31 research outputs found
Divergences and Symmetries in Higgs-Gauge Unification Theories
In theories with extra dimensions the Standard Model Higgs field can be
identified with the internal components of higher-dimensional gauge fields
(Higgs-gauge unification). The higher-dimensional gauge symmetry prevents the
Higgs mass from quadratic divergences, but at the fixed points of the orbifold
this symmetry is broken and divergences can arise if U(1) subgroups are
conserved. We show that another symmetry, remnant of the internal rotation
group after orbifold projection, can avoid the generation of such divergences.Comment: 6 pages, 1 figure. Talk given at the "XL Rencontres de Moriond on
Electroweak Interactions and Unified Theories", La Thuile, Italy, 5-12 Mar
200
Minimal muon anomalous magnetic moment
We classify all possible one-particle (scalar and fermion) extensions of the
Standard Model that can contribute to the anomalous magnetic moment of leptons.
We review the cases already discussed in the literature and complete the
picture by performing the calculation for a fermionic doublet with hypercharge
-3/2. We conclude that, out of the listed possibilities, only two scalar
leptoquarks and the pseudoscalar of a peculiar two-Higgs-doublet model could be
the responsibles for the muon anomalous magnetic moment discrepancy. Were this
the case, this particles could be seen in the next LHC run. To this aim,
especially to test the leptoquark hypothesis, we suggest to look for final
states with tops and muons.Comment: 15 pages, uses axodra
Massive vectors and loop observables: the case
We discuss the use of massive vectors for the interpretation of some recent
experimental anomalies, with special attention to the muon . We restrict
our discussion to the case where the massive vector is embedded into a
spontaneously broken gauge symmetry, so that the predictions are not affected
by the choice of an arbitrary energy cut-off. Extended gauge symmetries,
however, typically impose strong constraints on the mass of the new vector
boson and for the muon they basically rule out, barring the case of
abelian gauge extensions, the explanation of the discrepancy in terms of a
single vector extension of the standard model. We finally comment on the use of
massive vectors for -meson decay and di-photon anomalies.Comment: 25 pages, 1 figure. References added, to appear in JHE
Probing a slepton Higgs on all frontiers
We study several aspects of supersymmetric models with a symmetry
where the Higgs doublet is identified with the superpartner of a lepton. We
derive new, stronger bounds on the gaugino masses based on current
measurements, and also propose ways to probe the model up to scales of
at future colliders. Since the
symmetry cannot be exact, we analyze the effects of -symmetry
breaking on neutrino masses and proton decay. In particular, we find that
getting the neutrino mixing angles to agree with experiments in a minimal model
requires a UV cutoff for the theory at around .Comment: 33 pages, 5 figures; v2: added reference. Matches version published
in JHE
General bounds on non-standard neutrino interactions
We derive model-independent bounds on production and detection non-standard
neutrino interactions (NSI). We find that the constraints for NSI parameters
are around O(10^{-2}) to O(10^{-1}). Furthermore, we review and update the
constraints on matter NSI. We conclude that the bounds on production and
detection NSI are generally one order of magnitude stronger than their matter
counterparts.Comment: 18 pages, revtex4, 1 axodraw figure. Minor changes, matches published
versio
Global bounds on the Type-III Seesaw
We derive general bounds on the Type-III Seesaw parameters from a global fit to flavor and electroweak precision data. We explore and compare three Type-III Seesaw realizations: a general scenario, where an arbitrary number of heavy triplets is integrated out without any further assumption, and the more constrained cases in which only 3 or 2 (minimal scenario) additional heavy states are included. The latter assumption implies rather non-trivial correlations in the Yukawa flavor structure of the model so as to reproduce the neutrino masses and mixings as measured in neutrino oscillations experiments and thus qualitative differences can be found with the more general scenario. In particular, we find that, while the bounds on most elements of the dimension 6 operator coefficients are of order 10â4 for the general and 3-triplet cases, the 2-triplet scenario is more strongly constrained with bounds between 10â5 and 10â7 for the different flavours. We also discuss how these correlations affect the present CMS constraints on the Type-III Seesaw in the minimal 2-triplet scenario
Equivalent effective Lagrangians for Scherk-Schwarz compactifications
We discuss the general form of the mass terms that can appear in the
effective field theories of coordinate-dependent compactifications a la
Scherk-Schwarz. As an illustrative example, we consider an interacting
five-dimensional theory compactified on the orbifold S^1/Z_2, with a fermion
subject to twisted periodicity conditions. We show how the same physics can be
described by equivalent effective Lagrangians for periodic fields, related by
field redefinitions and differing only in the form of the five-dimensional mass
terms. In a suitable limit, these mass terms can be localized at the orbifold
fixed points. We also show how to reconstruct the twist parameter from any
given mass terms of the allowed form. Finally, after mentioning some possible
generalizations of our results, we re-discuss the example of brane-induced
supersymmetry breaking in five-dimensional Poincare' supergravity, and comment
on its relation with gaugino condensation in M-theory.Comment: 17 pages, 3 figures. Published versio
Loop bounds on non-standard neutrino interactions
We reconsider the bounds on non-standard neutrino interactions with matter
which can be derived by constraining the four-charged-lepton operators induced
at the loop level. We find that these bounds are model dependent. Naturalness
arguments can lead to much stronger constraints than those presented in
previous studies, while no completely model-independent bounds can be derived.
We will illustrate how large loop-contributions to four-charged-lepton
operators are induced within a particular model that realizes gauge invariant
non-standard interactions and discuss conditions to avoid these bounds. These
considerations mainly affect the constraint on the
non-standard coupling strength \eps_{e\mu}, which is lost. The only
model-independent constraints that can be derived are .
However, significant cancellations are required in order to saturate this
bound.Comment: Minor changes, version to be published in JHEP. 17 pages, 3 Axodraw
figures, REVTeX
Low energy effects of neutrino masses
While all models of Majorana neutrino masses lead to the same dimension five
effective operator, which does not conserve lepton number, the dimension six
operators induced at low energies conserve lepton number and differ depending
on the high energy model of new physics. We derive the low-energy dimension six
operators which are characteristic of generic Seesaw models, in which neutrino
masses result from the exchange of heavy fields which may be either fermionic
singlets, fermionic triplets or scalar triplets. The resulting operators may
lead to effects observable in the near future, if the coefficients of the
dimension five and six operators are decoupled along a certain pattern, which
turns out to be common to all models. The phenomenological consequences are
explored as well, including their contributions to and new
bounds on the Yukawa couplings for each model.Comment: modifications: couplings in appendix B, formulas (121)-(122) on rare
leptons decays (to match with published version) and consequently bounds in
table
Fermion Generations, Masses and Mixing Angles from Extra Dimensions
We discuss a toy model in six dimensions that predicts two fermion
generations, natural mass hierarchy and intergenerational mixing. Matter is
described by vector-like six dimensional fermions, one per each irreducible
standard model representation. Two fermion generations arise from the
compactification mechanism, through orbifold projection. They are localized in
different regions of the compact space by a six dimensional mass term. Flavour
symmetry is broken via Yukawa couplings, with a Higgs vacuum expectation value
not constant in the extra space. A hierarchical spectrum is obtained from order
one dimensionless parameters of the six dimensional theory. The Cabibbo angle
arises from the soft breaking of six dimensional parity symmetry. We also
briefly discuss how the present model could be extended to cover the realistic
case.Comment: 22 pages, 2 figures. References added, modified comment on neutrino
masses, published versio