little Higgs models with T-parity have a new source of lepton flavour violation. In this paper we consider the anomalous magnetic moment of the muon \gmtwo and the lepton flavour violating decays \mutoeg and \tautomug in Little Higgs model with T-parity \cite{Goyal:2006vq}. Our results shows that present experimental constraints of \mutoeg is much more useful to constrain the new sources of flavour violation which are present in T-parity models.
In the absence of a fundamental principle preventing charged lepton flavour violation, one expects that extensions of the Standard Model accommodating neutrino masses and mixings should also allow for charged lepton flavour violating processes such as $\ell_i \to \ell_jγ$, $\ell_i \to \ell_j \ell_k \ell_m $ and $μ- e$ conversion in nuclei, for which the rates depend in general on the mechanism of neutrino mass generation. In addition to low-energy experiments, there are also searches for lepton flavour violation at colliders, where new physics can be directly probed through flavour violating p…
If neutrinos are massive, `lepton flavour eigenstates' are absent from the amplitudes of all Standard Model processes. Measurements of $Γ(π\to e ν)/Γ(π\to μν)$ and of the MNS matrix are shown to exclude the existence of such states. Incoherent production of neutrinos of different flavour modifies the usual oscillation phase derived assuming production of a lepton flavour eigenstate. The new prediction can be tested in long line-base experiments by comparison of `$ν_μ$ disappearance' of neutrinos from pion and kaon decay.
We study the effects of dimension six effective operators on the flavour violating production and decay of leptons at the International Linear Collider. Analytic expressions for the cross sections, decay widths and asymmetries of all flavour changing processes will be presented, as well as an analysis of the feasibility of their observation at the ILC.
We present the results of an extensive analysis of lepton flavour violating decays in the Littlest Higgs model with T-parity (LHT). As lepton flavour violation is highly suppressed in the Standard Model by small neutrino masses, the LHT effects turn out to be naturally huge and could be seen in the near future experiments.
When lepton flavour effects in thermal leptogenesis are active, they introduce important differences with respect to the case in which they are neglected, the so-called one-flavour approximation. We investigate analytically and numerically the transition from the one-flavour to the two-flavour case when the $τ$-lepton flavour becomes distinguishable from the other two flavours. We study the impact of the oscillations of the asymmetries in lepton flavour space on the final lepton asymmetries, for the hierarchical right-handed neutrino mass spectrum. Flavour oscillations project the lepton state…
If observed, charged lepton flavour violation is a clear sign of new physics - beyond the Standard Model minimally extended to accommodate neutrino oscillation data. After a brief review of several charged lepton flavour violation observables and their current experimental status, we consider distinct extensions of the Standard Model which could potentially give rise to observable signals, focusing on the case of models in which the mechanism of neutrino mass generation is the common source of neutral and charged lepton flavour violation.
The mass hierarchies of quarks and charged leptons as well as a large $\n_\m$-$\n_\t$ mixing angle are naturally explained by the Frogatt-Nielsen mechanism with a nonparallel family structure of chiral charges. We extend this mechanism to right-handed neutrinos. Their out-of-equilibrium decay generates a cosmological baryon asymmetry whose size is quantized in powers of the hierarchy parameter $\e^2$. For the simplest hierarchy pattern the neutrino mass $\bar{m}_\n= (m_{\n_\m}m_{\n_\t})^{1/2} \sim 10^{-2}$ eV, which is inferred from present indications for neutrino oscillations, implies a bary…
We discuss charged lepton flavour violating processes such as mu -> e gamma in supersymmetric extensions of the Standard Model and in models with gauge-Higgs unification and a warped extra dimension. In both classes of models the processes turn out to be compatible with present experimental bounds, but can be detected by future experiments, thanks to the presence of a flavour symmetry G_f which constrains the form of the relevant couplings. The symmetry G_f is chosen to be finite, discrete and non-abelian and also helps to predict the peculiar lepton mixing pattern.
This note is a concise theoretical summary of the first session on new physics searches at the high-intensity frontier of the IFAE2017 Conference. Recent theoretical developments related to muonic lepton-flavour violation and $g-2$ are reviewed.
Flavour models may display a relation between the CP-violating asymmetry for leptogenesis and low-energy parameters. If the flavour symmetry produces an exact mass independent lepton mixing scheme at leading order (with type I see-saw) the CP-violating asymmetry would vanish in the absence of corrections. We present a model displaying the link between deviations from the mixing scheme and leptogenesis.
We suppose that the observed neutrino masses can be parametrised by a lepton number violating dimension-five operator, and calculate the mixing of double insertions of this operator into lepton flavour changing dimension-six operators of the standard model effective theory. This allows to predict the log-enhanced, but $m_ν^2$-suppressed lepton flavour violation that is generic to high-scale Majorana neutrino mass models. We also consider the Two Higgs Doublet Model, where the second Higgs allows the construction of three additional dimension-five operators, and evaluate the corresponding anoma…
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 definition of `lepton flavour eigenstates' introduced in a recent paper hep-ph/0302045 is in disagreement with the Standard Model prediction for the structure of the leptonic charged current and implies the absence of neutrino oscillations following pion decay, in contradiction to experimental data
Neutrino masses imply the violation of lepton flavour and new physics beyond the Standard Model. However, flavour change has only been observed in oscillations. In analogy with the quark sector, we could deduce the existence of a principle of Minimal Flavour Violation also for Leptons. Such an extension is not straightforward, since the mechanisms generating neutrino masses are unknown and many scenarios can be envisaged. Thus, we explore some possible definitions of MFVL and propose a notion that can include many models. We build an R-parity violating neutrino mass model in agreement with our…
B-decays mediated by both charged currents and neutral currents have provided hints of violation of lepton flavour universality. By assuming an explanation of these anomalies in terms of new physics at the TeV scale affecting the third fermion generation, we review the relevant constraints. In particular we illustrate the effects of electroweak radiative corrections and their impact on a consistent interpretation of the data. The simplest scenario is ruled out and we briefly discuss the modifications required to simultaneously describe the whole set of anomalies.
We discuss a supersymmetric inverse seesaw model in which lepton flavour violating decays can be enhanced either by flavour violating slepton contributions or by the non-unitarity of the charged current mixing matrix. As an example we calculate Br(mu -> e gamma) taking into account both heavy lepton exchange as well as supersymmetric diagrams in a minimal supergravity framework. We find that the for the same parameters the rate can be enhanced with respect to seesaw model expectations, with or without supersymmetry.
We address the issue of lepton flavour violation and neutrino masses in the ``fat-brane'' paradigm, where flavour changing processes are suppressed by localising different fermion field wave-functions at different positions (in the extra dimensions) in a thick brane. We study the consequences of suppressing lepton number violating charged lepton decays within this scenario for lepton masses and mixing angles. In particular, we find that charged lepton mass matrices are constrained to be quasi-diagonal. We further consider whether the same paradigm can be used to naturally explain small Dirac n…
Many models for physics beyond the Standard Model predict lepton-flavour violating decays of charged leptons at a level which may become observable very soon. In the present paper we investigate the decays of a Tau into three charged leptons in a generic way, based on effective-field-theory methods, where the relevant operators are classified according to their chirality structure. We work out the decay distributions and discuss phenomenological implications.
The Minimal Lepton Flavour Violation (MLFV) framework is discussed after the recent indication for CP violation in the leptonic sector. Among the three distinct versions of MLFV, the one with degenerate right-handed neutrinos will be disfavoured, if this indication is confirmed. The predictions for leptonic radiative rare decays and muon conversion in nuclei are analysed, identifying strategies to disentangle the different MLFV scenarios. The claim that the present anomalies in the semi-leptonic $B$-meson decays can be explained within the MLFV context is critically re-examined concluding that…
Non-Standard neutrino Interactions (NSI) are vector contact interactions involving two neutrinos and two first generation fermions, which can affect neutrino propagation in matter. SU(2) gauge invariance suggests that NSI should be accompanied by more observable charged lepton contact interactions. However, these can be avoided at tree level in various ways. We focus on lepton flavour-changing NSI, suppose they are generated by New Physics heavier than $m_W$ that does not induce (charged) Lepton Flavour Violation (LFV) at tree level, and show that LFV is generated at one loop in most cases. Th…
Two guiding new phenomenological flavor and flavor-electroweak ideas are expounded in this paper: (I) Oppositeness relation between neutrino and Charged Lepton Mass-Degeneracy-Deviation quantities. With inputs from the mass and neutrino oscillation data, it enables two independent estimations of the neutrino masses m =(0.11-0.30)eV and m =(0.11-0.16) eV. Small value of the neutrino oscillation hierarchy parameter is a consistency condition. (II) Essential connection between lepton mass hierarchies and low energy electroweak coupling constants. It enables an independent estimation of the QD-neu…
The impact of rare lepton decays ($μ\to e γ$, $τ\to μγ$, >...) and rare $B$ and $K$ decays ($B\to \ell^+\ell^-$, $B\to \ellν$, $K\to πνν$, $K\to\ellν$, ...) in probing the existence of an underlying flavour symmetry is reviewd. Particualar emphasis is deveoted to the connections between quark and lepton flavour strucutres, in the context of generic effective field theory approach to physics beyond the SM.
In this review article, we highlight the impact of models incorporating flavour symmetries on charged lepton flavour violating (LFV) processes. Flavour symmetries provide a natural approach to explain the peculiar mass hierarchies and mixing patterns of the Standard Model fermions. New sources of LFV are generally present in new physics beyond the Standard Model, and flavour symmetries can make distinctive predictions for LFV observables. Their discovery can provide crucial information to distinguish flavour symmetries and new physics scenarios in general. Because of their high sensitivity, we…
