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)