4,161 research outputs found
Enhanced Gauge Symmetry in Type II String Theory
We show how enhanced gauge symmetry in type II string theory compactified on
a Calabi--Yau threefold arises from singularities in the geometry of the target
space. When the target space of the type IIA string acquires a genus curve
of singularities, we find that an gauge theory with
adjoint hypermultiplets appears at the singularity. The new massless states
correspond to solitons wrapped about the collapsing cycles, and their dynamics
is described by a twisted supersymmetric gauge theory on . We
reproduce this result from an analysis of the -dual -manifold. We check
that the predictions made by this model about the nature of the Higgs branch,
the monodromy of period integrals, and the asymptotics of the one-loop
topological amplitude are in agreement with geometrical computations. In one of
our examples we find that the singularity occurs at strong coupling in the
heterotic dual proposed by Kachru and Vafa.Comment: 43 pages using harvmac's `big' option. (Minor correction concerning
magnetically charged states.
Cranial sensory neuron development in the absence of brain-derived neurotrophic factor in BDNF/Bax double null mice
AbstractTo investigate the role of brain-derived neurotrophic factor (BDNF) in differentiation of cranial sensory neurons in vivo, we analyzed development of nodose (NG), petrosal (PG), and vestibular (VG) ganglion cells in genetically engineered mice carrying null mutations in the genes encoding BDNF and the proapoptotic Bcl-2 homolog Bax. In bax −/− mutants, ganglion cell numbers were increased significantly compared to wild-type animals, indicating that naturally occurring cell death in these ganglia is regulated by Bax signaling. Analysis of bdnf −/− bax −/− mutants revealed that, although the Bax null mutation completely rescued cell loss in the absence of BDNF, it did not rescue the lethality of the BDNF null phenotype. Moreover, despite rescue of BDNF-dependent neurons by the bax null mutation, sensory target innervation was abnormal in double null mutants. Vagal sensory innervation to baroreceptor regions of the cardiac outflow tract was completely absent, and the density of vestibular sensory innervation to the cristae organs was markedly decreased, compared to wild-type controls. Moreover, vestibular afferents failed to selectively innervate their hair cell targets within the cristae organs in the double mutants. These innervation failures occurred despite successful navigation of sensory fibers to the peripheral field, demonstrating that BDNF is required locally for afferent ingrowth into target tissues. In addition, the bax null mutation failed to rescue expression of the dopaminergic phenotype in a subset of NG and PG neurons. These data demonstrate that BDNF signaling is required not only to support survival of cranial sensory neurons, but also to regulate local growth of afferent fibers into target tissues and, in some cells, transmitter phenotypic expression is required
Cauchy, infinitesimals and ghosts of departed quantifiers
Procedures relying on infinitesimals in Leibniz, Euler and Cauchy have been
interpreted in both a Weierstrassian and Robinson's frameworks. The latter
provides closer proxies for the procedures of the classical masters. Thus,
Leibniz's distinction between assignable and inassignable numbers finds a proxy
in the distinction between standard and nonstandard numbers in Robinson's
framework, while Leibniz's law of homogeneity with the implied notion of
equality up to negligible terms finds a mathematical formalisation in terms of
standard part. It is hard to provide parallel formalisations in a
Weierstrassian framework but scholars since Ishiguro have engaged in a quest
for ghosts of departed quantifiers to provide a Weierstrassian account for
Leibniz's infinitesimals. Euler similarly had notions of equality up to
negligible terms, of which he distinguished two types: geometric and
arithmetic. Euler routinely used product decompositions into a specific
infinite number of factors, and used the binomial formula with an infinite
exponent. Such procedures have immediate hyperfinite analogues in Robinson's
framework, while in a Weierstrassian framework they can only be reinterpreted
by means of paraphrases departing significantly from Euler's own presentation.
Cauchy gives lucid definitions of continuity in terms of infinitesimals that
find ready formalisations in Robinson's framework but scholars working in a
Weierstrassian framework bend over backwards either to claim that Cauchy was
vague or to engage in a quest for ghosts of departed quantifiers in his work.
Cauchy's procedures in the context of his 1853 sum theorem (for series of
continuous functions) are more readily understood from the viewpoint of
Robinson's framework, where one can exploit tools such as the pointwise
definition of the concept of uniform convergence.
Keywords: historiography; infinitesimal; Latin model; butterfly modelComment: 45 pages, published in Mat. Stu
A multi-method approach to radial-velocity measurement for single-object spectra
The derivation of radial velocities from large numbers of spectra that
typically result from survey work, requires automation. However, except for the
classical cases of slowly rotating late-type spectra, existing methods of
measuring Doppler shifts require fine-tuning to avoid a loss of accuracy due to
the idiosyncrasies of individual spectra. The radial velocity spectrometer
(RVS) on the Gaia mission, which will start operating very soon, prompted a new
attempt at creating a measurement pipeline to handle a wide variety of spectral
types.
The present paper describes the theoretical background on which this software
is based. However, apart from the assumption that only synthetic templates are
used, we do not rely on any of the characteristics of this instrument, so our
results should be relevant for most telescope-detector combinations.
We propose an approach based on the simultaneous use of several alternative
measurement methods, each having its own merits and drawbacks, and conveying
the spectral information in a different way, leading to different values for
the measurement. A comparison or a combination of the various results either
leads to a "best estimate" or indicates to the user that the observed spectrum
is problematic and should be analysed manually.
We selected three methods and analysed the relationships and differences
between them from a unified point of view; with each method an appropriate
estimator for the individual random error is chosen. We also develop a
procedure for tackling the problem of template mismatch in a systematic way.
Furthermore, we propose several tests for studying and comparing the
performance of the various methods as a function of the atmospheric parameters
of the observed objects. Finally, we describe a procedure for obtaining a
knowledge-based combination of the various Doppler-shift measurements.Comment: 16 pages, 4 figure
Lifetime and Coherence of Two-Level Defects in a Josephson Junction
We measure the lifetime () and coherence () of two-level defect
states (TLSs) in the insulating barrier of a Josephson phase qubit and compare
to the interaction strength between the two systems. We find for the average
decay times a power law dependence on the corresponding interaction strengths,
whereas for the average coherence times we find an optimum at intermediate
coupling strengths. We explain both the lifetime and the coherence results
using the standard TLS model, including dipole radiation by phonons and
anti-correlated dependence of the energy parameters on environmental
fluctuations.Comment: 4 pages, 4 figures and supplementary material (3 pages, 2 figures, 1
table
Rising Inequality? Changes in the Distribution of Income and Consumption in the 1980s
This paper examines changes in the distribution of income and consumption in the United States during the 1980s. using data from the Current Population Survey (income) and Consumer Expenditure Survey (consumption). We reach three primary conclusions. First. changes in the distribution of consumption parallel changes in the distribution of income. The lowest quintile of the consumption distribution received 0.9 percentage points less of total consumption in 1988 than in 1980; the corresponding decline for income was 0.6 percentage points. Second. broad conclusions concerning recent changes in the consumption distribution are not very sensitive to the exact choice of a measure of family needs. Under a wide variety of alternative household equivalence scales. there is a widening in the consumption distribution in the 1980s. Third. the usc of consumption measures of well-being in place of measures based on current money income docs change conclusions concerning the extent of poverty in the United Stales. Using the official federal poverty thresholds. we find that the overall consumption poverty rate was three percentage points below the income poverty rate in 1988. Comparisons of the poverty rates of the elderly and the non-elderly are substantially affected by the choice of poverty measure. The consumption poverty rare for the elderly was only 60 percent of the rate for adults and one-third of the rate for children in 1988.
Direct Wigner tomography of a superconducting anharmonic oscillator
The analysis of wave-packet dynamics may be greatly simplified when viewed in
phase-space. While harmonic oscillators are often used as a convenient platform
to study wave-packets, arbitrary state preparation in these systems is more
challenging. Here, we demonstrate a direct measurement of the Wigner
distribution of complex photon states in an anharmonic oscillator - a
superconducting phase circuit, biased in the small anharmonicity regime. We
test our method on both non-classical states composed of two energy eigenstates
and on the dynamics of a phase-locked wavepacket. This method requires a simple
calibration, and is easily applicable in our system out to the fifth level.Comment: 5 figures, 1 table and supplementary materia
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