57,258 research outputs found
T-Parity Violation by Anomalies
Little Higgs theories often rely on an internal parity ("T-parity'') to
suppress non-standard electroweak effects or to provide a dark matter
candidate. We show that such a symmetry is generally broken by anomalies, as
described by the Wess-Zumino-Witten term. We study a simple SU(3) x SU(3)/SU(3)
Little Higgs scheme where we obtain a minimal form for the topological
interactions of a single Higgs field. The results apply to more general models,
including [SU(3) x SU(3)/SU(3)]^4, SU(5)/SO(5), and SU(6)/Sp(6).Comment: 17 page
Z -> b\bar{b} Versus Dynamical Electroweak Symmetry Breaking involving the Top Quark
In models of dynamical electroweak symmetry breaking which sensitively
involve the third generation, such as top quark condensation, the effects of
the new dynamics can show up experimentally in Z->b\bar{b}. We compare the
sensitivity of Z->b\bar{b} and top quark production at the Tevatron to models
of the new physics. Z->b\bar{b} is a relatively more sensitive probe to new
strongly coupled U(1) gauge bosons, while it is generally less sensitive a
probe to new physics involving color octet gauge bosons as is top quark
production itself. Nonetheless, to accomodate a significant excess in
Z->b\bar{b} requires choosing model parameters that may be ruled out within run
I(b) at the Tevatron.Comment: LaTex file, 19 pages + 2 Figs., Fermilab-Pub-94/231-
Chiral Hierarchies, Compositeness and the Renormalization Group
A wide class of models involve the fine--tuning of significant hierarchies
between a strong--coupling ``compositeness'' scale, and a low energy dynamical
symmetry breaking scale. We examine the issue of whether such hierarchies are
generally endangered by Coleman--Weinberg instabilities. A careful study using
perturbative two--loop renormalization group methods finds that consistent
large hierarchies are not generally disallowed.Comment: 22 pp + 5 figs (uuencoded and submitted separately),
SSCL-Preprint-490; FERMI-PUB-93/035-
Nickel layers on indium arsenide
We report here on the preparation and characterization of InAs substrates for in situ deposition of ferromagnetic contacts, a necessary precursor for semiconductor devices based on spin injection. InAs has been grown on InAs(111)A and (100) substrates by molecular-beam epitaxy and then metalized in situ in order to better understand the mechanisms that inhibit spin injection into a semiconductor. Initial x-ray characterization of the samples indicate the presence of nickel arsenides and indium–nickel compounds forming during deposition at temperatures above room temperature. Several temperature ranges have been investigated in order to determine the effect on nickel-arsenide formation. The presence of such compounds at the interface could greatly reduce the spin-injection efficiency and help elucidate previous unsuccessful attempts at measuring spin injection into InAs
Search for Narrow-Width ttbar Resonances in ppbar Collisions at center of mass energy = 1.8 TeV
We present a preliminary result on a search for narrow-width resonances that
decay into ttbar pairs using 130 pb^{-1} of lepton plus jets data in ppbar
collisions at center of mass energy = 1.8 TeV. No significant deviation from
Standard Model prediction is observed. 95% C.L. upper limits on the production
cross section of the narrow-width resonance times its branching fraction to
ttbar are presented for different resonance masses, M_X. We also exclude the
existence of a leptophobic topcolor particle, X, with M_X < 560 GeV/c^2 for a
width \Gamma_X = 0.012 M_X.Comment: 3 pages, 1 figure; Submitted for proceedings of 5th International
Conference on Quark Confinement and Hadron spectrum, held in Italy, from
11-14 Sep., 200
Adhesive for aluminum withstands cryogenic temperatures
Polyurethane adhesive mixed to various proportions with milled glass fibers match the thermal characteristics of 2014-T6 aluminum at cryogenic temperatures
Dynamics of Domain Walls for Split and Runaway Potentials
We demonstrate that the evolution of wall-like inhomogeneities in run-away
potentials, characteristic of dynamical supersymmetry breaking and moduli
stabilisation, is very similar to the evolution of domain wall networks
associated with double well potentials. Instabilities that would lead to a
rapid decay of domain walls can be significantly ameliorated by compensation
effects between a non-degeneracy of the vacua and a biased initial
distribution, which can be naturally expected in a wide class or particle
physics models that lead to out-of-equilibrium phase transitions. Within this
framework, it is possible to obtain domain walls that live long enough to be
relevant for the cosmic power spectrum and galaxy clustering, while being
compatible with the observed cosmic microwave background anisotropies.Comment: 30 pages, 9 figure
Scanning apertureless microscopy below the diffraction limit: Comparisons between theory and experiment
The exact nature of the signal in scanning apertureless microscopy techniques is the subject of much debate. We have sought to resolve this controversy by carrying out simulations and experiments on the same structures. Simulations of a model of tip–sample coupling are shown to exhibit features that are in agreement with experimental observations at dimensions below the diffraction limit. The simulation of the optical imaging process is carried out using atomic force microscope data as a topographical template and a tip–sample dipole coupling model as the source of optical signal. The simulations show a number of key fingerprints including a dependence on the polarization of the external laser source, the size of the tip, and index of refraction of the sample being imaged. The experimental results are found to be in agreement with many of the features of the simulations. We conclude that the results of the dipole coupling theory agree qualitatively with experimental data and that apertureless microscopy measures optical properties, not just topography
Tunnel switch diode based on AlSb/GaSb heterojunctions
We report on tunnel switch diodes based on AlSb barriers and GaSb p–n junctions grown by molecular beam epitaxy. These were the devices with thyristor like switching in the GaSb/AlSb system. The characteristic "S" shaped current–voltage curve was found to occur for structures with AlSb barriers less than 300 Å thick. The switching voltage and current density exhibited less sensitivity to barrier and epilayer thickness than was predicted by the punch-through model. The results were correlated with drift diffusion simulations which have been modified to account for the presence of a tunneling contact
- …