4,322 research outputs found
Tri-bimaximal Neutrino Mixing and Quark Masses from a Discrete Flavour Symmetry
We build a supersymmetric model of quark and lepton masses based on the
discrete flavour symmetry group T', the double covering of A_4. In the lepton
sector our model is practically indistinguishable from recent models based on
A_4 and, in particular, it predicts a nearly tri-bimaximal mixing, in good
agreement with present data. In the quark sector a realistic pattern of masses
and mixing angles is obtained by exploiting the doublet representations of T',
not available in A_4. To this purpose, the flavour symmetry T' should be broken
spontaneously along appropriate directions in flavour space. In this paper we
fully discuss the related vacuum alignment problem, both at the leading order
and by accounting for small effects coming from higher-order corrections. As a
result we get the relations: \sqrt{m_d/m_s}\approx |V_{us}| and
\sqrt{m_d/m_s}\approx |V_{td}/V_{ts}|.Comment: 27 pages, 1 figure; minor correction
The complete HEFT Lagragian after the LHC Run I
The complete effective chiral Lagrangian for a dynamical Higgs is presented and constrained by means of a global analysis including electroweak precision data together with Higgs and triple gauge-boson coupling data from the LHC Run I. The operators' basis up to next-to-leading order in the expansion consists of 148 (188 considering right-handed neutrinos) flavour universal terms and it is presented here making explicit the custodial nature of the operators. This effective Lagrangian provides the most general description of the physical Higgs couplings once the electroweak symmetry is assumed, and it allows for deviations from the SU(2)L doublet nature of the Standard Model Higgs. The comparison with the effective linear Lagrangian constructed with an exact SU(2)L doublet Higgs and considering operators with at most canonical dimension six is presented. A promising strategy to disentangle the two descriptions consists in analysing (i) anomalous signals present only in the chiral Lagrangian and not expected in the linear one, that are potentially relevant for LHC searches, and (ii) decorrelation effects between observables that are predicted to be correlated in the linear case and not in the chiral one. The global analysis presented here, which includes several kinematic distributions, is crucial for reducing the allowed parameter space and for controlling the correlations between parameters. This improves previous studies aimed at investigating the Higgs Nature and the origin of the electroweak symmetry breaking
Current-oscillator correlation and Fano factor spectrum of quantum shuttle with finite bias voltage and temperature
A general master equation is derived to describe an electromechanical
single-dot transistor in the Coulomb blockade regime. In the equation, Fermi
distribution functions in the two leads are taken into account, which allows
one to study the system as a function of bias voltage and temperature of the
leads. Furthermore, we treat the coherent interaction mechanism between
electron tunneling events and the dynamics of excited vibrational modes.
Stationary solutions of the equation are numerically calculated. We show
current through the oscillating island at low temperature appears step like
characteristics as a function of the bias voltage and the steps depend on mean
phonon number of the oscillator. At higher temperatures the current steps would
disappear and this event is accompanied by the emergence of thermal noise of
the charge transfer. When the system is mainly in the ground state, zero
frequency Fano factor of current manifests sub-Poissonian noise and when the
system is partially driven into its excited states it exhibits super-Poissonian
noise. The difference in the current noise would almost be removed for the
situation in which the dissipation rate of the oscillator is much larger than
the bare tunneling rates of electrons.Comment: 14 pages, 8 figure
Fermion Masses and Mixings in a S4 Based Model
It has been recently claimed that the symmetry group S4 yields to the
Tri-bimaximal neutrino mixing in a "natural" way from the group theory point of
view. Approving of this feature as an indication, we build a supersymmetric
model of lepton and quark masses based on this family symmetry group. In the
lepton sector, a correct mass hierarchy among the charged leptons is achieved
together to a neutrino mass matrix which can be diagonalized by the
Tri-bimaximal pattern. Our model results to be phenomenologically unequivalent
with respect to other proposals based on different flavour groups but still
predicting the Tri-bimaximal mixing. In the quark sector a realistic pattern
for masses and mixing angles is obtained. The flavour structures of the mass
matrices in both the sectors come from the spontaneously symmetry breaking of
S4, due to several scalar fields, which get non-zero vacuum expectation values.
A specific vacuum alignment is required and it is shown to be a natural results
of the minimization of the scalar potential and, moreover, to be stable under
the corrections from the higher order terms.Comment: 25 pages, LaTeX; added references and minor correctio
Intrinsic Deviation from the Tri-bimaximal Neutrino Mixing in a Class of A_4 Flavor Models
It is well known that the tri-bimaximal neutrino mixing pattern V_0 can be
derived from a class of flavor models with the non-Abelian A_4 symmetry. We
point out that small corrections to V_0, which are inherent in the A_4 models
and arise from both the charged-lepton and neutrino sectors, have been omitted
in the previous works. We show that such corrections may lead the 3 \times 3
neutrino mixing matrix V to a non-unitary deviation from V_0, but they cannot
result in a nonzero value of \theta_13 or any new CP-violating phases. Current
experimental constraints on the unitarity of V allow us to constrain the model
parameters to some extent.Comment: 11 pages, no figures; a reference added, accepted for publication in
Phys. Lett.
Towards Minimal S4 Lepton Flavor Model
We study lepton flavor models with the 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 triplet flavon is not realistic, but we can construct realistic models
with two triplet flavons, or one triplet and one doublet flavons.Comment: 18 pages, 4 figures, references are adde
Tri-Bimaximal Lepton Mixing and Leptogenesis
In models with flavour symmetries added to the gauge group of the Standard
Model the CP-violating asymmetry necessary for leptogenesis may be related with
low-energy parameters. A particular case of interest is when the flavour
symmetry produces exact Tri-Bimaximal lepton mixing leading to a vanishing
CP-violating asymmetry. In this paper we present a model-independent discussion
that confirms this always occurs for unflavoured leptogenesis in type I see-saw
scenarios, noting however that Tri-Bimaximal mixing does not imply a vanishing
asymmetry in general scenarios where there is interplay between type I and
other see-saws. We also consider a specific model where the exact Tri-Bimaximal
mixing is lifted by corrections that can be parametrised by a small number of
degrees of freedom and analyse in detail the existing link between low and
high-energy parameters - focusing on how the deviations from Tri-Bimaximal are
connected to the parameters governing leptogenesis.Comment: 29 pages, 6 figures; version 2: references added, minor correction
Lepton Flavour Violation in a Supersymmetric Model with A4 Flavour Symmetry
We compute the branching ratios for mu-> e gamma, tau-> mu gamma and tau -> e
gamma in a supersymmetric model invariant under the flavour symmetry group A4 X
Z3 X U(1)_{FN}, in which near tri-bimaximal lepton mixing is naturally
predicted. At leading order in the small symmetry breaking parameter u, which
is of the same order as the reactor mixing angle theta_{13}, we find that the
branching ratios generically scale as u^2. Applying the current bound on the
branching ratio of mu -> e gamma shows that small values of u or tan(beta) are
preferred in the model for mass parameters m_{SUSY} and m_{1/2} smaller than
1000 GeV. The bound expected from the on-going MEG experiment will provide a
severe constraint on the parameter space of the model either enforcing u approx
0.01 and small tan(beta) or m_{SUSY} and m_{1/2} above 1000 GeV. In the special
case of universal soft supersymmetry breaking terms in the flavon sector a
cancellation takes place in the amplitudes and the branching ratios scale as
u^4, allowing for smaller slepton masses. The branching ratios for tau -> mu
gamma and tau -> e gamma are predicted to be of the same order as the one for
mu -> e gamma, which precludes the possibility of observing these tau decays in
the near future.Comment: 44 page
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