1,720 research outputs found
Higgsless Electroweak Symmetry Breaking in Warped Backgrounds: Constraints and Signatures
We examine the phenomenology of a warped 5-dimensional model based on
SU(2) SU(2) U(1) model which implements
electroweak symmetry breaking through boundary conditions, without the presence
of a Higgs boson. We use precision electroweak data to constrain the general
parameter space of this model. Our analysis includes independent and
gauge couplings, radiatively induced UV boundary gauge kinetic terms, and all
higher order corrections from the curvature of the 5-d space. We show that this
setup can be brought into good agreement with the precision electroweak data
for typical values of the parameters. However, we find that the entire range of
model parameters leads to violation of perturbative unitarity in gauge boson
scattering and hence this model is not a reliable perturbative framework.
Assuming that unitarity can be restored in a modified version of this scenario,
we consider the collider signatures. It is found that new spin-1 states will be
observed at the LHC and measurement of their properties would identify this
model. However, the spin-2 graviton Kaluza-Klein resonances, which are a
hallmark of the Randall-Sundrum model, are too weakly coupled to be detected.Comment: More detailed analysis, added references, 43 pages, 15 figures, LaTe
Warped Higgsless Models with IR--Brane Kinetic Terms
We examine a warped Higgsless model
in 5-- with IR(TeV)--brane kinetic terms. It is shown that adding a brane
term for the gauge field does not affect the scale (
TeV) where perturbative unitarity in is violated.
This term could, however, enhance the agreement of the model with the precision
electroweak data. In contrast, the inclusion of a kinetic term corresponding to
the custodial symmetry of the theory delays the unitarity violation
in scattering to energy scales of TeV for a significant
fraction of the parameter space. This is about a factor of 4 improvement
compared to the corresponding scale of unitarity violation in the Standard
Model without a Higgs. We also show that null searches for extra gauge bosons
at the Tevatron and for contact interactions at LEP II place non-trivial bounds
on the size of the IR-brane terms.Comment: 23 pages, 8 figure
Treatment of a Giant Haemangioma of the Liver With Kasabach-Merritt Syndrome by Orthotopic Liver Transplant
We describe a case of giant cavernous haemangioma of the liver with disseminated intravascular coagulopathy (Kasabach-Merritt syndrome) which was cured by orthotopic liver transplant
Low-scale warped extra dimension and its predilection for multiple top quarks
Within warped extra dimension models that explain flavor through geometry,
flavor changing neutral current constraints generally force the Kaluza-Klein
scale to be above many TeV. This creates tension with a natural electroweak
scale. On the other hand, a much lower scale compatible with precision
electroweak and flavor changing neutral current constraints is allowed if we
decouple the Kaluza-Klein states of Standard Model gauge bosons from light
fermions bulk mass parameters). The main
signature for this approach is four top quark production via the Kaluza-Klein
excitations' strong coupling to top quarks. We study single lepton, like-sign
dilepton, and trilepton observables of four-top events at the Large Hadron
Collider. The like-sign dilepton signature typically has the largest discovery
potential for a strongly coupled right-handed top case (M_{KK} \sim 2-2.5
\TeV), while single lepton is the better when the left-handed top couples most
strongly (M_{KK} \sim 2 \TeV). We also describe challenging lepton-jet
collimation issues in the like-sign dilepton and trilepton channels. An
alternative single lepton observable is considered which takes advantage of the
many bottom quarks in the final state. Although searches of other particles may
compete, we find that four top production via Kaluza-Klein gluons is most
promising in a large region of this parameter space.Comment: 35 pages, 8 figures. discussions improved, references adde
Heavy-light decay topologies as a new strategy to discover a heavy gluon
We study the collider phenomenology of the lightest Kaluza-Klein excitation
of the gluon, G*, in theories with a warped extra dimension. We do so by means
of a two-site effective lagrangian which includes only the lowest-lying spin-1
and spin-1/2 resonances. We point out the importance of the decays of G* to one
SM plus one heavy fermion, that were overlooked in the previous literature. It
turns out that, when kinematically allowed, such heavy-light decays are
powerful channels for discovering the G*. In particular, we present a
parton-level Montecarlo analysis of the final state Wtb that follows from the
decay of G* to one SM top or bottom quark plus its heavy partner. We find that
at \sqrt{s} = 7 TeV and with 10 fb^{-1} of integrated luminosity, the LHC can
discover a KK gluon with mass in the range M_{G*} = (1.8 - 2.2) TeV if its
coupling to a pair of light quarks is g_{G*qqbar} = (0.2-0.5) g_3. The same
process is also competitive for the discovery of the top and bottom partners as
well. We find, for example, that the LHC at \sqrt{s} = 7 TeV can discover a 1
TeV KK bottom quark with an integrated luminosity of (5.3 - 0.61) fb^{-1} for
g_{G*qqbar} = (0.2-0.5) g_3.Comment: 36 pages, 13 figures. v2: a few typos corrected, comments added,
version published in JHE
Monte Carlo Exploration of Warped Higgsless Models
We have performed a detailed Monte Carlo exploration of the parameter space
for a warped Higgsless model of electroweak symmetry breaking in 5 dimensions.
This model is based on the gauge group
in an AdS bulk with arbitrary gauge kinetic terms on both the Planck and
TeV branes. Constraints arising from precision electroweak measurements and
collider data are found to be relatively easy to satisfy. We show, however,
that the additional requirement of perturbative unitarity up to the cut-off,
TeV, in elastic scattering in the absence of dangerous
tachyons eliminates all models. If successful models of this class exist, they
must be highly fine-tuned.Comment: 26 pages, 7 figures; new fig and additional text adde
Theories for the Fermi Scale
I give a short review of our present understanding of new theories of the
electroweak scale, with emphasis on recent progress. Plenary talk at the EPS
2007 Conference at Manchester.Comment: 14 pages, EPS 2007 Conferenc
KK Parity in Warped Extra Dimension
We construct models with a Kaluza-Klein (KK) parity in a five- dimensional
warped geometry, in an attempt to address the little hierarchy problem present
in setups with bulk Standard Model fields. The lightest KK particle (LKP) is
stable and can play the role of dark matter. We consider the possibilities of
gluing two identical slices of 5D AdS in either the UV (IR-UV-IR model) or the
IR region (UV-IR-UV model) and discuss the model-building issues as well as
phenomenological properties in both cases. In particular, we find that the
UV-IR-UV model is not gravitationally stable and that additional mechanisms
might be required in the IR-UV-IR model in order to address flavor issues.
Collider signals of the warped KK parity are different from either the
conventional warped extra dimension without KK parity, in which the new
particles are not necessarily pair-produced, or the KK parity in flat universal
extra dimensions, where each KK level is nearly degenerate in mass. Dark matter
and collider properties of a TeV mass KK Z gauge boson as the LKP are
discussed.Comment: 35 pages, 11 figure
Search for light custodians in a clean decay channel at the LHC
Models of warped extra dimensions with custodial symmetry usually predict the
existence of a light Kaluza-Klein fermion arising as a partner of the
right-handed top quark, sometimes called light custodians which we will denote
. The production of these particles at the LHC can give rise to
multi-W events which could be observed in same-sign dilepton channels, but its
mass reconstruction is challenging. In this letter we study the possibility of
finding a signal for the pair production of this new particle at the LHC
focusing on a rarer, but cleaner decay mode of a light custodian into a
boson and a -quark. In this mode it would be possible to reconstruct the
light custodian mass.
In addition to the dominant standard model QCD production processes, we
include the contribution of a Kaluza-Klein gluon first mode. We find that the
stands out from the background as a peak in the invariant
mass. However, when taking into account only the electronic and muonic decay
modes of the boson and tagging efficiencies, the LHC will have access
only to the very light range of masses, GeV.Comment: 8 pages, 2 figure
Electron-multiplying CCDs for future soft X-ray spectrometers
CCDs have been used in several high resolution soft X-ray spectrometers for both space and terrestrial applications such as the Reflection Grating Spectrometer on XMM-Newton and the Super Advanced X-ray Emission Spectrometer at the Paul Scherrer Institut in Switzerland. However, with their ability to use multiplication gain to amplify signal and suppress readout noise, EM-CCDs are being considered instead of CCDs for future soft X-ray spectrometers. When detecting low energy X-rays, EM-CCDs are able to increase the Signal-to-Noise ratio of the device, making the X-rays much easier to detect. If the signal is also significantly split between neighbouring pixels, the increase in the size of the signal will make complete charge collection and techniques such as centroiding easier to accomplish. However, multiplication gain from an EM-CCD does cause a degradation of the energy resolution of the device and there are questions about how the high field region in an EM-CCD will behave over time in high radiation environments. This paper analyses the possible advantages and disadvantages of using EM-CCDs for high resolution soft X-ray spectroscopy and suggests in which situations using them would not only be possible, but also beneficial to the instrument
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