707 research outputs found
Mass and Width of a Heavy Higgs Boson
The gauge dependence of the Higgs-boson mass and width in the on-shell scheme
of renormalization is studied in the heavy-Higgs-boson approximation. The
corresponding expansions in the pole scheme are analyzed adopting three
frequently employed parametrizations. The convergence properties and other
theoretical features of the on-shell and pole expansions, as well as their
relative merits, are discussed.Comment: 8 pages (Latex), 1 figure (Postscript
Absorptive part of meson--baryon scattering amplitude and baryon polarization in chiral perturbation theory
We compute the spin asymmetry and polarization of the final-state baryon in
its rest frame in two-body meson--baryon low-energy scattering with unpolarized
initial state, to lowest non-trivial order in BChPT. The required absorptive
amplitudes are obtained analytically at one-loop level. We discuss the
polarization results numerically for several meson--baryon processes. Even at
low energies above threshold, where BChPT can reasonably be expected to be
applicable, sizable values of polarization are found for some processes
Calculation of Infrared-Divergent Feynman Diagrams with Zero Mass Threshold
Two-loop vertex Feynman diagrams with infrared and collinear divergences are
investigated by two independent methods. On the one hand, a method of
calculating Feynman diagrams from their small momentum expansion extended to
diagrams with zero mass thresholds is applied. On the other hand, a numerical
method based on a two-fold integral representation is used. The application of
the latter method is possible by using lightcone coordinates in the parallel
space. The numerical data obtained with the two methods are in impressive
agreement.Comment: 20 pages, Latex with epsf-figures, References updated, to appear in
Z.Phys.
Computational Simulations of a Mach 0.745 Transonic Truss-Braced Wing Design
A joint effort between the NASA Ames and Langley Research Centers was undertaken to analyze the Mach 0.745 variant of the Boeing Transonic Truss-Braced Wing (TTBW) Design. Two different flow solvers, LAVA and USM3D, were used to predict the TTBW flight performance. Sensitivity studies related to mesh resolution and numerical schemes were conducted to define best practices for this type of geometry and flow regime. Validation efforts compared the numerical simulation results of various modeling methods against experimental data taken from the NASA Ames 11-foot Unitary Wind Tunnel experimental data. The fidelity of the computational representation of the wind tunnel experiment, such as utilizing a porous wall boundary condition to model the ventilated test section, was varied to examine how different tunnel effects influence CFD predictions. LAVA and USM3D results both show an approximate 0.5 angle of attack shift from experimental lift curve data. This drove an investigation that revealed that the trailing edge of the experimental model was rounded in comparison to the CAD model, due to manufacturing tolerances, which had not been accounted for in the initial simulations of the experiment. Simulating the TTBW with an approximation of this rounded trailing-edge reduces error by approximately 60%. An accurate representation of the tested TTBW geometry, ideally including any wing twists and deflections experienced during the test under various loading conditions, will be necessary for proper validation of the CFD
No Fossil Disk in the T Tauri Multiple System V773 Tau
We present new multi-epoch near-infrared and optical high-angular images of
the V773 Tau pre-main sequence triple system, a weak-line T Tauri (WTTS) system
in which the presence of an evolved, ``fossil'' protoplanetary disk has been
inferred on the basis of a significant infrared excess. Our images reveal a
fourth object bound to the system, V773 Tau D. While it is much fainter than
all other components at 2 micron, it is the brightest source in the system at
4.7 micron. We also present medium-resolution K band adaptive optics
spectroscopy of this object, which is featureless with the exception of a weak
Br gamma emission line. Based on this spectrum and on the spectral energy
distribution of the system, we show that V773 Tau D is another member of the
small class of ``infrared companions'' (IRCs) to T Tauri stars. It is the least
luminous, and probably the least massive, component of the system, as opposed
to most other IRCs, which suggests that numerous low-luminosity IRCs such as
V773 Tau D may still remain to be discovered. Furthermore, it is the source of
the strong IR excess in the system. We therefore reject the interpretation of
this excess as the signature of a fossil (or ``passive'') disk and further
suggest that these systems may be much less frequent than previously thought.
We further show that V773 Tau C is a variable classical T Tauri star (CTTS)
and that its motion provides a well constrained orbital model. We show that
V773 Tau D can be dynamically stable within this quadruple system if its orbit
is highly inclined. Finally, V773 Tau is the first multiple system to display
such a variety of evolutionary states (WTTS, CTTS, IRC), which may be the
consequence of the strong star-star interactions in this compact quadruple
system.Comment: Accepted for publication in Astrophysical Journal, 29 pages, 2
tables, 5 figure
Radial velocities of giant stars: an investigation of line profile variations
Since 1999, a radial velocity survey of 179 red giant stars is ongoing at
Lick Observatory with a one month cadence. At present ~20-100 measurements have
been collected per star with an accuracy of 5 to 8 m/s. Of the stars monitored,
145 (80%) show radial velocity (RV) variations at a level >20 m/s, of which 43
exhibit significant periodicities. Here, we investigate the mechanism causing
the observed radial velocity variations. Firstly, we search for a correlation
between the radial velocity amplitude and an intrinsic parameter of the star,
in this case surface gravity (log g). Secondly, we investigate line profile
variations and compare these with theoretical predictions.Comment: To appear in the proceedings of the Helas II workshop:
Helioseismology, Asteroseismology and MHD Connections; published in the
Journal of Physics: Conference Series, ed. L. Gizon; 7 pages, 5 figure
Precise radial velocities of giant stars. IV. A correlation between surface gravity and radial velocity variation and a statistical investigation of companion properties
Since 1999, we have been conducting a radial velocity survey of 179 K giants
using the CAT at UCO/Lick observatory. At present ~20-100 measurements have
been collected per star with a precision of 5 to 8 m/s. Of the stars monitored,
145 (80%) show radial velocity (RV) variations at a level >20 m/s, of which 43
exhibit significant periodicities. Our aim is to investigate possible
mechanism(s) that cause these observed RV variations. We intend to test whether
these variations are intrinsic in nature, or possibly induced by companions, or
both. In addition, we aim to characterise the parameters of these companions. A
relation between log g and the amplitude of the RV variations is investigated
for all stars in the sample. Furthermore, the hypothesis that all periodic RV
variations are caused by companions is investigated by comparing their inferred
orbital statistics with the statistics of companions around main sequence
stars. A strong relation is found between the amplitude of the RV variations
and log g in K giant stars, as suggested earlier by Hatzes & Cochran (1998).
However, most of the stars exhibiting periodic variations are located above
this relation. These RV variations can be split in a periodic component which
is not correlated with log g and a random residual part which does correlate
with log g. Compared to main-sequence stars, K giants frequently exhibit
periodic RV variations. Interpreting these RV variations as being caused by
companions, the orbital param eters are different from the companions orbiting
dwarfs. Intrinsic mechanisms play an important role in producing RV variations
in K giants stars, as suggested by their dependence on log g. However, it
appears that periodic RV variations are additional to these intrinsic
variations, consistent with them being caused by companions.Comment: 10 pages, accepted by A&
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