375 research outputs found
Vacuum Induced CP Violation Generating a Complex CKM Matrix with Controlled Scalar FCNC
We propose a viable minimal model with spontaneous CP violation in the
framework of a Two Higgs Doublet Model. The model is based on a generalised
Branco-Grimus-Lavoura model with a flavoured symmetry, under
which two of the quark families are even and the third one is odd. The
lagrangian respects CP invariance, but the vacuum has a CP violating phase,
which is able to generate a complex CKM matrix, with the rephasing invariant
strength of CP violation compatible with experiment. The question of scalar
mediated flavour changing neutral couplings is carefully studied. In particular
we point out a deep connection between the generation of a complex CKM matrix
from a vacuum phase and the appearance of scalar FCNC. The scalar sector is
presented in detail, showing that the new scalars are necessarily lighter than
1 TeV. A complete analysis of the model including the most relevant constraints
is performed, showing that it is viable and that it has definite implications
for the observation of New Physics signals in, for example, flavour changing
Higgs decays or the discovery of the new scalars at the LHC. We give special
emphasis to processes like , as well as , which are relevant for the LHC and the ILC.Comment: 36 pages, 11 figure
Vector-like Quarks at the Origin of Light Quark Masses and Mixing
We show how a novel fine-tuning problem present in the Standard Model can be
solved through the introduction of a single flavour symmetry G, together with
three quarks, three quarks, as well as a complex singlet
scalar. The symmetry G is extended to the additional fields and it is an exact
symmetry of the Lagrangian, only spontaneously broken by the vacuum. Specific
examples are given and a phenomenological analysis of the main features of the
model is presented. It is shown that even for vector-like quarks with masses
accessible at the LHC, one can have realistic quark masses and mixing, while
respecting the strict constraints on process arising from flavour changing
neutral currents (FCNC). The vector-like quark decay channels are also
described.Comment: 25 pages, no figure
Small violations of unitarity, the phase in Bs mixing and visible t->cZ decays at the LHC
We show that it is possible to accommodate the observed size of the phase in
--, mixing in the framework of a model with violation of
unitarity. This violation is associated to the presence of a new
isosinglet quark , which mixes both with and and has a mass
not exceeding 500 GeV. The crucial point is the fact that this framework allows
for of order , to be
contrasted with the situation in the Standard Model, where is
constrained to be of order . We point out that this scenario implies
rare top decays at a rate observable at the LHC and
significantly different from unity. In this framework, one may also account for
the observed size of -- mixing without having to invoke long
distance contributions. It is also shown that in the present scenario, the
observed size of -- mixing constrains
to be of order
, which is significantly smaller than what is allowed in generic
models with violations of unitarity.Comment: 20 pages. References added. Additional observables considered,
updated numerical examples, conclusions unchange
Controlled Flavour Changing Neutral Couplings in Two Higgs Doublet Models
We propose a class of Two Higgs Doublet Models where there are Flavour
Changing Neutral Currents (FCNC) at tree level, but under control due to the
introduction of a discrete symmetry in the full Lagrangian. It is shown that in
this class of models, one can have simultaneously FCNC in the up and down
sectors, in contrast to the situation encountered in BGL models. The intensity
of FCNC is analysed and it is shown that in this class of models one can
respect all the strong constraints from experiment without unnatural
fine-tuning. It is pointed out that the additional sources of flavour and CP
violation are such that they can enhance significantly the generation of the
Baryon Asymmetry of the Universe, with respect to the Standard Model.Comment: 29 pages, 3 figure
Symmetry Constrained Two Higgs Doublet Models
We study Two-Higgs-Doublet Models (2HDM) where Abelian symmetries have been
introduced, leading to a drastic reduction in the number of free parameters in
the 2HDM. Our analysis is inspired in BGL models, where, as the result of a
symmetry of the Lagrangian, there are tree-level scalar mediated
Flavour-Changing-Neutral-Currents, with the flavour structure depending only on
the CKM matrix. A systematic analysis is done on the various possible schemes,
which are classified in different classes, depending on the way the extra
symmetries constrain the matrices of couplings defining the flavour structure
of the scalar mediated neutral currents. All the resulting flavour textures of
the Yukawa couplings are stable under renormalisation since they result from
symmetries imposed at the Lagrangian level. We also present a brief
phenomenological analysis of the most salient features of each class of
symmetry constrained 2HDM.Comment: 30 pages, 5 Table
CP violation and limits on New Physics including recent measurements
We analyse present constraints on the SM parameter space and derive, in a
model independent way, various bounds on New Physics contributions to
-- and -- mixings. Our analyses include
information on a large set of asymmetries, leading to the measurement of the
CKM phases and , as well as recent data from D0 and CDF
related to the -- system such as the measurement of , and . We examine in detail several
observables such as the asymmetries , , the width differences
and and discuss the r\^ole they play
in establishing the limits on New Physics. The present data clearly favour the
SM, with the New Physics favoured region placed around the SM solution. A New
Physics solution significantly different from the SM is still allowed, albeit
quite disfavoured (2.6% probability). We analyse the presently available
indirect knowledge on the phase entering in --
mixing and study the impact of a future measurement of to be
achieved at LHC, through the measurement of the time-dependent CP asymmetry in
decays.Comment: 29 pages, 31 figures; updated analyses and reference
Extending Trinity to the Scalar Sector through Discrete Flavoured Symmetries
We conjecture the existence of a relation between elementary scalars and
fermions, making it plausible the existence of three Higgs doublets. We
introduce a Trinity Principle (TP) which, given the fact that there are no
massless quarks, requires the existence of a minimum of three Higgs doublets.
The TP states that each line of the mass matrix of a quark of a given charge
should receive the contribution from one and only one scalar doublet and
furthermore a given scalar doublet should contribute to one and only one line
of the mass matrix of a quark of a given charge. This principle is analogous to
the Natural Flavour Conservation (NFC) of Glashow and Weinberg with the key
distinction that NFC required the introduction of a flavour blind symmetry,
while the TP requires a flavoured symmetry, to be implemented in a natural way.
We provide two examples which satisfy the Trinity Principle based on
and flavoured symmetries, and
show that they are the minimal multi-Higgs extensions of the Standard Model
where CP can be imposed as a symmetry of the full Lagrangian and broken by the
vacuum, without requiring soft-breaking terms. We show that the vacuum phases
are sufficient to generate a complex CKM matrix, in agreement with experiment.
The above mentioned flavoured symmetries lead to a strong reduction in the
number of parameters in the Yukawa interactions, enabling a control of the
Scalar Flavour Changing Neutral Couplings (SFCNC). We analyse some of the other
physical implications of the two models, including an estimate of the
enhancement of the Baryon Asymmetry of the Universe provided by the new sources
of CP violation, and a discussion of the the strength of their tree-level
SFCNC.Comment: 24 page
CP-odd and CP-even Weak-Basis Invariants in the Presence of Vector-Like Quarks
We propose a minimal set of weak-basis invariants in an extension of the SM
where one up-type isosinglet vector-like quark is introduced, which allows us
to obtain all the physical content of the CKM matrix. We present CP-odd
invariants of lower order in mass than the one in the SM, which may have
important consequences for Baryogenesis. We study the extreme chiral limit,
where the two lightest generations have vanishing mass, showing that in this
extension, contrary to the SM, CP violation can be observed in collisions much
above the electroweak scale.Comment: 18 page
Mixing asymmetries in B meson systems, the D0 like-sign dimuon asymmetry and generic New Physics
The measurement of a large like-sign dimuon asymmetry AbSL by the D0 experiment at the Tevatron departs noticeably from Standard Model expectations and it may be interpreted as a hint of physics beyond the Standard Model contributing to ΔB≠0 transitions. In this work we analyse how the natural suppression of AbSL in the SM can be circumvented by New Physics. We consider generic Standard Model extensions where the charged current mixing matrix is enlarged with respect to the usual 3×3 unitary Cabibbo-Kobayashi-Maskawa matrix, and show how, within this framework, a significant enhancement over Standard Model expectations for AbSL is easily reachable through enhancements of the semileptonic asymmetries AdSL and AsSL of both B0d - B¯0d and B0s - B¯0s systems. Despite being insufficient to reproduce the D0 measurement, such deviations from SM expectations may be probed by the LHCb experiment
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