43,168 research outputs found
Two charges on plane in a magnetic field: special trajectories
A classical mechanics of two Coulomb charges on a plane and
subject to a constant magnetic field perpendicular to a plane is
considered. Special "superintegrable" trajectories (circular and linear) for
which the distance between charges remains unchanged are indicated as well as
their respectful constants of motion. The number of the independent constants
of motion for special trajectories is larger for generic ones. A classification
of pairs of charges for which special trajectories occur is given. The special
trajectories for three particular cases of two electrons, (electron -
positron), (electron - -particle) are described explicitly.Comment: 22 pages, 5 figure
Pion transition form factor in the Regge approach and incomplete vector-meson dominance
The concept of incomplete vector-meson dominance and Regge models is applied
to the transition form factor of the pion. First, we argue that variants of the
chiral quark model fulfilling the chiral anomaly may violate the Terazawa-West
unitarity bounds, as these bounds are based on unverified assumptions for the
real parts of the amplitudes, precluding a possible presence of polynomial
terms. A direct consequence is that the transition form factor need not
necessarily vanish at large values of the photon virtuality. Moreover, in the
range of the BaBar experiment, the Terazawa-West bound is an order of magnitude
above the data, thus is of formal rather than practical interest. Then we
demonstrate how the experimental data may be properly explained with incomplete
vector-meson dominance in a simple model with one state, as well as in more
sophisticated Regge models. Generalizations of the simple Regge model along the
lines of Dominguez result in a proper description of the data, where one may
adjust the parameters in such a way that the Terazawa-West bound is satisfied
or violated. We also impose the experimental constraint from the Z -> pi0 gamma
decay. Finally, we point out that the photon momentum asymmetry parameter may
noticeably influence the precision analysis.Comment: 11 pages, 7 figure
The performance of socially responsible mutual funds: the role of fees and management companies
In this paper, we shed light on the debate about the financial performance of socially responsible
investment (SRI) mutual funds by separately analyzing the contributions of before-fee performance and
fees to SRI funds' performance and by investigating the role played by fund management companies in
the determination of those variables. We apply the matching estimator methodology to obtain our results
and find that in the period 1997-2005, US SRI funds had significantly higher fees and better before- and
after-fee performance than conventional funds with similar characteristics. Differences, however, were
driven exclusively by SRI funds run by management companies specialized in socially responsible
investment
Probabilistic Inference from Arbitrary Uncertainty using Mixtures of Factorized Generalized Gaussians
This paper presents a general and efficient framework for probabilistic
inference and learning from arbitrary uncertain information. It exploits the
calculation properties of finite mixture models, conjugate families and
factorization. Both the joint probability density of the variables and the
likelihood function of the (objective or subjective) observation are
approximated by a special mixture model, in such a way that any desired
conditional distribution can be directly obtained without numerical
integration. We have developed an extended version of the expectation
maximization (EM) algorithm to estimate the parameters of mixture models from
uncertain training examples (indirect observations). As a consequence, any
piece of exact or uncertain information about both input and output values is
consistently handled in the inference and learning stages. This ability,
extremely useful in certain situations, is not found in most alternative
methods. The proposed framework is formally justified from standard
probabilistic principles and illustrative examples are provided in the fields
of nonparametric pattern classification, nonlinear regression and pattern
completion. Finally, experiments on a real application and comparative results
over standard databases provide empirical evidence of the utility of the method
in a wide range of applications
Comparing univariate and multivariate models to forecast portfolio value-at-risk
This article addresses the problem of forecasting portfolio value-at-risk (VaR) with multivariate GARCH models vis-Ã -vis univariate models. Existing literature has tried to answer this question by analyzing only small portfolios and using a testing framework not appropriate for ranking VaR models. In this work we provide a more comprehensive look at the problem of portfolio VaR forecasting by using more appropriate statistical tests of comparative predictive ability. Moreover, we compare univariate vs. multivariate VaR models in the context of diversified portfolios containing a large number of assets and also provide evidence based on Monte Carlo experiments. We conclude that, if the sample size is moderately large, multivariate models outperform univariate counterparts on an out-of-sample basis.Market risk, Backtesting, Conditional predictive ability, GARCH, Volatility, Capital requirements, Basel II
Type Ia supernova counts at high z: signatures of cosmological models and progenitors
Determination of the rates at which supernovae of Type Ia (SNe Ia) occur in
the early Universe can give signatures of the time spent by the binary
progenitor systems to reach explosion and of the geometry of the Universe.
Observations made within the Supernova Cosmology Project are already providing
the first numbers. Here it is shown that, for any assumed SNe Ia progenitor,
SNe Ia counts up to are useful tests of the SNe Ia
progenitor systems and cosmological tracers of a possible non-zero value of the
cosmological constant, . The SNe Ia counts at high redshifts compare
differently with those at lower redshifts depending on the cosmological model.
Flat --dominated universes would show a more significant
increase of the SNe Ia counts at than a flat,
universe. Here we consider three sorts of universes: a flat universe with
, , ;
an open universe with , ,
; and a flat, --dominated universe with , , ). On the
other hand, the SNe Ia counts from one class of binary progenitors (double
degenerate systems) should not increase steeply in the to range,
contrary to what should be seen for other binary progenitors. A measurement of
the SNe Ia counts up to is within reach of ongoing SNe Ia searches
at high redshifts.Comment: 16 pages, incl. 2 figures. To appear in ApJ (Letters
Features of the Extension of a Statistical Measure of Complexity to Continuous Systems
We discuss some aspects of the extension to continuous systems of a
statistical measure of complexity introduced by Lopez-Ruiz, Mancini and Calbet
(LMC) [Phys. Lett. A 209 (1995) 321]. In general, the extension of a magnitude
from the discrete to the continuous case is not a trivial process and requires
some choice. In the present study, several possibilities appear available. One
of them is examined in detail. Some interesting properties desirable for any
magnitude of complexity are discovered on this particular extension.Comment: 22 pages, 0 figure
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