228 research outputs found
The littlest Higgs is a cruiserweight
We study the exact (one-loop) effective potential of the littlest Higgs model
and determine the dependence of physical quantities, such as the vacuum
expectation value v_W and mass m_h of the Higgs boson, on the fundamental
parameters of the Lagrangian--masses, couplings of new states, the fundamental
scale f of the sigma model, and the coefficients of operators quadratically
sensitive to the cutoff of the theory. On the one hand, we show that it is
possible to have the electroweak ground state and a relatively large cutoff
\Lambda = 4\pi f with f in the 2 TeV range without requiring unnaturally small
coefficients for quadratically divergent quantities, and with only moderate
cancellations between the contribution of different sectors to the effective
potential of the Higgs. On the other hand, this cannot be achieved while at the
same time keeping m_h close to its (bantamweight) current lower bound of 114.4
GeV. The natural expectation for m_h is O(f), mainly because of large
logarithmically divergent contributions to the effective potential of the
top-quark sector. Even a fine-tuning at the level of O(10^{-2}) in the
coefficients of the quadratic divergences is not enough to produce small
physical Higgs masses, and the natural expectation is in the 800 GeV range
(cruiserweight) for f \sim 2 TeV. We conclude that the littlest Higgs model is
a solution of the little hierarchy problem, in the sense that it stabilizes the
electroweak symmetry breaking scale to be a factor of 100 less than the cutoff
of the theory, but this requires a quite large physical mass for the Higgs, and
hence precision electroweak studies should be redone accordingly. We also study
finite temperature corrections.Comment: 18 pages, 9 figures, RevTex4. Final version accepted for publication
in Phys. Rev.
The S-parameter in Holographic Technicolor Models
We study the S parameter, considering especially its sign, in models of
electroweak symmetry breaking (EWSB) in extra dimensions, with fermions
localized near the UV brane. Such models are conjectured to be dual to 4D
strong dynamics triggering EWSB. The motivation for such a study is that a
negative value of S can significantly ameliorate the constraints from
electroweak precision data on these models, allowing lower mass scales (TeV or
below) for the new particles and leading to easier discovery at the LHC. We
first extend an earlier proof of S>0 for EWSB by boundary conditions in
arbitrary metric to the case of general kinetic functions for the gauge fields
or arbitrary kinetic mixing. We then consider EWSB in the bulk by a Higgs VEV
showing that S is positive for arbitrary metric and Higgs profile, assuming
that the effects from higher-dimensional operators in the 5D theory are
sub-leading and can therefore be neglected. For the specific case of AdS_5 with
a power law Higgs profile, we also show that S ~ + O(1), including effects of
possible kinetic mixing from higher-dimensional operator (of NDA size) in the
theory. Therefore, our work strongly suggests that S is positive in
calculable models in extra dimensions.Comment: 21 pages, 2 figures. v2: references adde
Holographic Technidilaton and LHC searches
We analyze in detail the phenomenology of a model of dynamical electroweak
symmetry breaking inspired by walking technicolor, by using the techniques of
the bottom-up approach to holography. The model admits a light composite scalar
state, the dilaton, in the spectrum. We focus on regions of parameter space for
which the mass of such dilaton is 125 GeV, and for which the bounds on the
precision electroweak parameter S are satisfied. This requires that the
next-to-lightest composite state is the techni-rho meson, with a mass larger
than 2.3 TeV. We compute the couplings controlling the decay rates of the
dilaton to two photons and to two (real or virtual) Z and W bosons. For generic
choices of the parameters, we find a suppression of the decay into heavy gauge
bosons, in respect to the analog decay of the standard-model Higgs. We find a
dramatic effect on the decay into photons, which can be both strongly
suppressed or strongly enhanced, the latter case corresponding to the large-N
regime of the dual theory. There is a correlation between this decay rate of
the dilaton into photons and the mass splitting between the techni-rho meson
and its axial-vector partner: if the decay is enhanced in respect to the
standard-model case, then the heavy spin-1 resonances are nearly degenerate in
mass, otherwise their separation in mass is comparable to the mass scale
itself.Comment: Very minor typos corrected. References adde
Wilson Loops in string duals of Walking and Flavored Systems
We consider the VEV of Wilson loop operators by studying the behavior of
string probes in solutions of Type IIB string theory generated by Nc D5 branes
wrapped on an internal manifold. In particular, we focus on solutions to the
background equations that are dual to field theories with a walking gauge
coupling as well as for flavored systems. We present in detail our walking
solution and emphasize various general aspects of the procedure to study Wilson
loops using string duals. We discuss the special features that the strings show
when probing the region associated with the walking of the field theory
coupling.Comment: 28 pages. Various figures. Version to be published
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