223 research outputs found
Fermionic extensions of the Standard Model in light of the Higgs couplings
As the Higgs boson properties settle, the constraints on the Standard Model
extensions tighten. We consider all possible new fermions that can couple to
the Higgs, inspecting sets of up to four chiral multiplets. We confront them
with direct collider searches, electroweak precision tests, and current
knowledge of the Higgs couplings. The focus is on scenarios that may depart
from the decoupling limit of very large masses and vanishing mixing, as they
offer the best prospects for detection. We identify exotic chiral families that
may receive a mass from the Higgs only, still in agreement with the
signal strength. A mixing between the Standard Model
and non-chiral fermions induces order deviations in the Higgs
couplings. The mixing can be as large as in case of custodial
protection of the couplings or accidental cancellation in the oblique
parameters. We also notice some intriguing effects for much smaller values of
, especially in the lepton sector. Our survey includes a number of
unconventional pairs of vector-like and Majorana fermions coupled through the
Higgs, that may induce order one corrections to the Higgs radiative couplings.
We single out the regions of parameters where and are
unaffected, while the signal strength is significantly modified,
turning a few times larger than in the Standard Model in two cases. The second
run of the LHC will effectively test most of these scenarios.Comment: v2: a few clarifications and references added, improved treatment of
the constraint from Z-b-bbar, Higgs couplings updated by using the combined
ATLAS+CMS fi
Common origin of \theta_{13} and \Delta m^2_{12} in a model of neutrino mass with quaternion symmetry
The smallness of the 1-3 lepton mixing angle and of the
neutrino mass-squared-difference ratio can be
understood as the departure from a common limit where they both vanish. We
discuss in general the conditions for realizing the mass degeneracy of a pair
of neutrinos and show that the vanishing of a CP violating phase is needed. We
find that the discrete quaternion group Q of eight elements is the simplest
family symmetry which correlates the smallness of to the
value of . In such a model we predict if the ordering of the neutrino mass spectrum is
normal, and if it is inverted.Comment: revtex, 7 pages, 6 pdf figures; Appendix added discussing in detail
the alignment of VEV
Dynamical Clockwork Axions
The clockwork mechanism is a novel method for generating a large separation
between the dynamical scale and interaction scale of a theory. We demonstrate
how the mechanism can arise from a sequence of strongly-coupled sectors. This
framework avoids elementary scalar fields as well as ad hoc continuous global
symmetries, both of which are subject to serious stability issues. The
clockwork factor, , is determined by the consistency of the strong dynamics.
The preserved global of the clockwork appears as an accidental symmetry,
resulting from discrete or gauge symmetries, and it is spontaneously
broken by the chiral condensates. We apply such a dynamical clockwork to
construct models with an effectively invisible QCD axion from TeV-scale strong
dynamics. The axion couplings are determined by the localisation of the
Standard Model interactions along the clockwork sequence. The TeV spectrum
includes either coloured hadrons or vector-like quarks. Dark matter can be
accounted for by the axion or the lightest neutral baryons, which are
accidentally stable.Comment: 41 page
Fermion masses and mixing in models with SO(10) x A_4 symmetry
We study the flavour sector in models where the three families of matter are
unified in a representation of the group. The
necessary ingredients to realize tri-bi-maximal mixing in the lepton sector are
identified systematically. The non-renormalizable operators contributing to the
fermion mass matrices play an important role. We also present a mechanism to
explain the inter-family mass hierarchy of quarks and charged leptons, which
relies on a `universal seesaw' mechanism and is compatible with tri-bi-maximal
mixing.Comment: 24 pages, revte
Flavour violation in supersymmetric SO(10) unification with a type II seesaw mechanism
We study flavour violation in a supersymmetric SO(10) implementation of the
type II seesaw mechanism, which provides a predictive realization of triplet
leptogenesis. The experimental upper bounds on lepton flavour violating
processes have a significant impact on the leptogenesis dynamics, in particular
they exclude the strong washout regime. Requiring successful leptogenesis then
constrains the otherwise largely unknown overall size of flavour-violating
observables, thus yielding testable predictions. In particular, the branching
ratio for mu -> e gamma lies within the reach of the MEG experiment if the
superpartner spectrum is accessible at the LHC, and the supersymmetric
contribution to epsilon_K can account for a significant part of the
experimental value. We show that this scenario can be realized in a consistent
SO(10) model achieving gauge symmetry breaking and doublet-triplet splitting in
agreement with the proton decay bounds, improving on the MSSM prediction for
alpha_3(m_Z), and reproducing the measured quark and lepton masses.Comment: 40 pages, 10 figures. Accepted for publication in JHE
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