4,900 research outputs found
Variation of the Diameter of the Sun as Measured by the Solar Disk Sextant (SDS)
The balloon-borne Solar Disk Sextant (SDS) experiment has measured the
angular size of the Sun on seven occasions spanning the years 1992 to 2011. The
solar half-diameter -- observed in a 100-nm wide passband centred at 615 nm --
is found to vary over that period by up to 200 mas, while the typical estimated
uncertainty of each measure is 20 mas. The diameter variation is not in phase
with the solar activity cycle; thus, the measured diameter variation cannot be
explained as an observational artefact of surface activity. Other possible
instrument-related explanations for the observed variation are considered but
found unlikely, leading us to conclude that the variation is real. The SDS is
described here in detail, as is the complete analysis procedure necessary to
calibrate the instrument and allow comparison of diameter measures across
decades.Comment: 41 pages; appendix and 2 figures added plus some changes in text
based on referee's comments; to appear in MNRA
Application of the Covariant Spectator Theory to the study of heavy and heavy-light mesons
As an application of the Covariant Spectator Theory (CST) we calculate the
spectrum of heavy-light and heavy-heavy mesons using covariant versions of a
linear confining potential, a one- gluon exchange, and a constant interaction.
The CST equations possess the correct one-body limit and are therefore
well-suited to describe mesons in which one quark is much heavier than the
other. We find a good fit to the mass spectrum of heavy-light and heavy-heavy
mesons with just three parameters (apart from the quark masses). Remarkably,
the fit parameters are nearly unchanged when we fit to experimental
pseudoscalar states only or to the whole spectrum. Because pseudoscalar states
are insensitive to spin-orbit interactions and do not determine spin-spin
interactions separately from central interactions, this result suggests that it
is the covariance of the kernel that correctly predicts the spin-dependent
quark-antiquark interaction
Covariant spectator theory of quark-antiquark bound states: Mass spectra and vertex functions of heavy and heavy-light mesons
We use the covariant spectator theory with an effective quark-antiquark
interaction, containing Lorentz scalar, pseudoscalar, and vector contributions,
to calculate the masses and vertex functions of, simultaneously, heavy and
heavy-light mesons. We perform least-square fits of the model parameters,
including the quark masses, to the meson spectrum and systematically study the
sensitivity of the parameters with respect to different sets of fitted data. We
investigate the influence of the vector confining interaction by using a
continuous parameter controlling its weight. We find that vector contributions
to the confining interaction between 0% and about 30% lead to essentially the
same agreement with the data. Similarly, the light quark masses are not very
tightly constrained. In all cases, the meson mass spectra calculated with our
fitted models agree very well with the experimental data. We also calculate the
mesons wave functions in a partial wave representation and show how they are
related to the meson vertex functions in covariant form.Comment: 23 pages, 10 figures. Minor corrections of previous version. To be
published in Phys. Rev.
Singularity-free two-body equation with confining interactions in momentum space
We are developing a covariant model for all mesons that can be described as
quark-antiquark bound states in the framework of the Covariant Spectator Theory
(CST) in Minkowski space. The kernel of the bound-state equation contains a
relativistic generalization of a linear confining potential which is singular
in momentum space and makes its numerical solution more difficult. The same
type of singularity is present in the momentum-space Schr\"odinger equation,
which is obtained in the nonrelativistic limit. We present an alternative,
singularity-free form of the momentum-space Schr\"odinger equation which is
much easier to solve numerically and which yields accurate and stable results.
The same method will be applied to the numerical solution of the CST
bound-state equations.Comment: 4 pages, 2 figures, talk presented at the 22nd European Conference on
Few-Body Problems in Physics (EFB22), Krakow, Poland, 9 - 13 September 201
Superbubbles
Individual massive stars with M sub bol -6 have huge stellar winds that create interstellar bubbles. Stars with masses greater than 8 solar mass are considered supernova progenitors. These massive stars are numerous in OB associations where few supernova remnants are detected. Model calculations describing the evolution of an association show: that large, hot cavities are formed by pushing the ambient gas into neutral shells; that the shell radii change with galactocentric radius; that only thirty percent of the interstellar medium is in the form of supercavities; and that a consequence is that only a small fraction of supernovae form supernova remnants
Relativistic phenomenology of meson spectra with a covariant quark model in Minkowski space
In this work, we perform a covariant treatment of quark-antiquark systems. We
calculate the spectra and wave functions using a formalism based on the
Covariant Spectator Theory (CST). Our results not only reproduce very well the
experimental data with a very small set of global parameters, but they also
allow a direct test of the predictive power of covariant kernels
Quarkonia and heavy-light mesons in a covariant quark model
Preliminary calculations using the Covariant Spectator Theory (CST) employed
a scalar linear confining interaction and an additional constant vector
potential to compute the mesonic mass spectra. In this work we generalize the
confining interaction to include more general structures, in particular a
vector and also a pseudoscalar part, as suggested by a recent study. A
one-gluon-exchange kernel is also implemented to describe the short-range part
of the interaction. We solve the simplest CST approximation to the complete
Bethe-Salpeter equation, the one-channel spectator equation, using a numerical
technique that eliminates all singularities from the kernel. The parameters of
the model are determined through a fit to the experimental pseudoscalar meson
spectra, with a good agreement for both quarkonia and heavy-light states.Comment: 4 pages, 2 figures; 21st International Conference on Few-Body
Problems in Physics, May 18 - 22, 2015, Chicago, US
Optimal control of a dengue epidemic model with vaccination
We present a SIR+ASI epidemic model to describe the interaction between human
and dengue fever mosquito populations. A control strategy in the form of
vaccination, to decrease the number of infected individuals, is used. An
optimal control approach is applied in order to find the best way to fight the
disease.Comment: This is a preprint of a paper accepted for presentation at ICNAAM
2011, Halkidiki, Greece, 19-25 September 2011, and to appear in AIP
Conference Proceedings, volume 138
Modeling and Optimal Control Applied to a Vector Borne Disease
A model with six mutually-exclusive compartments related to Dengue disease is
presented. In this model there are three vector control tools: insecticides
(larvicide and adulticide) and mechanical control. The problem is studied using
an Optimal Control (OC) approach. The human data for the model is based on the
Cape Verde Dengue outbreak. Some control measures are simulated and their
consequences analyzed
Insecticide control in a Dengue epidemics model
A model for the transmission of dengue disease is presented. It consists of
eight mutually-exclusive compartments representing the human and vector
dynamics. It also includes a control parameter (insecticide) in order to fight
the mosquitoes. The main goal of this work is to investigate the best way to
apply the control in order to effectively reduce the number of infected humans
and mosquitoes. A case study, using data of the outbreak that occurred in 2009
in Cape Verde, is presented.Comment: Accepted 28/07/2010 in the special session "Numerical Optimization"
of the 8th International Conference of Numerical Analysis and Applied
Mathematics (ICNAAM 2010), Rhodes, Greece, 19-25 September 201
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