727 research outputs found
Axigluons cannot explain the observed top quark forward-backward asymmetry
We study an SU(3)^2 axigluon model introduced by Frampton, Shu, and Wang to
explain the recent Fermilab Tevatron observation of a significant positive
enhancement in the top quark forward-backward asymmetry relative to standard
model predictions. First, we demonstrate that data on neutral B_d-meson mixing
excludes the region of model parameter space where the top asymmetry is
predicted to be the largest. Keeping the gauge couplings below the critical
value that would lead to fermion condensation imposes further limits at large
axigluon mass, while precision electroweak constraints on the model are
relatively mild. Furthermore, by considering an extension to an SU(3)^3 color
group, we demonstrate that embedding the model in an extra-dimensional
framework can only dilute the axigluon effect on the forward-backward
asymmetry. We conclude that axigluon models are unlikely to be the source of
the observed top quark asymmetry.Comment: 12 pages, 7 eps figures included. Minor changes to conform with
published versio
Gap Equations and Electroweak Symmetry Breaking
Recently a new dynamical symmetry breaking model of electroweak interactions
was proposed based on interacting fermions. Two fermions of different SU(2)
representations form a symmetry breaking condensate and generate the lepton and
quark masses. The weak gauge bosons get their usual standard model masses from
a gauge invariant Lagrangian of a composite doublet scalar field. The new
fermion fields become massive by condensation. In this note the gap equations
are given in the linearized (mean field) approximation and the conditions for
symmetry breaking and mass generation are presented. Perturbative unitarity
constrains the self-couplings and the masses of the new fermions, a raw
spectrum is given.Comment: 10 pages, 4 figure
Constraints and Hamiltonian in Light-Front Quantized Field Theory
Self-consistent Hamiltonian formulation of scalar theory on the null plane is
constructed following Dirac method. The theory contains also {\it constraint
equations}. They would give, if solved, to a nonlinear and nonlocal
Hamiltonian. The constraints lead us in the continuum to a different
description of spontaneous symmetry breaking since, the symmetry generators now
annihilate the vacuum. In two examples where the procedure lacks
self-consistency, the corresponding theories are known ill-defined from
equal-time quantization. This lends support to the method adopted where both
the background field and the fluctuation above it are treated as dynamical
variables on the null plane. We let the self-consistency of the Dirac procedure
determine their properties in the quantized theory. The results following from
the continuum and the discretized formulations in the infinite volume limit do
agree.Comment: 11 pages, Padova University preprint DFPF/92/TH/52 (December '92
Gauged Nambu-Jona-Lasinio model with extra dimensions
We investigate phase structure of the D (> 4)-dimensional gauged
Nambu-Jona-Lasinio (NJL) model with extra dimensions
compactified on TeV scale, based on the improved ladder Schwinger-Dyson (SD)
equation in the bulk. We assume that the bulk running gauge coupling in the SD
equation for the SU(N_c) gauge theory with N_f massless flavors is given by the
truncated Kaluza-Klein effective theory and hence has a nontrivial ultraviolet
fixed point (UVFP). We find the critical line in the parameter space of two
couplings, the gauge coupling and the four-fermion coupling, which is similar
to that of the gauged NJL model with fixed (walking) gauge coupling in four
dimensions. It is shown that in the presence of such walking gauge interactions
the four-fermion interactions become ``nontrivial'' even in higher dimensions,
similarly to the four-dimensional gauged NJL model. Such a nontriviality holds
only in the restricted region of the critical line (``nontrivial window'') with
the gauge coupling larger than a non-vanishing value (``marginal triviality
(MT)'' point), in contrast to the four-dimensional case where such a
nontriviality holds for all regions of the critical line except for the pure
NJL point. In the nontrivial window the renormalized effective potential yields
a nontrivial interaction which is conformal invariant. The exisitence of the
nontrivial window implies ``cutoff insensitivity'' of the physics prediction in
spite of the ultraviolet dominance of the dynamics. In the formal limit D -> 4,
the nontrivial window coincides with the known condition of the nontriviality
of the four-dimensional gauged NJL model, .Comment: 34 pages, 6 figures, references added, to appear in Phys.Rev.D. The
title is changed in PR
Z Boson Propagator Correction in Technicolor Theories with ETC Effects Included
We calculate the Z boson propagator correction, as described by the S
parameter, in technicolor theories with extended technicolor interactions
included. Our method is to solve the Bethe-Salpeter equation for the requisite
current-current correlation functions. Our results suggest that the inclusion
of extended technicolor interactions has a relatively small effect on S.Comment: 15pages, 8 figure
ACD estimation of the S parameter revisited
The analytic continuation by duality (ACD) technique has been used to
estimate the electroweak S parameter in technicolor models. In this letter, we
investigate the reliability of this method by applying it to some toy models
with known spectra. We find that in most instances the technique cannot be
trusted to give a reliable result.Comment: 15 pages, LaTeX, 2 postscript figures. Uses seceqn.sty and epsfig.st
Revealing Short-period Exoplanets and Brown Dwarfs in the Galactic Bulge using the Microlensing Xallarap Effect with the \textit{Nancy Grace Roman Space Telescope}
The \textit{Nancy Grace Roman Space Telescope} (\textit{ Roman}) will provide
an enormous number of microlensing light curves with much better photometric
precisions than ongoing ground-based observations. Such light curves will
enable us to observe high-order microlensing effects which have been previously
difficult to detect. In this paper, we investigate \textit{Roman}'s potential
to detect and characterize short-period planets and brown dwarfs (BDs) in
source systems using the orbital motion of source stars, the so-called xallarap
effect. We analytically estimate the measurement uncertainties of xallarap
parameters using the Fisher matrix analysis. We show that the \textit{Roman}
Galactic Exoplanet Survey (RGES) can detect warm Jupiters with masses down to
0.5 and orbital period of 30 days via the xallarap effect.
Assuming a planetary frequency function from \citet{Cumming+2008}, we find
\textit{Roman} will detect hot and warm Jupiters and close-in
BDs around microlensed source stars during the microlensing survey. These
detections are likely to be accompanied by the measurements of the companion's
masses and orbital elements, which will aid in the study of the physical
properties for close-in planet and BD populations in the Galactic bulge.Comment: Accepted for publication in The Astronomical Journa
The Zero Temperature Chiral Phase Transition in SU(N) Gauge Theories
We investigate the zero temperature chiral phase transition in an SU(N) gauge
theory as the number of fermions is varied. We argue that there exists a
critical number of fermions , above which there is no chiral symmetry
breaking or confinement, and below which both chiral symmetry breaking and
confinement set in. We estimate and discuss the nature of the phase
transition.Comment: 13 pages, LaTeX, version published in PR
Narrow Technihadron Production at the First Muon Collider
In modern technicolor models, there exist very narrow spin-zero and spin-one
neutral technihadrons---, and ---with masses of a
few 100 GeV. The large coupling of to , the direct
coupling of and to the photon and , and the superb
energy resolution of the First Muon Collider may make it possible to resolve
these technihadrons and produce them at extraordinarily large rates.Comment: 11 pages, latex, including 2 postscript figure
A Comment on the Zero Temperature Chiral Phase Transition in Gauge Theories
Recently Appelquist, Terning, and Wijewardhana investigated the zero
temperature chiral phase transition in SU(N) gauge theory as the number of
fermions N_f is varied. They argued that there is a critical number of fermions
N^c_f, above which there is no chiral symmetry breaking and below which chiral
symmetry breaking and confinement set in. They further argued that that the
transition is not second order even though the order parameter for chiral
symmetry breaking vanishes continuously as N_f approaches N^c_f on the broken
side. In this note I propose a simple physical picture for the spectrum of
states as N_f approaches N^c_f from below (i.e. on the broken side) and argue
that this picture predicts very different and non-universal behavior than is
the case in an ordinary second order phase transition. In this way the
transition can be continuous without behaving conventionally. I further argue
that this feature results from the (presumed) existence of an infrared
Banks-Zaks fixed point of the gauge coupling in the neighborhood of the chiral
transition and therefore depends on the long-distance nature of the non-abelian
gauge force.Comment: 7 pages, 2 figure
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