Flavon-induced connections between lepton flavour mixing and charged lepton flavour violation processes

Journal of High Energy Physics, Oct 2016

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 are τ − → μ + e − e − , e + μ − μ −, and the other 3-body and all radiative decays are suppressed by small Z 3-breaking effects. These processes also depend on the representation the flavon is in, whether pseudo-real (case i) or complex (case ii). We calculate the decay rates for all processes for each case and derive their strong connection with lepton flavour mixing. In case i, sum rules for the branching ratios of these processes are obtained, with typical examples Br(τ − → μ + e − e −) ≈ Br(τ − → e + μ − μ −) and Br(τ − → e −γ) ≈ Br(τ − → μ −γ). In case ii, we observe that the mixing between two Z 3-covariant flavons plays an important role. All processes are suppressed by charged lepton masses and current experimental con- straints allow the electroweak scale and the flavon masses to be around hundreds of GeV. Our discussion can be generalised in other flavour models with different flavour symmetries.

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Flavon-induced connections between lepton flavour mixing and charged lepton flavour violation processes

Received: July Flavon-induced connections between lepton avour mixing and charged lepton avour violation processes Silvia Pascoli 0 1 2 Ye-Ling Zhou 0 1 2 0 Open Access , c The Authors 1 Durham DH1 3LE , United Kingdom 2 Institute for Particle Physics Phenomenology, Department of Physics, Durham University In leptonic avour models with discrete tween avons and leptons can result in special avour structures after they gain vacuum expectation values. At the same time, they can also contribute to the other lepton- avourviolating processes. We study the avon-induced LFV 3-body charged lepton decays and radiative decays and we take as example the A4 discrete symmetry. In A4 models, a Z3 residual symmetry roughly holds in the charged lepton sector for the realisation of tribimaximal mixing at leading order. The only processes allowed by this symmetry are mixing; and; charged; lepton; avour; violation; processes; Discrete Symmetries; Neutrino Physics; E ective eld theories; Beyond Stan- - , and the other 3-body and all radiative decays are suppressed by small Z3-breaking e ects. These processes also depend on the representation the all processes for each case and derive their strong connection with lepton avour mixing. examples Br( ) and Br( dard Model Charged lepton avour violation in avour models ei 31=2A A Radiative decays mediated by complex scalars Introduction which is parametrised by [12] UPMNS = B@ s12c23 c12s13s23e c12s13c23e s12s13s23e s12s13c23e c13s23 AC B@0 ei 21=2 1 Introduction 2 3 4 Flavour mixing in the A4 symmetry Residual symmetries and tri-bimaximal mixing The breaking of the residual symmetries Z3-preserving LFV charged lepton decays Z3-breaking LFV charged lepton decays Z3-breaking LFV charged lepton decays in concrete models Model constructions Z3-breaking LFV charged lepton decays in Model I Z3-breaking LFV charged lepton decays in Model II in which cij cos ij and sij sin ij . The three mixing angles have been measured to a good accuracy. Their current best- t and 1 values from a global analysis of the available data [13, 14] are given by sin2 12 = 0:308+00::001132 ; sin2 13 = 0:0217+00::00001130 0:0221+00::00001100 sin2 23 = 0:574+00::012464 0:579+00::002229 ; maximal, and the reactor angle 13 takes a value around 0.1, 13 The origin of this distinct mixing structure remains unexplained. Discrete avour the three avours together. The tetrahedral group A4 [15], which is the smallest group type. There are other commonly-used groups, such as S4 [16], A5 [17], (48) [18, 19], (96) [20]. At a lower energy scale, the avour symmetry is broken, leading to by Gl and G up to Majorana phases and column or row permutations of the PMNS matrix. In the semi-direct approach, Gl and G cannot fully determine avour mixing chosen to be Gl = Z3 and G A4, arises accidentally in the neutrino sector. In models of S4, the residual Z3; Z2; Z20 all mixing (s12 = s23 = p2=p (96) [20]. It should symmetries should be slightly broken. A common approach to realise the breaking of Gf is to introduce avons, new scalars model constructions [18, 19, 34{38, 41{45, 47{49, 52, 53]. The slight breaking of the residual symmetries can be provided by additional intercouplings between avons and leptons [21, 54, 55]. In our recent paper [51], we observe the modi cation to the TBM avour mixing pattern, in particular conserving the origin of non-zero 13 and Dirac-type CP violation. The interactions of avons and leptons, in addition to ensuring special Yukawa struccussed [56{62], see also [63{66]. For instance, some avons are formed by multi-Higgs to identify the origins of leptonic avour mixing. The LFV decays of charged leptons induced by avons can be divided into two breaking it. try are In A4 models, the only processes allowed by the Z3 residual symme , and all other 3-body and radiative decays are forbidden [67, 68]. The latter can take place if the Z3 symmetry is broken [69], Current experimental bounds of the branching ratios of LFV 3-body de e ; e+e e , and ratiative decays are in general around 10 8, measured by Belle [70] and BaBar [71], respectively. The upper limit of the 3-body decay C.L., from the SINDRUM experiment [72]. The most stringent measurement is in the MEG experiment, with branching ratio 10 13 at 90% C.L. [73]. A MEG upgrade (MEG II) is envisaged to reach the upper limit of the branching ratio to 4 decay is 1:0 to the scale of avour symmetry. In this paper, we develop a generic method to analyse charged LFV processes in modsion of charged LFV processes induced by avons. We derive the expressions of leading current oscillation data. Flavour mixing in the A4 symmetry Residual symmetries and tri-bimaximal mixing S, T 2ST , T ST 2. It is the smallest discrete group which has a 3-dimensional irreducible reduced as 3 symmetric and anti-symmetric components, respectively. We work in the Altarelli-Feruglio basis [27], where T is diagonal. T and S are respectively given by T = B 0 !2 0 C (...truncated)


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Silvia Pascoli, Ye-Ling Zhou. Flavon-induced connections between lepton flavour mixing and charged lepton flavour violation processes, Journal of High Energy Physics, 2016, pp. 145, Volume 2016, Issue 10, DOI: 10.1007/JHEP10(2016)145