3 research outputs found
The Little Hierarchy in Universal Extra Dimensions
In the standard model in universal extra dimensions (UED) the mass of the
Higgs field is driven to the cutoff of the higher-dimensional theory. This
re-introduces a small hierarchy since the compactification scale 1/R should not
be smaller than the weak scale. In this paper we study possible solutions to
this problem by considering five-dimensional theories where the Higgs field
potential vanishes at tree level due to a global symmetry. We consider two
avenues: a Little Higgs model and a Twin Higgs model. An obstacle for the
embedding of these four-dimensional models in five dimensions is that their
logarithmic sensitivity to the cutoff will result in linear divergences in the
higher dimensional theory. We show that, despite the increased cutoff
sensitivity of higher dimensional theories, it is possible to control the Higgs
mass in these two scenarios. For the Little Higgs model studied, the
phenomenology will be significantly different from the case of the standard
model in UED. This is due to the fact that the compactification scale
approximately coincides with the scale where the masses of the new states
appear. For the case of the Twin Higgs model, the compactification scale may be
considerably lower than the scale where the new states appear. If it is as low
as allowed by current limits, it would be possible to experimentally observe
the standard model Kaluza-Klein states as well as a new heavy quark. On the
other hand, if the compactification scale is higher, then the phenomenology at
colliders would coincide with the one for the standard model in UED.Comment: 25 pages, 2 figure
Rare K and B Decays in the Littlest Higgs Model without T-Parity
We analyze rare K and B decays in the Littlest Higgs (LH) model without
T-parity. We find that the final result for the Z^0-penguin contribution
contains a divergence that is generated by the one-loop radiative corrections
to the currents corresponding to the dynamically broken generators. Including
an estimate of these logarithmically enhanced terms, we calculate the branching
ratios for the decays K^+ -> pi^+ nu bar nu, K_L -> pi^0 nu bar nu, B_{s,d} ->
mu^+ mu^- and B -> X_{s,d} nu bar nu. We find that for the high energy scale
f=O(2-3) TeV, as required by the electroweak precision studies, the enhancement
of all branching ratios amounts to at most 15% over the SM values. On the
technical side we identify a number of errors in the existing Feynman rules in
the LH model without T-parity that could have some impact on other analyses
present in the literature. Calculating penguin and box diagrams in the unitary
gauge, we find divergences in both contributions that are cancelled in the sum
except for the divergence mentioned above.Comment: 39 pages, 8 figures, typos corrected, comment on (2.17) and (2.18)
added, references added, results unchange
Direct Detection of Dark Matter in Supersymmetric Models
We evaluate neutralino-nucleon scattering rates in several well-motivated
supersymmetric models, and compare against constraints on the neutralino relic
density, BF( b\to s\gamma ) as well as the muon anomalous magnetic moment a_\mu
. In the mSUGRA model, the indirect constraints favor the hyperbolic
branch/focus point (HB/FP) region of parameter space, and in fact this region
is just where neutralino-nucleon scattering rates are high enough to be
detected in direct dark matter search experiments! In Yukawa unified SUSY
SO(10) models with scalar mass non-universality, the relic density of
neutralinos is almost always above experimental bounds, while the corresponding
direct detection rates are below experimental levels. Conversely, in five
dimensional SO(10) models where gauge symmetry breaking is the result of
compactification of the extra dimension, and supersymmetry breaking is
communicated via gaugino mediation, the relic density is quite low, while
direct detection rates can be substantial.Comment: 25 page latex file including 18 EPS figures; revised version with
references added and cross sections rescaled; figures changed. A copy of the
paper with better resolution figures can be found at
http://www.hep.fsu.edu/~belyaev/projects/directz1