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45
result(s) for
"Lavoura, Luís"
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Fitting the Zbb¯ vertex in the two-Higgs-doublet model and in the three-Higgs-doublet model
A
bstract
We investigate the new contributions to the parameters
g
L
and
g
R
of the
Zb
b
¯
vertex in a multi-Higgs-doublet model (MHDM). We emphasize that those contributions generally worsen the fit of those parameters to the experimental data. We propose a solution to this problem, wherein
g
R
has the opposite sign from the one predicted by the Standard Model; this solution, though, necessitates light scalars and large Yukawa couplings in the MHDM.
Journal Article
Oblique corrections from leptoquarks
A
bstract
We present general formulas for the oblique-correction parameters
S
,
T
,
U
,
V
,
W
, and
X
in a model of New Physics having arbitrary numbers of scalar leptoquarks of the five permissible types. We allow for a general mixing among the scalars of the various electric charges, viz. −4
/
3, −1
/
3, 2
/
3, and 5
/
3. We then extend the formulas to the case of a New Physics model with additional scalars in any representations of the gauge group SU(2)
×
U(1), mixing arbitrarily among themselves.
Journal Article
Two-body lepton-flavour-violating decays in a 2HDM with soft family-lepton-number breaking
A
bstract
We evaluate the decays
ℓ
1
±
→
ℓ
2
±
γ
,
Z
→
ℓ
1
+
ℓ
2
−
, and
h
→
ℓ
1
+
ℓ
2
−
, where
ℓ
1
and
ℓ
2
are charged leptons with different flavours and
h
is the scalar particle with mass 125.25 GeV, in a two-Higgs-doublet model where all the Yukawa-coupling matrices conserve the lepton flavours but the Majorana mass terms of the right-handed neutrinos break the flavour lepton numbers. We find that (1) the decays
ℓ
1
±
→
ℓ
2
±
γ
require large Yukawa couplings and very light right-handed neutrinos in order to be visible, (2) the decays
Z
→
ℓ
1
+
ℓ
2
−
will be invisible in all the planned experiments, except in a very restricted range of circumstances, but (3) the decays
h
→
ℓ
1
+
ℓ
2
−
may be detected in future experiments for rather relaxed sets of input parameters.
Journal Article
Oblique corrections from triplet quarks
A
bstract
We present general formulas for the oblique-correction parameters
S
,
T
,
U
,
V, W
, and
X
in an extension of the Standard Model having arbitrary numbers of singlet, doublet, and triplet quarks with electric charges −4
/
3, −1
/
3, 2
/
3, and 5
/
3 that mix with the standard quarks of the same charge.
Journal Article
Fitting the$$ Zb\\overline{b} $$vertex in the two-Higgs-doublet model and in the three-Higgs-doublet model
We investigate the new contributions to the parameters g L and g R of the$$ Zb\\overline{b} $$Zb b ¯ vertex in a multi-Higgs-doublet model (MHDM). We emphasize that those contributions generally worsen the fit of those parameters to the experimental data. We propose a solution to this problem, wherein g R has the opposite sign from the one predicted by the Standard Model; this solution, though, necessitates light scalars and large Yukawa couplings in the MHDM.
Journal Article
On the Addition of a Large Scalar Multiplet to the Standard Model
We consider the addition of a single $SU(2)$ multiplet of complex scalar fields to the Standard Model (SM). We explicitly consider the various possible values of the weak isospin J of that multiplet, up to and including $J = 7/2$. We allow the multiplet to have arbitrary weak hypercharge. The scalar fields of the multiplet are assumed to have no vacuum expectation value; the mass differences among the components of the multiplet originate in its coupling, present in the scalar potential (SP), to the Higgs doublet of the SM. We derive exact bounded-from-below and unitarity conditions on the SP, thereby constraining those mass differences. We compare those constraints to the ones that may be derived from the oblique parameters.
Journal Article
More models for lepton mixing with four constraints
A
bstract
We propose new lepton-mixing textures that may be enforced through well-defined symmetries in renormalizable models. Each of our textures has four sum rules for the neutrino mass observables. The models are based on the type-I seesaw mechanism; their charged-lepton mass matrices are diagonal because of the symmetries imposed. Each model has three versions, depending on the identification of the charged leptons. Testing all the models, we have found that five of them agree with the data at the 1
σ
level when the neutrino-mass ordering is normal, and two models agree with the data for an inverted ordering. We detail the predictions of each of those seven models.
Journal Article
The centers of discrete groups as stabilizers of dark matter
Abstract
The most usual option to stabilize dark matter (DM) is a $\\mathbb {Z}_2$ symmetry. In general, though, DM may be stabilized by any $\\mathbb {Z}_N$ with N ≥ 2. We consider the way that $\\mathbb {Z}_N$ is a subgroup of the internal-symmetry group G of a model; we entertain the possibility that $\\mathbb {Z}_N$ is the center of G, yet G is not of the form $\\mathbb {Z}_N \\times G^\\prime$, where G′ is a group smaller (i.e., of lower order) than G. We examine all the discrete groups of order smaller than 2001 and we find that many of them cannot be written as the direct product of a cyclic group and some other group, yet they have a non-trivial center that might be used in model building to stabilize DM.
Journal Article
Lepton mixing and the charged-lepton mass ratios
A
bstract
We construct a class of renormalizable models for lepton mixing that generate predictions given in terms of the charged-lepton mass ratios. We show that one of those models leads, when one takes into account the known experimental values, to almost maximal
CP
-breaking phases and to almost maximal neutrinoless double-beta decay. We study in detail the scalar potential of the models, especially the bounds imposed by unitarity on the values of the quartic couplings.
Journal Article