1,490 research outputs found
A Light Supersymmetric Axion in an Anomalous Abelian Extension of the Standard Model
We present a supersymmetric extension of the Standard Model (USSM-A) with an
anomalous U(1) and Stueckelberg axions for anomaly cancellation, generalizing
similar non-supersymmetric constructions. The model, built by a bottom-up
approach, is expected to capture the low-energy supersymmetric description of
axionic symmetries in theories with gauged anomalous abelian interactions,
previously explored in the non-supersymmetric case for scenarios with
intersecting branes. The choice of a USSM-like superpotential, with one extra
singlet superfield and an extra abelian symmetry, allows a physical axion-like
particle in the spectrum. We describe some general features of this
construction and in particular the modification of the dark-matter sector which
involves both the axion and several neutralinos with an axino component. The
axion is expected to be very light in the absence of phases in the
superpotential but could acquire a mass which can also be in the few GeV range
or larger. In particular, the gauging of the anomalous symmetry allows
independent mass/coupling interaction to the gauge fields of this particle, a
feature which is absent in traditional (invisible) axion models. We comment on
the general implications of our study for the signature of moduli from string
theory due to the presence of these anomalous symmetries.Comment: 46 pages, 28 figures. Revised version, accepted for a publication on
Phys.Rev.
Low-Flow-Coefficient Centrifugal Compressor Design for Supercritical COâ‚‚
This paper presents a design strategy for very low flow coefficient multistage compressors operating with supercritical CO₂ for carbon capture and sequestration (CCS) and enhanced oil recovery (EOR). At flow coefficients less than 0.01, the stage efficiency is much reduced due to dissipation in the gas-path and more prominent leakage and windage losses. Instead of using a vaneless diffuser as is standard design practice in such applications, the current design employs a vaned diffuser to decrease the meridional velocity and to widen the gas path. The aim is to achieve a step change in performance. The impeller exit width is increased in a systematic parameter study to explore the limitations of this design strategy and to define the upper limit in efficiency gain. The design strategy is applied to a full-scale reinjection compressor currently in service. Three-dimensional, steady, supercritical CO₂ computational fluid dynamics (CFD) simulations of the full stage with leakage flows are carried out with the National Institute of Standards and Technology (NIST) real gas model. The design study suggests that a nondimensional impeller exit width parameter b₂* =(b₂ /R)φ of six yields a 3.5 point increase in adiabatic efficiency relative to that of a conventional compressor design with vaneless diffuser. Furthermore, it is shown that in such stages the vaned diffuser limits the overall stability and that the onset of rotating stall is likely caused by vortex shedding near the diffuser leading edge. The inverse of the nondimensional impeller exit width parameter b₂* can be interpreted as the Rossby number. The investigation shows that, for very low flow coefficient designs, the Coriolis accelerations dominate the relative flow accelerations, which leads to inverted swirl angle distributions at impeller exit. Combined with the twoorders- of-magnitude higher Reynolds number for supercritical CO₂ , the leading edge vortex shedding occurs at lower flow coefficients than in air suggesting an improved stall margin
SU(3) sextet model with Wilson fermions
We investigate the spectrum and IR properties of the SU(3) "sextet" model
with two Dirac fermions in the two-index symmetric representation via lattice
simulations. This model is a prime candidate for a realization of Walking
Technicolor, which features a minimal matter content and it is expected to be
inside or very close to the lower boundary of the conformal window. We use the
Wilson discretization for the fermions and map the phase structure of the
lattice model. We study several spectral and gradient flow observables both in
the bulk and the weak coupling phases. While in the bulk phase we find clear
signs of chiral symmetry breaking, in the weak coupling phase there is no clear
indication for it, and instead the chiral limit of the model seems compatible
with an IR-conformal behavior.Comment: 32 pages, many figure
Role of electric charge in shaping equilibrium configurations of fluid tori encircling black holes
Astrophysical fluids may acquire non-zero electrical charge because of strong
irradiation or charge separation in a magnetic field. In this case,
electromagnetic and gravitational forces may act together and produce new
equilibrium configurations, which are different from the uncharged ones.
Following our previous studies of charged test particles and uncharged perfect
fluid tori encircling compact objects, we introduce here a simple test model of
a charged perfect fluid torus in strong gravitational and electromagnetic
fields. In contrast to ideal magnetohydrodynamic models, we consider here the
opposite limit of negligible conductivity, where the charges are tied
completely to the moving matter. This is an extreme limiting case which can
provide a useful reference against which to compare subsequent more complicated
astrophysically-motivated calculations. To clearly demonstrate the features of
our model, we construct three-dimensional axisymmetric charged toroidal
configurations around Reissner-Nordstr\"om black holes and compare them with
equivalent configurations of electrically neutral tori.Comment: 14 pages, 7 figure
Kinetics of Carboxylmethylation of the Charge Isoforms of Myelin Basic Protein by Protein Methyltransferase II
The charge isoforms (C1-C5) of bovine myelin basic protein (MBP) were used as substrates for the rat brain enzyme protein carboxylmethyltransferase (PM II). The objective of these experiments was to ascertain whether the kinetic behavior of the MBP isoforms reflected differences in the structures of this molecular family. Initial velocity plots as a function of the MBP-isoform concentration showed significnt differences ( p > 0.05) among the assayed isoforms except for isoforms C2 and C4. Under the conditions of our experiment all the curves exhibited a consistent sigmoidicity. The kinetic data were best fitted by a model, previously described for the enzyme D-Î’-hydroxybutyrate dehydrogenase, in which two independent sites must be randomly occupied before any catalytic activity can occur. This mechanism is substantially different from that proposed by other investigators for similar PM II enzymes and other substrates. The differences in the rates of isoform carboxylmethylation are largely accounted for by the different apparent dissociation constants K s and is explained on the basis of inherent structural differences among the charge isoforms.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65821/1/j.1471-4159.1989.tb09257.x.pd
The statistical geometry of scale-free random trees
The properties of scale-free random trees are investigated using both
preconditioning on non-extinction and fixed size averages, in order to study
the thermodynamic limit. The scaling form of volume probability is found, the
connectivity dimensions are determined and compared with other exponents which
describe the growth. The (local) spectral dimension is also determined, through
the study of the massless limit of the Gaussian model on such trees.Comment: 21 pages, 2 figures, revtex4, minor changes (published version
Tunneling-percolation origin of nonuniversality: theory and experiments
A vast class of disordered conducting-insulating compounds close to the
percolation threshold is characterized by nonuniversal values of transport
critical exponent t, in disagreement with the standard theory of percolation
which predicts t = 2.0 for all three dimensional systems. Various models have
been proposed in order to explain the origin of such universality breakdown.
Among them, the tunneling-percolation model calls into play tunneling processes
between conducting particles which, under some general circumstances, could
lead to transport exponents dependent of the mean tunneling distance a. The
validity of such theory could be tested by changing the parameter a by means of
an applied mechanical strain. We have applied this idea to universal and
nonuniversal RuO2-glass composites. We show that when t > 2 the measured
piezoresistive response \Gamma, i. e., the relative change of resistivity under
applied strain, diverges logarithmically at the percolation threshold, while
for t = 2, \Gamma does not show an appreciable dependence upon the RuO2 volume
fraction. These results are consistent with a mean tunneling dependence of the
nonuniversal transport exponent as predicted by the tunneling-percolation
model. The experimental results are compared with analytical and numerical
calculations on a random-resistor network model of tunneling-percolation.Comment: 13 pages, 12 figure
Mediation of supersymmetry breaking in extra dimensions
We review the mechanisms of supersymmetry breaking mediation that occur in
sequestered models, where the visible and the hidden sectors are separated by
an extra dimension and communicate only via gravitational interactions. By
locality, soft breaking terms are forbidden at the classical level and reliably
computable within an effective field theory approach at the quantum level. We
present a self-contained discussion of these radiative gravitational effects
and the resulting pattern of soft masses, and give an overview of realistic
model building based on this set-up. We consider both flat and warped extra
dimensions, as well as the possibility that there be localized kinetic terms
for the gravitational fields.Comment: LaTex, 15 pages; brief review prepared for MPLA. v2: minor
correction
Heat energy balance in the convective atmospheric boundary layer at Xianghe (Beijing Area), China
Minimal gauge-Higgs unification with a flavour symmetry
We show that a flavour symmetry a la Froggatt-Nielsen can be naturally
incorporated in models with gauge-Higgs unification, by exploiting the heavy
fermions that are anyhow needed to realize realistic Yukawa couplings. The case
of the minimal five-dimensional model, in which the SU(2)_L x U(1)_Y
electroweak group is enlarged to an SU(3)_W group, and then broken to U(1)_em
by the combination of an orbifold projection and a Scherk-Schwarz twist, is
studied in detail. We show that the minimal way of incorporating a U(1)_F
flavour symmetry is to enlarge it to an SU(2)_F group, which is then completely
broken by the same orbifold projection and Scherk-Schwarz twist. The general
features of this construction, where ordinary fermions live on the branes
defined by the orbifold fixed-points and messenger fermions live in the bulk,
are compared to those of ordinary four-dimensional flavour models, and some
explicit examples are constructed.Comment: LaTex, 37 pages, 2 figures; some clarifying comments and a few
references adde
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