239 research outputs found
Stable lepton mass matrices
Abstract: We study natural lepton mass matrices, obtained assuming the stability of physical flavour observables with respect to the variations of individual matrix elements. We identify all four possible stable neutrino textures from algebraic conditions on their entries. Two of them turn out to be uniquely associated to specific neutrino mass patterns. We then concentrate on the semi-degenerate pattern, corresponding to an overall neutrino mass scale within the reach of future experiments. In this context we show that i) the neutrino and charged lepton mixings and mass matrices are largely constrained by the requirement of stability, ii) naturalness considerations give a mild preference for the Majorana phase most relevant for neutrinoless double-\u3b2 decay, \u3b1 3c \u3c0/2, and iii) SU(5) unification allows to extend the implications of stability to the down quark sector. The above considerations would benefit from an experimental determination of the PMNS ratio 1aU32/U31 1a, i.e. of the Dirac phase \u3b4. \ua9 2016, The Author(s)
On the effect of resonances in composite Higgs phenomenology
We consider a generic composite Higgs model based on the coset SO(5)/SO(4)
and study its phenomenology beyond the leading low-energy effective lagrangian
approximation. Our basic goal is to introduce in a controllable and simple way
the lowest-lying, possibly narrow, resonances that may exist is such models. We
do so by proposing a criterion that we call partial UV completion. We
characterize the simplest cases, corresponding respectively to a scalar in
either singlet or tensor representation of SO(4) and to vectors in the adjoint
of SO(4). We study the impact of these resonances on the signals associated to
high-energy vector boson scattering, pointing out for each resonance the
characteristic patterns of depletion and enhancement with respect to the
leading-order chiral lagrangian. En route we derive the O(p^4) general chiral
lagrangian and discuss its peculiar accidental and approximate symmetries.Comment: v3: a few typos corrected. Conclusions unchange
Anomalous Couplings in Double Higgs Production
The process of gluon-initiated double Higgs production is sensitive to
non-linear interactions of the Higgs boson. In the context of the Standard
Model, studies of this process focused on the extraction of the Higgs trilinear
coupling. In a general parametrization of New Physics effects, however, an even
more interesting interaction that can be tested through this channel is the
(ttbar hh) coupling. This interaction vanishes in the Standard Model and is a
genuine signature of theories in which the Higgs boson emerges from a
strongly-interacting sector. In this paper we perform a model-independent
estimate of the LHC potential to detect anomalous Higgs couplings in
gluon-fusion double Higgs production. We find that while the sensitivity to the
trilinear is poor, the perspectives of measuring the new (ttbar hh) coupling
are rather promising.Comment: 22 pages, 9 figures. v2: plots of Figs.8 and 9 redone to include
experimental uncertainty on the Higgs couplings, references adde
Higgs Low-Energy Theorem (and its corrections) in Composite Models
The Higgs low-energy theorem gives a simple and elegant way to estimate the
couplings of the Higgs boson to massless gluons and photons induced by loops of
heavy particles. We extend this theorem to take into account possible nonlinear
Higgs interactions resulting from a strong dynamics at the origin of the
breaking of the electroweak symmetry. We show that, while it approximates with
an accuracy of order a few percents single Higgs production, it receives
corrections of order 50% for double Higgs production. A full one-loop
computation of the gg->hh cross section is explicitly performed in MCHM5, the
minimal composite Higgs model based on the SO(5)/SO(4) coset with the Standard
Model fermions embedded into the fundamental representation of SO(5). In
particular we take into account the contributions of all fermionic resonances,
which give sizeable (negative) corrections to the result obtained considering
only the Higgs nonlinearities. Constraints from electroweak precision and
flavor data on the top partners are analyzed in detail, as well as direct
searches at the LHC for these new fermions called to play a crucial role in the
electroweak symmetry breaking dynamics.Comment: 30 pages + appendices and references, 12 figures. v2: discussion of
flavor constraints improved; references added; electroweak fit updated,
results unchanged. Matches published versio
Renormalisation group corrections to neutrino mixing sum rules
Neutrino mixing sum rules are common to a large class of models based on the
(discrete) symmetry approach to lepton flavour. In this approach the neutrino
mixing matrix is assumed to have an underlying approximate symmetry form
\tildeU_\nu, which is dictated by, or associated with, the employed
(discrete) symmetry. In such a setup the cosine of the Dirac CP-violating phase
can be related to the three neutrino mixing angles in terms of a sum
rule which depends on the symmetry form of \tildeU_\nu. We consider five
extensively discussed possible symmetry forms of \tildeU_\nu: i)
bimaximal (BM) and ii) tri-bimaximal (TBM) forms, the forms corresponding to
iii) golden ratio type A (GRA) mixing, iv) golden ratio type B (GRB) mixing,
and v) hexagonal (HG) mixing. For each of these forms we investigate the
renormalisation group corrections to the sum rule predictions for in
the cases of neutrino Majorana mass term generated by the Weinberg (dimension
5) operator added to i) the Standard Model, and ii) the minimal SUSY extension
of the Standard Model
Supersymmetry with a pNGB Higgs and partial compositeness
We study the consequences of combining SUSY with a pseudo Nambu-Goldstone boson Higgs coming from an SO(5)/SO(4) coset and "partial compositeness". In particular, we focus on how electroweak symmetry breaking and the Higgs mass are reproduced in models where the symmetry SO(5) is linearly realized. The global symmetry forbids tree-level contributions to the Higgs potential coming from D-terms, differently from what happens in most of the SUSY little-Higgs constructions. While the stops are generally heavy, light fermion top partners below 1TeV are predicted. In contrast to what happens in non-SUSY composite Higgs models, they are necessary to reproduce the correct top, rather than Higgs, mass. En passant, we point out that, independently of SUSY, models where tR is fully composite and embedded in the 5 of SO(5) generally predict a too light Higgs. Open Access, \ua9 2014 The Authors
Post-LS3 Experimental Options in ECN3
The Experimental Cavern North 3 (ECN3) is an underground experimental cavern
on the CERN Pr\'evessin site. ECN3 currently hosts the NA62 experiment, with a
physics programme devoted to rare kaon decays and searches of hidden particles
approved until Long Shutdown 3 (LS3). Several options are proposed on the
longer term in order to make best use of the worldwide unique potential of the
high-intensity/high-energy proton beam extracted from the Super Proton
Synchrotron (SPS) in ECN3. The current status of their study by the CERN
Physics Beyond Colliders (PBC) Study Group is presented, including
considerations on beam requirements and upgrades, detector R&D and
construction, schedules and cost, as well as physics potential within the CERN
and worldwide landscape.Comment: 113 pages, 39 figure
Exploring holographic Composite Higgs models
Simple Composite Higgs models predict new vector-like fermions not too far from the electroweak scale, yet LHC limits are now sensitive to the TeV scale. Motivated by this tension, we explore the holographic dual of the minimal model, MCHM5, to try and alleviate this tension without increasing the fine-tuning in the Higgs potential. Interestingly, we find that lowering the UV cutoff in the 5D picture allows for heavier top partners and less fine-tuning. In the 4D dual this corresponds to increasing the number of âcoloursâ N , thus increasing the decay constant of the Goldstone Higgs. This is essentially a âLittle Randall-Sundrum Modelâ, which are known to reduce some flavour and electroweak constraints. Furthermore, in anticipation of the ongoing efforts at the LHC to put bounds on the top Yukawa, we demonstrate that deviations from the SM can be suppressed or enhanced with respect to what is expected from mere symmetry arguments in 4D. We conclude that the 5D holographic realisation of the MCHM5 with a small UV cutoff is not in tension with the current experimental data
Predictions for the Leptonic Dirac CP Violation Phase: a Systematic Phenomenological Analysis
We derive predictions for the Dirac phase present
in the unitary neutrino mixing
matrix , where and are
unitary matrices which arise from the diagonalisation
respectively of the charged lepton and the neutrino mass matrices.
We consider forms of and allowing us to express
as a function of three
neutrino mixing angles,
present in ,
and the angles contained in .
We consider several forms of
determined by, or associated with, symmetries,
tri-bimaximal, bimaximal, etc.,
for which the angles in are
fixed. For each of these forms and forms of
allowing to reproduce the measured values of the neutrino
mixing angles,
we construct the likelihood function
for , using i) the latest results of the global
fit analysis of neutrino oscillation data,
and ii) the prospective sensitivities
on the neutrino mixing angles.
Our results, in particular, confirm the conclusion
reached in earlier similar studies
that the measurement of the Dirac phase
in the neutrino mixing matrix, together with an improvement
of the precision on the mixing angles,
can provide unique information about the
possible existence of symmetry
in the lepton sector
Generalised geometrical CP violation in a T' lepton flavour model
We analyse the interplay of generalised CP transformations and the non-Abelian discrete group T \u2032 and use the semi-direct product G f = T \u2032 caH CP, as family symmetry acting in the lepton sector. The family symmetry is shown to be spontaneously broken in a geometrical manner. In the resulting flavour model, naturally small Majorana neutrino masses for the light active neutrinos are obtained through the type I see-saw mechanism. The known masses of the charged leptons, lepton mixing angles and the two neutrino mass squared differences are reproduced by the model with a good accuracy. The model allows for two neutrino mass spectra with normal ordering (NO) and one with inverted ordering (IO). For each of the three spectra the absolute scale of neutrino masses is predicted with relatively small uncertainty. The value of the Dirac CP violation (CPV) phase \u3b4 in the lepton mixing matrix is predicted to be \u3b4 = \u3c0/2 or 3\u3c0/2. Thus, the CP violating effects in neutrino oscillations are predicted to be maximal (given the values of the neutrino mixing angles) and experimentally observable. We present also predictions for the sum of the neutrino masses, for the Majorana CPV phases and for the effective Majorana mass in neutrinoless double beta decay. The predictions of the model can be tested in a variety of ongoing and future planned neutrino experiments
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