220 research outputs found
Dissecting the Wjj Anomaly: Diagnostic Tests of a Leptophobic Z'
We examine the scenario where a leptophobic Z' boson accounts for the excess
of events in the Wjj channel as observed by CDF. We assume generation
independent couplings for the Z' and obtain allowed regions for the four
hadronic couplings using the cross section range quoted by CDF as well as
constraints from dijet production at UA2. These coupling regions translate into
well-determined rates for the associated production of Z/\gamma+Z' at the
Tevatron and LHC, as well as W+Z' at the LHC,that are directly correlated with
the Wjj rate observed at the Tevatron. The Wjj rate at the LHC is large and
this channel should be observed soon once the SM backgrounds are under control.
The rates for Z/\gamma+Z' associated production are smaller, and these
processes should not yet have been observed at the Tevatron given the expected
SM backgrounds. In addition, we also show that valuable coupling information is
obtainable from the distributions of other kinematic variables, e.g., M_{WZ'},
p_T^W, and \cos \theta_W^*. Once detected, these associated production
processes and the corresponding kinematic distributions examined here will
provide further valuable information on the Z' boson couplings.Comment: 20 pages, 10 figures. Analysis and discussion update
Large Extra Dimensions at Linear Colliders
In this talk, I first present the motivation for theories wherein extra
spacetime dimensions can be compactified to have large magnitudes. In
particular, I discuss the Arkani-Hamed, Dimopoulos, Dvali (ADD) scenario. I
present the constraints that have been derived on these models from current
experiments and the expectations from future colliders. I concentrate
particularly on the possibilities of probing these extra dimensions at future
linear colliders.Comment: Talk given at the Third International Workshop on Electron-Electron
Interactions at TeV Energies (e- e- 99), Santa Cruz, California, 10-12 Dec
1999. 7 pages, LaTeX, style files attache
Bulk Gauge Fields in the Randall-Sundrum Model
We explore the consequences of placing the Standard Model gauge fields in the
bulk of the recently proposed localized gravity model of Randall and Sundrum.
We find that the Kaluza Klein excitations of these fields are necessarily
strongly coupled and we demonstrate that current precision electroweak data
constrain the lowest states to lie above TeV. Taking the weak scale
to be TeV, the resulting implications on the model parameters force
the bulk curvature, , to be larger than the higher dimensional Planck
scale, , violating the consistency of the theory. In turn, to preserve
|R_5|\lsim M^2, the weak scale must be pushed to \gsim 100 TeV. Hence we
conclude that it is disfavored to place the Standard Model gauge fields in the
bulk of this model as it is presently formulated.Comment: Improved results, 14 pages, Latex fil
Bounds on Dark Matter Interactions with Electroweak Gauge Bosons
We investigate scenarios in which dark matter interacts with the Standard
Model primarily through electroweak gauge bosons. We employ an effective field
theory framework wherein the Standard Model and the dark matter particle are
the only light states in order to derive model-independent bounds. Bounds on
such interactions are derived from dark matter production by weak boson fusion
at the LHC, indirect detection searches for the products of dark matter
annihilation and from the measured invisible width of the . We find that
limits on the UV scale, , reach weak scale values for most operators
and values of the dark matter mass, thus probing the most natural scenarios in
the WIMP dark matter paradigm. Our bounds suggest that light dark matter
(m_{\chi}\lsim m_Z/2 or m_{\chi}\lsim 100-200\gev, depending on the
operator) cannot interact only with the electroweak gauge bosons of the
Standard Model, but rather requires additional operator contributions or dark
sector structure to avoid overclosing the universe.Comment: 45 pages, 26 figure
Electroweak precision constraints on the Lee-Wick Standard Model
We perform an analysis of the electroweak precision observables in the
Lee-Wick Standard Model. The most stringent restrictions come from the S and T
parameters that receive important tree level and one loop contributions. In
general the model predicts a large positive S and a negative T. To reproduce
the electroweak data, if all the Lee-Wick masses are of the same order, the
Lee-Wick scale is of order 5 TeV. We show that it is possible to find some
regions in the parameter space with a fermionic state as light as 2.4-3.5 TeV,
at the price of rising all the other masses to be larger than 5-8 TeV. To
obtain a light Higgs with such heavy resonances a fine-tuning of order a few
per cent, at least, is needed. We also propose a simple extension of the model
including a fourth generation of Standard Model fermions with their Lee-Wick
partners. We show that in this case it is possible to pass the electroweak
constraints with Lee-Wick fermionic masses of order 0.4-1.5 TeV and Lee-Wick
gauge masses of order 3 TeV.Comment: 24 pages, 7 figure
Higgs Properties in the Fourth Generation MSSM: Boosted Signals Over the 3G Plan
The generalization of the MSSM to the case of four chiral fermion generations
(4GMSSM) can lead to significant changes in the phenomenology of the otherwise
familiar Higgs sector. In most of the 3GMSSM parameter space, the lighter
CP-even is GeV and mostly Standard Model-like while
are all relatively heavy. Furthermore, the ratio of Higgs vevs,
, is relatively unconstrained. In contrast to this, in the 4GMSSM,
heavy fourth generation fermion loops drive the masses of to large
values while the CP-odd boson, , can remain relatively light and is restricted to the range 1/2 \lsim \tan \beta \lsim 2 due to
perturbativity requirements on Yukawa couplings. We explore this scenario in
some detail, concentrating on the collider signatures of the light CP-odd Higgs
at both the Tevatron and LHC. We find that while may lead to a
potential signal in the channel at the LHC, may first be
observed in the channel due to a highly loop-enhanced cross
section that can be more than an order of magnitude greater than that of a SM
Higgs for masses of and . We find that the
CP-even states are highly mixed and can have atypical branching
fractions. Precision electroweak constraints, particularly for the light
parameter space region, are examined in detail.Comment: 20 pages, 7 figures; typos fixed, refs adde
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
Two Graviton Production at and Hadron Hadron Colliders in the Randall-Sundrum Model
We compute the pair production cross section of two Kaluza Klein modes in the
Randall-Sundrum model at and hadron hadron colliders. These processes
are interesting because they get dominant contribution from the graviton
interaction at next to leading order. Hence they provide a nontrivial test of
the low scale gravity models. All the Feynman rules at next to leading order
are also presented. These rules may be useful for many phenomenological
applications including the computation of higher order loop corrections.Comment: 24 pages, 11 figures, some typos correcte
Searching for Lee-Wick Gauge Bosons at the LHC
In an extension of the Standard Model(SM) based on the ideas of Lee and Wick,
Grinstein, O'Connell and Wise have found an interesting way to remove the usual
quadratically divergent contributions to the Higgs mass induced by radiative
corrections. Phenomenologically, the model predicts the existence of Terascale,
negative-norm copies of the usual SM fields with rather unique properties:
ghost-like propagators and negative decay widths, but with otherwise SM-like
couplings. The model is both unitary and causal on macroscopic scales. In this
paper we examine whether or not such states with these unusual properties can
be uniquely identified as such at the LHC. We find that in the extended strong
and electroweak gauge boson sector of the model, which is the simplest one to
analyze, such an identification can be rather difficult. Observation of heavy
gluon-like resonances in the dijet channel offers the best hope for this
identification.Comment: 17 pages, 4 figs; discussion adde
Collider Production of TeV Scale Black Holes and Higher-Curvature Gravity
We examine how the production of TeV scale black holes at colliders is
influenced by the presence of Lovelock higher-curvature terms in the action of
models with large extra dimensions. Such terms are expected to arise on rather
general grounds, e.g., from string theory and are often used in the literature
to model modifications to the Einstein-Hilbert action arising from quantum
and/or stringy corrections. While adding the invariant which is quadratic in
the curvature leads to quantitative modifications in black hole properties,
cubic and higher invariants are found to produce significant qualitative
changes, e.g., classically stable black holes. We use these higher-order
curvature terms to construct a toy model of the black hole production cross
section threshold. For reasonable parameter values we demonstrate that detailed
measurements of the properties of black holes at future colliders will be
highly sensitive to the presence of the Lovelock higher-order curvature terms.Comment: 37 pages, 11 figures, references adde
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