343 research outputs found
Curing the Ills of Higgsless Models: the S Parameter and Unitarity
We consider various constraints on Higgsless models of electroweak symmetry
breaking based on a bulk SU(2)_L x SU(2)_R x U(1)_{B-L} gauge group in warped
space. First we show that the S parameter which is positive if fermions are
localized on the Planck brane can be lowered (or made vanishing) by changing
the localization of the light fermions. If the wave function of the light
fermions is almost flat their coupling to the gauge boson KK modes will be
close to vanishing, and therefore contributions to the S parameter will be
suppressed. At the same time the experimental bounds on such Z' and W' gauge
bosons become very weak, and their masses can be lowered to make sure that
perturbative unitarity is not violated in this theory before reaching energies
of several TeV. The biggest difficulty of these models is to incorporate a
heavy top quark mass without violating any of the experimental bounds on bottom
quark gauge couplings. In the simplest models of fermion masses a sufficiently
heavy top quark also implies an unacceptably large correction to the Zb\bar{b}
vertex and a large splitting between the KK modes of the top and bottom quarks,
yielding large loop corrections to the T-parameter. We present possible
directions for model building where perhaps these constraints could be obeyed
as well.Comment: 21 pages, LaTeX, 5 figures. References and acknowledgements adde
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
An action for the (2,0) self-dual tensor multiplet in a conformal supergravity background
We present the action for a self-dual tensor in six dimensions, coupled to a
(2,0) conformal supergravity background. This action gives rise to the expected
equations of motion. An alternative look upon one of the gauge symmetries
clarifies its role in the supersymmetry transformation rules and the
realisation of the algebra.Comment: 14 pages, accepted by Class. Quant. Gra
No-Go Theorem for Horizon-Shielded Self-Tuning Singularities
We derive a simple no-go theorem relating to self-tuning solutions to the
cosmological constant for observers on a brane, which rely on a singularity in
an extra dimension. The theorem shows that it is impossible to shield the
singularity from the brane by a horizon, unless the positive energy condition
(rho+p >= 0) is violated in the bulk or on the brane. The result holds
regardless of the kinds of fields which are introduced in the bulk or on the
brane, whether Z_2 symmetry is imposed at the brane, or whether higher
derivative terms of the Gauss-Bonnet form are added to the gravitational part
of the action. However, the no-go theorem can be evaded if the three-brane has
spatial curvature. We discuss explicit realizations of such solutions which
have both self-tuning and a horizon shielding the singularity.Comment: 7 pages, 4 figures, revtex; added reference and minor correction
Higgsless Electroweak Theory following from the Spherical Geometry
A new formulation of the Electroweak Model with 3-dimensional spherical
geometry in the target space is suggested. The free Lagrangian in the spherical
field space along with the standard gauge field Lagrangian form the full
Higgsless Lagrangian of the model, whose second order terms reproduce the same
fields with the same masses as the Standard Electroweak Model. The vector
bosons and electron masses are generated automatically, so there is no need in
special mechanism.Comment: 6 page
Spacelike Branes
Scalar field theories with appropriate potentials in Minkowski space can have
time-dependent classical solutions containing topological defects which
correspond to S-branes - i.e. branes all of whose tangential dimensions are
spacelike. It is argued that such S-branes arise in string theory as
time-dependent solutions of the worldvolume tachyon field of an unstable
D-brane or D-brane-anti-D-brane pair. Using the known coupling of the spacetime
RR fields to the worldvolume tachyon it is shown that these S-branes carry a
charge, defined as the integral of a RR field strength over a sphere
(containing a time as well as spatial dimensions) surrounding the S-brane. This
same charge is carried by SD-branes, i.e. Dirichlet branes arising from open
string worldsheet conformal field theories with a Dirichlet boundary condition
on the timelike dimension. The corresponding SD-brane boundary state is
constructed. Supergravity solutions carrying the same charges are also found
for a few cases.Comment: 23 pages, harvmac(b), no figures, v2 references added and minor
changes, v4: more references adde
A High Luminosity e+e- Collider to study the Higgs Boson
A strong candidate for the Standard Model Scalar boson, H(126), has been
discovered by the Large Hadron Collider (LHC) experiments. In order to study
this fundamental particle with unprecedented precision, and to perform
precision tests of the closure of the Standard Model, we investigate the
possibilities offered by An e+e- storage ring collider. We use a design
inspired by the B-factories, taking into account the performance achieved at
LEP2, and imposing a synchrotron radiation power limit of 100 MW. At the most
relevant centre-of-mass energy of 240 GeV, near-constant luminosities of 10^34
cm^{-2}s^{-1} are possible in up to four collision points for a ring of 27km
circumference. The achievable luminosity increases with the bending radius, and
for 80km circumference, a luminosity of 5 10^34 cm^{-2}s^{-1} in four collision
points appears feasible. Beamstrahlung becomes relevant at these high
luminosities, leading to a design requirement of large momentum acceptance both
in the accelerating system and in the optics. The larger machine could reach
the top quark threshold, would yield luminosities per interaction point of
10^36 cm^{-2}s^{-1} at the Z pole (91 GeV) and 2 10^35 cm^{-2}s^{-1} at the W
pair production threshold (80 GeV per beam). The energy spread is reduced in
the larger ring with respect to what is was at LEP, giving confidence that beam
polarization for energy calibration purposes should be available up to the W
pair threshold. The capabilities in term of physics performance are outlined.Comment: Submitted to the European Strategy Preparatory Group 01-04-2013 new
version as re-submitted to PRSTA
BPS pp-wave brane cosmological solutions in string theory
We construct time dependent BPS pp-wave brane solutions in the context of
M-theory and type II supergravity. It is found that N-brane solutions we
considered satisfy the crossing rule as S-brane solutions but 1/8 supersymmetry
remains. By applying them to the cosmological setting, inflationary solutions
are obtained. During this inflation, the size of the extradimensions becomes
smaller than our four-dimensional spacetime dynamically. We also discuss the
mechanism for terminating this inflation and recovering the hot big-bang
universe.Comment: 10 page
Regular S-Brane Backgrounds
We construct time-dependent S-brane solutions to the supergravity field
equations in various dimensions which (unlike most such geometries) do not
contain curvature singularities. The configurations we consider are less
symmetric than are earlier solutions, with our simplest solution being obtained
by a simple analytical continuation of the Kerr geometry. We discuss in detail
the global structure and properties of this background. We then generalize it
to higher dimensions and to include more complicated field configurations -
like non vanishing scalars and antisymmetric tensor gauge potentials - by the
usual artifice of applying duality symmetries.Comment: 22 pages, 3 figures. Typos in eq.(2.6) correcte
4D Constructions of Supersymmetric Extra Dimensions and Gaugino Mediation
We present 4D gauge theories which at low energies coincide with higher
dimensional supersymmetric (SUSY) gauge theories on a transverse lattice. We
show that in the simplest case of pure 5D SUSY Yang-Mills there is an
enhancement of SUSY in the continuum limit without fine-tuning. This result no
longer holds in the presence of matter fields, in which case fine-tuning is
necessary to ensure higher dimensional Lorentz invariance and supersymmetry. We
use this construction to generate 4D models which mimic gaugino mediation of
SUSY breaking. The way supersymmetry breaking is mediated in these models to
the MSSM is by assuming that the physical gauginos are a mixture of a number of
gauge eigenstate gauginos: one of these couples to the SUSY breaking sector,
while another couples to the MSSM matter fields. The lattice can be as coarse
as just two gauge groups while still obtaining the characteristic
gaugino-mediated soft breaking terms.Comment: 32 pages LaTeX; missing factor in two-loop gauge-mediated scalar mass
estimate fixed, comments on unification revise
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