11 research outputs found
The Higgs boson from an extended symmetry
The variety of ideas put forward in the context of a "composite" picture for
the Higgs boson calls for a simple and effective description of the related
phenomenology. Such a description is given here by means of a "minimal" model
and is explicitly applied to the example of a Higgs-top sector from an SO(5)
symmetry. We discuss the spectrum, the ElectroWeak Precision Tests, B-physics
and naturalness. We show the difficulty to comply with the different
constraints. The extended gauge sector relative to the standard SU(2)xU(1), if
there is any, has little or no impact on these considerations. We also discuss
the relation of the "minimal" model with its "little Higgs" or "holographic"
extensions based on the same symmetry.Comment: 22 pp; v3: Small corrections, version to be published in Phys. Rev.
Higgs doublet as a Goldstone boson in perturbative extensions of the Standard Model
We investigate the idea of the Higgs doublet as a pseudo-Goldstone boson in
perturbative extensions of the Standard Model, motivated by the desire to
ameliorate its hierarchy problem without conflict with the electroweak
precision data. Two realistic supersymmetric models with global SU(3) symmetry
are proposed, one for large and another for small values of tan\beta. The two
models demonstrate two different mechanisms for EWSB and the Higgs mass
generation. Their experimental signatures are quite different. Our
constructions show that a pseudo-Goldstone Higgs doublet in perturbative
extensions is just as plausible as in non-perturbative ones.Comment: 16pp, typos corrected, phenomenological discussion expande
Fermion masses and proton decay in a minimal five-dimensional SO(10) model
We propose a minimal SO(10) model in 5 space-time dimensions. The single
extra spatial dimension is compactified on the orbifold S^1/(Z_2 x Z_2')
reducing the gauge group to that of Pati-Salam. The breaking down to the
standard model group is obtained through an ordinary Higgs mechanism taking
place at the Pati-Salam brane, giving rise to a proper gauge coupling
unification. We achieve a correct description of fermion masses and mixing
angles by describing first and second generations as bulk fields, and by
embedding the third generation into four multiplets located at the Pati-Salam
brane. The Yukawa sector is simple and compact and predicts a neutrino spectrum
of normal hierarchy type. Concerning proton decay, dimension five operators are
absent and the essentially unique localization of matter multiplets implies
that the minimal couplings between the super-heavy gauge bosons and matter
fields are vanishing. Non-minimal interactions are allowed but the resulting
dimension six operators describing proton decay are too suppressed to produce
observable effects, even in future, super-massive detectors.Comment: 21 pages, 3 figure
Composite GUTs: models and expectations at the LHC
We investigate grand unified theories (GUTs) in scenarios where electroweak
(EW) symmetry breaking is triggered by a light composite Higgs, arising as a
Nambu-Goldstone boson from a strongly interacting sector. The evolution of the
standard model (SM) gauge couplings can be predicted at leading order, if the
global symmetry of the composite sector is a simple group G that contains the
SM gauge group. It was noticed that, if the right-handed top quark is also
composite, precision gauge unification can be achieved. We build minimal
consistent models for a composite sector with these properties, thus
demonstrating how composite GUTs may represent an alternative to supersymmetric
GUTs. Taking into account the new contributions to the EW precision parameters,
we compute the Higgs effective potential and prove that it realizes
consistently EW symmetry breaking with little fine-tuning. The G group
structure and the requirement of proton stability determine the nature of the
light composite states accompanying the Higgs and the top quark: a coloured
triplet scalar and several vector-like fermions with exotic quantum numbers. We
analyse the signatures of these composite partners at hadron colliders:
distinctive final states contain multiple top and bottom quarks, either alone
or accompanied by a heavy stable charged particle, or by missing transverse
energy.Comment: 55 pages, 13 figures, final version to be published in JHE
Proton lifetime from SU(5) unification in extra dimensions
We provide detailed estimates of the proton lifetime in the context of simple supersymmetric SU(5) grand unified models with an extra compact spatial dimension, described by the orbifold S1/(Z2 × Z2') and by a large compactification scale Mc ≈ 1014÷1016 GeV. We focus on a class of models where the grand unified symmetry is broken by the compactification mechanism and where baryon violation proceeds mainly through gauge vector boson exchange so that the proton lifetime scales as Mc4. We carefully compute Mc from a next-to-leading analysis of gauge coupling unification and we find that Mc can only be predicted up to an overall factor 10±1. The simplest model, where the dominant decay mode is π0e+ and has no flavour suppression, is strongly constrained by existing data, but not totally ruled out. We also analyze models where some of the matter fields are localized in the extra space and proton decay is flavour suppressed. In models associated to anarchy in the neutrino sector the preferred decay channel is K+ and the lifetime can be within the reach of the next generation of experiments