807 research outputs found
Dust in Interstellar Clouds, Evolved Stars and Supernovae
Outflows of pre-main-sequence stars drive shocks into molecular material
within 0.01 - 1 pc of the young stars. The shock-heated gas emits infrared,
millimeter and submillimeter lines of many species including. Dust grains are
important charge carriers and play a large role in coupling the magnetic field
and flow of neutral gas. Some effects of the dust on the dynamics of oblique
shocks began to emerge in the 1990s. However, detailed models of these shocks
are required for the calculation of the grain sputtering contribution to gas
phase abundances of species producing observed emissions. We are developing
such models. Some of the molecular species introduced into the gas phase by
sputtering in shocks or by thermally driven desorption in hot cores form on
grain surfaces. Recently laboratory studies have begun to contribute to the
understanding of surface reactions and thermally driven desorption important
for the chemistry of star forming clouds. Dusty plasmas are prevalent in many
evolved stars just as well as in star forming regions. Radiation pressure on
dust plays a significant role in mass loss from some post-main-sequence stars.
The mechanisms leading to the formation of carbonaceous dust in the stellar
outflows are similar to those important for soot formation in flames. However,
nucleation in oxygen-rich outflows is less well understood and remains a
challenging research area. Dust is observed in supernova ejecta that have not
passed through the reverse shocks that develop in the interaction of ejecta
with ambient media. Dust is detected in high redshift galaxies that are
sufficiently young that the only stars that could have produced the dust were
so massive that they became supernovae. Consequently, the issue of the survival
of dust in strong supernova shocks is of considerable interest.Comment: 4 pages, to be published in the proceedings of Fifth International
Conference on Physics of Dusty Plasma
Pyramidal micromirrors for microsystems and atom chips
Concave pyramids are created in the (100) surface of a silicon wafer by anisotropic etching in potassium hydroxide. High quality micromirrors are then formed by sputtering gold onto the smooth silicon (111) faces of the pyramids. These mirrors show great promise as high quality optical devices suitable for integration into micro-optoelectromechanical systems and atom chips. We have shown that structures of this shape can be used to laser-cool and hold atoms in a magneto-optical trap
Thirty Years of Precision Electroweak Physics
We discuss the development of the theory of electroweak radiative corrections
and its role in testing the Standard Model, predicting the top quark mass,
constraining the Higgs boson mass, and searching for deviations that may signal
the presence of new physics.Comment: 19 pages, acknowledgments added, J.J.Sakurai Prize Talk, APS Meeting,
Albuquerque, N.M., April 2002. To appear in a future issue of Journal of
Physics
The Active Traveling Wave in the Cochlea
A sound stimulus entering the inner ear excites a deformation of the basilar
membrane which travels along the cochlea towards the apex. It is well
established that this wave-like disturbance is amplified by an active system.
Recently, it has been proposed that the active system consists of a set of
self-tuned critical oscillators which automatically operate at an oscillatory
instability. Here, we show how the concepts of a traveling wave and of
self-tuned critical oscillators can be combined to describe the nonlinear wave
in the cochlea.Comment: 5 pages, 2 figure
Decays of Scalar and Pseudoscalar Higgs Bosons into Fermions: Two-loop QCD Corrections to the Higgs-Quark-Antiquark Amplitude
As a first step in the aim of arriving at a differential description of
neutral Higgs boson decays into heavy quarks, , to second
order in the QCD coupling , we have computed the
amplitude at the two-loop level in QCD for a general neutral Higgs boson which
has both scalar and pseudoscalar couplings to quarks. This amplitude is given
in terms of a scalar and a pseudoscalar vertex form factor, for which we
present closed analytic expressions in terms of one-dimensional harmonic
polylogarithms of maximum weight 4. The results hold for arbitrary
four-momentum squared, , of the Higgs boson and of the heavy quark mass,
. Moreover we derive the approximate expressions of these form factors near
threshold and in the asymptotic regime .Comment: 56 pages, 2 figure
Feynman Rules for the Rational Part of the Standard Model One-loop Amplitudes in the 't Hooft-Veltman Scheme
We study Feynman rules for the rational part of the Standard Model
amplitudes at one-loop level in the 't Hooft-Veltman scheme.
Comparing our results for quantum chromodynamics and electroweak 1-loop
amplitudes with that obtained based on the Kreimer-Korner-Schilcher (KKS)
scheme, we find the latter result can be recovered when our
scheme becomes identical (by setting in our expressions)
with the KKS scheme. As an independent check, we also calculate Feynman rules
obtained in the KKS scheme, finding our results in complete agreement with
formulae presented in the literature. Our results, which are studied in two
different schemes, may be useful for clarifying the
problem in dimensional regularization. They are helpful to eliminate or find
ambiguities arising from different dimensional regularization schemes.Comment: Version published in JHEP, presentation improved, 41 pages, 10
figure
Gauge and parametrization dependencies of the one-loop counterterms in the Einstein gravity.
The parametrization and gauge dependencies of the one-loop counterterms on
the mass-shell in the Einstein gravity are investigated. The physical meaning
of the loop calculation results on the mass shell and the parametrization
dependence of the renormgroup functions in the nonrenormalizable theories are
discussed.Comment: 14 pages in LATEX (Some references added
Wilson Expansion of QCD Propagators at Three Loops: Operators of Dimension Two and Three
In this paper we construct the Wilson short distance operator product
expansion for the gluon, quark and ghost propagators in QCD, including
operators of dimension two and three, namely, A^2, m^2, m A^2, \ovl{\psi} \psi
and m^3. We compute analytically the coefficient functions of these operators
at three loops for all three propagators in the general covariant gauge. Our
results, taken in the Landau gauge, should help to improve the accuracy of
extracting the vacuum expectation values of these operators from lattice
simulation of the QCD propagators.Comment: 20 pages, no figure
Transient evolution of C-type shocks in dusty regions of varying density
Outflows of young stars drive shocks into dusty, molecular regions. Most
models of such shocks assume that they are steady and propagating perpendicular
to the magnetic field. Real shocks often violate both of these assumptions and
the media through which they propagate are inhomogeneous. We use the code
employed previously to produce the first time-dependent simulations of
fast-mode, oblique C-type shocks interacting with density perturbations. We
include a self-consistent calculation of the thermal and ionisation balances
and a fluid treatment of grains. We identify features that develop when a
multifluid shock encounters a density inhomogeneity to investigate whether any
part of the precursor region ever behaves in a quasi-steady fashion. If it does
the shock may be modelled approximately without solving the time-dependent
hydromagnetic equations. Simulations were made for initially steady oblique
C-type shocks encountering density inhomogeneities. For a semi-finite
inhomogeneity with a density larger than the surrounding medium, a transmitted
shock evolves from being J-type to a steady C-type shock on a timescale
comparable to the ion-flow time through it. A sufficiently upstream part of the
precursor of an evolving J-type shock is quasi-steady. The ion-flow timescale
is also relevant for the evolution of a shock moving into a region of
decreasing density. The models for shocks propagating into regions in which the
density increases and then decreases to its initial value cannot be entirely
described in terms of the results obtained for monotonically increasing and
decreasing densities. For the latter model, the long-term evolution to a C-type
shock cannot be approximated by quasi-steady models.Comment: 11 pages, 9 figure
Inclusive production of charged pions in p+C collisions at 158 GeV/c beam momentum
The production of charged pions in minimum bias p+C interactions is studied
using a sample of 377000 inelastic events obtained with the NA49 detector at
the CERN SPS at 158 GeV/c beam momentum. The data cover a phase space area
ranging from 0 to 1.8 GeV/c in transverse momentum and from -0.1 to 0.5 in
Feynman x. Inclusive invariant cross sections are given on a grid of 270 bins
per charge thus offering for the first time a dense coverage of the projectile
hemisphere and of the cross-over region into the target fragmentation zone.Comment: 31 pages, 30 figures, submitted to European Journal of Physic
- âŠ