32,407 research outputs found
Pion-to-vacuum vector and axial vector amplitudes and weak decays of pions in a magnetic field
We propose a model-independent parametrization for the one-pion-to-vacuum
matrix elements of the vector and axial vector hadronic currents in the
presence of an external uniform magnetic field. It is shown that, in general,
these hadronic matrix elements can be written in terms of several gauge
covariant Lorentz structures and form factors. Within this framework we obtain
a general expression for the weak decay and discuss
the corresponding limits of strong and weak external magnetic fields.Comment: 33 page
Sketch-based Influence Maximization and Computation: Scaling up with Guarantees
Propagation of contagion through networks is a fundamental process. It is
used to model the spread of information, influence, or a viral infection.
Diffusion patterns can be specified by a probabilistic model, such as
Independent Cascade (IC), or captured by a set of representative traces.
Basic computational problems in the study of diffusion are influence queries
(determining the potency of a specified seed set of nodes) and Influence
Maximization (identifying the most influential seed set of a given size).
Answering each influence query involves many edge traversals, and does not
scale when there are many queries on very large graphs. The gold standard for
Influence Maximization is the greedy algorithm, which iteratively adds to the
seed set a node maximizing the marginal gain in influence. Greedy has a
guaranteed approximation ratio of at least (1-1/e) and actually produces a
sequence of nodes, with each prefix having approximation guarantee with respect
to the same-size optimum. Since Greedy does not scale well beyond a few million
edges, for larger inputs one must currently use either heuristics or
alternative algorithms designed for a pre-specified small seed set size.
We develop a novel sketch-based design for influence computation. Our greedy
Sketch-based Influence Maximization (SKIM) algorithm scales to graphs with
billions of edges, with one to two orders of magnitude speedup over the best
greedy methods. It still has a guaranteed approximation ratio, and in practice
its quality nearly matches that of exact greedy. We also present influence
oracles, which use linear-time preprocessing to generate a small sketch for
each node, allowing the influence of any seed set to be quickly answered from
the sketches of its nodes.Comment: 10 pages, 5 figures. Appeared at the 23rd Conference on Information
and Knowledge Management (CIKM 2014) in Shanghai, Chin
Ill-posedness in the Einstein equations
It is shown that the formulation of the Einstein equations widely in use in
numerical relativity, namely, the standard ADM form, as well as some of its
variations (including the most recent conformally-decomposed version), suffers
from a certain but standard type of ill-posedness. Specifically, the norm of
the solution is not bounded by the norm of the initial data irrespective of the
data. A long-running numerical experiment is performed as well, showing that
the type of ill-posedness observed may not be serious in specific practical
applications, as is known from many numerical simulations.Comment: 13 pages, 3 figures, accepted for publication in Journal of
Mathematical Physics (to appear August 2000
Construction of Simulation Wavefunctions for Aqueous Species: D3O+
This paper investigates Monte Carlo techniques for construction of compact
wavefunctions for the internal atomic motion of the D3O+ ion. The polarization
force field models of Stillinger, et al and of Ojamae, et al. were used.
Initial pair product wavefunctions were obtained from the asymptotic high
temperature many-body density matrix after contraction to atom pairs using
Metropolis Monte Carlo. Subsequent characterization shows these pair product
wavefunctions to be well optimized for atom pair correlations despite that fact
that the predicted zero point energies are too high. The pair product
wavefunctions are suitable to use within variational Monte Carlo, including
excited states, and density matrix Monte Carlo calculations. Together with the
pair product wavefunctions, the traditional variational theorem permits
identification of wavefunction features with significant potential for further
optimization. The most important explicit correlation variable found for the
D3O+ ion was the vector triple product {\bf r}({\bf
r}{\bf r}). Variational Monte Carlo with 9 of such
explicitly correlated functions yielded a ground state wavefunction with an
error of 5-6% in the zero point energy.Comment: 17 pages including 6 figures, typos correcte
Minimal Stability in Maximal Supergravity
Recently, it has been shown that maximal supergravity allows for
non-supersymmetric AdS critical points that are perturbatively stable. We
investigate this phenomenon of stability without supersymmetry from the
sGoldstino point of view. In particular, we calculate the projection of the
mass matrix onto the sGoldstino directions, and derive the necessary conditions
for stability. Indeed we find a narrow window allowing for stable SUSY breaking
points. As a by-product of our analysis, we find that it seems impossible to
perturb supersymmetric critical points into non-supersymmetric ones: there is a
minimal amount of SUSY breaking in maximal supergravity.Comment: 27 pages, 1 figure. v2: two typos corrected, published versio
The effect of the Coriolis force on Kelvin-Helmholtz-driven mixing in protoplanetary disks
We study the stability of proto-planetary disks with vertical velocity
gradients in their equilibrium rotation rates; such gradients are expected to
develop when dust settles into the midplane. Using a linear stability analysis
of a simple three-layer model, we show that the onset of instability occurs at
a larger value of the Richardson number, and therefore for a thicker layer,
when the effects of Coriolis forces are included. This analysis also shows that
even-symmetry (midplane-crossing) modes develop faster than odd-symmetry ones.
These conclusions are corroborated by a large number of nonlinear numerical
simulations with two different parameterized prescriptions for the initial
(continuous) dust distributions. Based on these numerical experiments, the
Richardson number required for marginal stability is more than an order of
magnitude larger than the traditional 1/4 value. The dominant modes that grow
have horizontal wavelengths of several initial dust scale heights, and in
nonlinear stages mix solids fairly homogeneously over a comparable vertical
range. We conclude that gravitational instability may be more difficult to
achieve than previously thought, and that the vertical distribution of matter
within the dust layer is likely globally, rather than locally, determined.Comment: Accepted for publication in Ap
Exploring elite soccer teams’ performances during different match-status periods of close matches’ comebacks
The aim of the present study was to examine winning and losing teams’ performances during the four different match-status periods that occur in close soccer matches’ comebacks (1° drawing; 2° winning/losing; 3° drawing; and 4° losing/winning). The variables (i.e., shots, passing effectiveness and ball possession) were gathered from 17 matches of the Spanish professional soccer league. Relative-phase analysis of ball possession between teams revealed a shift from anti-phase to in-phase relations from period 1 to 4. Pass efficacy revealed a particular trend of anti-phase relations in period 2 and the analysis of shots revealed similar phase relations between periods. Statistically significant differences were observed between winning and losing teams in Period 3 for ball possession and passing effectiveness. Also, statistically significant differences among periods were observed for winning teams in ball possession with period 4 as the most differentiated from the other periods. Besides, winning teams also showed significant differences between periods in passing effectiveness (period 4 vs 3), and in shots (period 3 vs periods 1, 2 and 4). On the other hand, ball possession showed significant differences for losing teams with periods 3 and 4 different than periods 1 and 2. The current findings can be used when controlling match-status scenarios and key performance indicators along the match
Youth-Adult Differences in the Demand for Unionization: Are American, British and Canadian Workers All That Different?
This paper examines demand for union membership amongst young workers in Britain, Canada and the United States. The paper benchmarks youth demands for collective
representation against those of adult workers and finds that a large and significant representation gap exists in all three countries. Using a model of representation advanced by Farber (1982) and Riddell (1993) we find that a majority of the union density differential between young and adult workers is due to supply-side constraints rather than a lower desire for unionisation on the part of the young. This finding lends credence to two conjectures made in the paper; the first is that tastes for collective representation do not differ among workers (either by nationality or by age) and second that union representation can be fruitfully modelled as an experience good. The experience good properties of union
membership explain the persistence of union density differentials amongst youth and adults both over time and across countries
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