54,342 research outputs found
Analytical Behaviour of Positronium Decay Amplitudes
Positronium annihilation amplitudes that are computed by assuming a
factorization approximation with on-shell intermediate leptons do not exhibit
good analytical behaviour. Using dispersion techniques, we find new
contributions that interfere with the known results to restore analytical
properties. Those new amplitudes which cannot be obtained using standard
factorized amplitude formalism, contribute at order alpha^2. Therefore they
have to be evaluated before any theoretical conclusion can be drawn upon the
orthopositronium lifetime puzzle.Comment: LaTeX, 22 pages, 3 eps figure
Parapositronium Decay and Dispersion Relations
Positronium decay rates are computed at the one-loop level, using
convolution-type factorized amplitudes. The dynamics of this factorization is
probed with dispersion relations, showing that unallowed approximations are
usually made, and some ordre alpha^2 corrections missed. Further, we discuss
the relevance of the Schrodinger wavefunction as the basis for perturbative
calculations. Finally, we apply our formalism to the parapositronium two-photon
decay.Comment: LaTeX, 13 pages, 1 eps figur
Dynamics of swimming bacteria at complex interfaces
Flagellated bacteria exploiting helical propulsion are known to swim along
circular trajectories near surfaces. Fluid dynamics predicts this circular
motion to be clockwise (CW) above a rigid surface (when viewed from inside the
fluid) and counter-clockwise (CCW) below a free surface. Recent experimental
investigations showed that complex physicochemical processes at the nearby
surface could lead to a change in the direction of rotation, both at solid
surfaces absorbing slip-inducing polymers and interfaces covered with
surfactants. Motivated by these results, we use a far-field hydrodynamic model
to predict the kinematics of swimming near three types of interfaces: clean
fluid-fluid interface, slipping rigid wall, and a fluid interface covered by
incompressible surfactants. Representing the helical swimmer by a superposition
of hydrodynamic singularities, we first show that in all cases the surfaces
reorient the swimmer parallel to the surface and attract it, both of which are
a consequence of the Stokes dipole component of the swimmer flow field. We then
show that circular motion is induced by a higher-order singularity, namely a
rotlet dipole, and that its rotation direction (CW vs. CCW) is strongly
affected by the boundary conditions at the interface and the bacteria shape.
Our results suggest thus that the hydrodynamics of complex interfaces provide a
mechanism to selectively stir bacteria
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Synthesis of molybdenum oxide nanoparticles by nanosecond laser ablation
Phothermal therapy (PTT) is one of the most promising techniques to treat cancer. Finding the ideal PTT agent nanomaterial has remained a challenge and has brought the interest of several researchers. In this work, we report the synthesis of molybdenum oxide (MoOx) nanoparticles (NPs), which exhibit absorption in the biological optical window ~840 nm, by using the laser ablation of solids in liquids (LASL) technique with nanosecond (ns) pulses. A Nd:YAG laser was used to synthesize the NPs in deionized (DI) water, free of surfactants or additives, which were optically characterized by absorption spectroscopy and TEM-EDX microscopy. Semi spherical NPs with a suitable average size and shape for potential use as PTT agents were obtained by laser ablation and ablation + fragmentation. The calculated band gap is 3.1 eV, which corresponds to MoO3. Micro-Raman spectroscopy studies determined that these NPs are composed of amorphous molybdenum oxide hydrates (MoO3 · xH2O)
New Precision Electroweak Tests of SU(5) x U(1) Supergravity
We explore the one-loop electroweak radiative corrections in supergravity via explicit calculation of vacuum-polarization and
vertex-correction contributions to the and
parameters. Experimentally, these parameters are obtained from a global fit to
the set of observables , and . We
include -dependent effects, which induce a large systematic negative shift
on for light chargino masses (m_{\chi^\pm_1}\lsim70\GeV). The
(non-oblique) supersymmetric vertex corrections to \Zbb, which define the
parameter, show a significant positive shift for light chargino
masses, which for can be nearly compensated by a negative
shift from the charged Higgs contribution. We conclude that at the 90\%CL, for
m_t\lsim160\GeV the present experimental values of and
do not constrain in any way supergravity in
both no-scale and dilaton scenarios. On the other hand, for m_t\gsim160\GeV
the constraints on the parameter space become increasingly stricter. We
demonstrate this trend with a study of the m_t=170\GeV case, where only a
small region of parameter space, with \tan\beta\gsim4, remains allowed and
corresponds to light chargino masses (m_{\chi^\pm_1}\lsim70\GeV). Thus
supergravity combined with high-precision LEP data would
suggest the presence of light charginos if the top quark is not detected at the
Tevatron.Comment: LaTeX, 11 Pages+4 Figures(not included), the figures available upon
request as an uuencoded file(0.4MB) or 4 PS files from [email protected],
CERN-TH.7078/93, CTP-TAMU-68/93, ACT-24/9
Sample Size Requirements and Considerations for Models to Assess Human-Machine System Performance
Hierarchical Linear Models (HLMs), also known as multi-level models, are an extension of multiple regression analysis and can aid in the understanding of human and machine workloads of a system. These models allow for prediction and testing in systems with hierarchies of two or more levels. The complex interrelated variability of these multi-level models exists in operational settings, such as the Air Force Distributed Common Ground System Full Motion Video (AF DCGS FMV) community which is composed of individuals (Level-1), groups (Level-2), units (Level-3), and organizations (Level-4). Through the development of sample size requirements and considerations for multi-level models, this research determined necessary requirements and strategies to assess human-machine system performance based requests for statistical testing and evaluation. This research compares sample size recommendations to previous Level-2 HLM recommendations and extends recommendations to Level-3 and Level-4 HLMs based on varying effect sizes, level predictor variance, and error variance scenarios. Depending on the application, results demonstrate that sample size requirements may be smaller than what literature previously reported. Further, when sample size requirements cannot be met, this research develops and assesses re-sampling methods as a means to augment small samples for estimation. The operational community, DCGS FMV, has a small population of 1,000 and limited access to analysts. This research provides distributions based on Subject Matter Expert (SME) opinion for re-sampling methods to the DCGS FMV Level-3 and Level-4 HLMs. These findings provide a foundation on which sample size recommendations and extrapolation techniques for HLM are made. Such recommendations and techniques with SME input provide simulation based HLM assessments that can give initial recommendations with limited associated cost, analyst and researcher time, and resources. These are initial steps towards advancing the Air Force\u27s capabilities and mission understanding for human-machine based systems
Spatially resolved H_2 emission from a very low-mass star
Molecular outflows from very low-mass stars (VLMSs) and brown dwarfs have
been studied very little. So far, only a few CO outflows have been observed,
allowing us to map the immediate circumstellar environment. We present the
first spatially resolved H2 emission around IRS54 (YLW52), a ~0.1-0.2 Msun
Class I source. By means of VLT SINFONI K-band observations, we probed the H2
emission down to the first ~50 AU from the source. The molecular emission shows
a complex structure delineating a large outflow cavity and an asymmetric
molecular jet. Thanks to the detection of several H2 transitions, we are able
to estimate average values along the jet-like structure (from source position
to knot D) of Av~28 mag, T~2000-3000 K, and H2 column density N(H2)~1.7x10^17
cm^-2. This allows us to estimate a mass loss rate of ~2x10^-10 Msun/yr for the
warm H2 component . In addition, from the total flux of the Br Gamma line, we
infer an accretion luminosity and mass accretion rate of 0.64 Lsun and ~3x10^-7
Msun/yr, respectively. The outflow structure is similar to those found in
low-mass Class I and CTTS. However, the Lacc/Lbol ratio is very high (~80%),
and the mass accretion rate is about one order of magnitude higher when
compared to objects of roughly the same mass, pointing to the young nature of
the investigated source.Comment: accepted as a Letter in A&
SUSY signals at HERA in the no-scale flipped SU(5) supergravity model
Sparticle production and detection at HERA are studied within the recently
proposed no-scale flipped supergravity model. Among the various
reaction channels that could lead to sparticle production at HERA, only the
following are within its limit of sensitivity in this model: , where are the
two lightest neutralinos and is the lightest chargino. We study the
elastic and deep-inelastic contributions to the cross sections using the
Weizs\"acker-Williams approximation. We find that the most promising
supersymmetric production channel is right-handed selectron ()
plus first neutralino (), with one hard electron and missing energy
signature. The channel leads to comparable rates but also
allows jet final states. A right-handedly polarized electron beam at HERA would
shut off the latter channel and allow preferentially the former one. With an
integrated luminosity of {\cal L}=100\ipb, HERA can extend the present LEPI
lower bounds on by
\approx25\GeV, while {\cal L}=1000\ipb will make HERA competitive with
LEPII. We also show that the Leading Proton Spectrometer (LPS) at HERA is an
excellent supersymmetry detector which can provide indirect information about
the sparticle masses by measuring the leading proton longitudinal momentum
distribution.Comment: 11 pages, 8 figures (available upon request as uuencoded file or
separate ps files), tex (harvmac) CTP-TAMU-15/93, CERN/LAA/93-1
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