29,314 research outputs found
Nonrelativistic Bound States in Quantum Field Theory
Get PDFNonrelativistic bound states are studied using an effective field theory.
Large logarithms in the effective theory can be summed using the velocity
renormalization group. For QED, one can determine the structure of the leading
and next-to-leading order series for the energy, and compute corrections up to
order alpha^8 ln^3 alpha, which are relevant for the present comparison between
theory and experiment. For QCD, one can compute the velocity renormalization
group improved quark potentials. Using these to compute the renormalization
group improved t-tbar production cross-section near threshold gives a result
with scale uncertainties of 2%, a factor of 10 smaller than existing fixed
order calculations.Comment: Lattice 2000 (plenary). 13 page
Ephedrine requirements are reduced during spinal anaesthesia for caesarean section in preeclampsia
Get PDFPart of the Portfolio Thesis by Geoffrey H. Sharwood-Smith: The inferior vena caval compression theory of hypotension in obstetric spinal anaesthesia: studies in normal and preeclamptic pregnancy, a literature review and revision of fundamental concepts, available at http://hdl.handle.net/10023/1815Background:
Despite controversy over the haemodynamically safest blockade for caesarean section in women with severe preeclampsia, an increasing number of anaesthetists now opt for spinal anaesthesia. In a previous study we found that spinal compared to epidural anaesthesia offered an equally safe but more effective option for these patients. The current study was designed to compare the hypotension induced by spinal anaesthesia, as measured by ephedrine requirement, between 20 normotensive and 20 severely preeclamptic but haemodynamically stabilised women.
Method:
Standardised spinal anaesthesia was instituted and ephedrine was given in boluses of 6 mg if the systolic pressure fell >20% from the baseline, or if the patient exhibited symptoms of hypotension.
Results:
The mean ephedrine requirement of the normotensive group (27.9 ± 11.6 mg) was significantly greater (P < 0.01) than that of the preeclamptic group (16.4 ± 15.0 mg).
Conclusion:
This suggests that the hypotension induced by spinal anaesthesia in women with severe but haemodynamically stabilised preeclampsia, is less than that of normotensive patients.Publisher PD
A Perspective on Hadron Physics
Full text linkThe phenomena of confinement and dynamical chiral symmetry breaking are basic
to understanding hadron observables. They can be explored using Dyson-Schwinger
equations. The existence of a systematic, nonperturbative and symmetry
preserving truncation of these equations enables the proof of exact results in
QCD, and their illustration using simple but accurate models. We provide a
sketch of the material qualitative and quantitative success that has been
achieved in the study of pseudoscalar and vector mesons. Efforts are now
turning to the study of baryons, which we exemplify via a calculation of
nucleon weak and pionic form factors.Comment: 16 pages, 5 figures. Contribution to the proceedings of the "Xth
Mexican Workshop on Particles and Fields," Morelia, Mexico, 6-12 Nov. 200
Catalytic surface effects on space thermal protection system during Earth entry of flights STS-2 through STS-5
Get PDFAn on going orbiter experiment catalytic surface effects experiment being conducted on the Space Shuttle is discussed. The catalytic surface effects experiment was peformed on four of the five flights of Columbia. Temperature time histories and distributions along the midfuselage and wing of the orbiter were used to determine the surface catalytic efficiency of the baseline high temperature reusable surface insulation. Correlation parameters are shown that allow the comparison of all flight data with predictions from the design and surface emittance decreased as a result of contaminants during the five flights of the Space Shuttle
Very-large-scale motions in rough-bed open-channel flow
Get PDFAcknowledgements The study has been supported by two EPSRC/UK grants, ‘High-resolution numerical and experimental studies of turbulence-induced sediment erosion and near-bed transport’ (EP/G056404/1) and ‘Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification’ (EP/K041169/1). Discussions with I. Marusic and comments of three anonymous reviewers are greatly appreciated.Peer reviewedPublisher PD
The Sigma Commutator from Lattice QCD
Get PDFAs a direct source of information on chiral symmetry breaking within QCD, the
sigma commutator is of considerable importance. Since hadron structure is a
non-perturbative problem, numerical calculations on a space-time lattice are
currently the only rigorous approach. With recent advances in the calculation
of hadron masses within full QCD, it is of interest to see whether the sigma
commutator can be calculated directly from the dependence of the nucleon mass
on the input quark mass. We show that, provided the correct chiral behaviour of
QCD is respected in the extrapolation to realistic quark masses, one can indeed
obtain a fairly reliable determination of the sigma commutator using present
lattice data. For two-flavour dynamical fermion QCD the sigma commutator lies
between 45 and 55 MeV based on recent data from CP-PACS and UKQCD.Comment: 4 pages, 3 figures, uses espcrc1.sty and epsfig.sty. Contribution to
the proceedings of the International Conference on Quark Nuclear Physics held
in Adelaide Feb. 200
Generation of slow intense optical solitons in a resonance photonic crystal
Full text linkWe demonstrate interesting and previously unforeseen properties of a pair of
gap solitons in a resonant photonic crystal which are predicted and explained
in a physically transparent form using both analytical and numerical methods.
The most important result is the fact that an oscillating gap soliton created
by the presence of a localized population inversion inside the crystal can be
manipulated by means of a proper choice of bit rate, phase and amplitude
modulation. Developing this idea, we are able to obtain qualitatively different
regimes of a resonant photonic crystal operation. In particular, a noteworthy
observation is that both the delay time and amplitude difference must exceed a
certain level to ensure effective control over the soliton dynamics
Spatiotemporal symmetries in the disynaptic canal-neck projection
Get PDFThe vestibular system in almost all vertebrates, and in particular in humans, controls
balance by employing a set of six semicircular canals, three in each inner ear, to detect angular
accelerations of the head in three mutually orthogonal coordinate planes. Signals from the canals are
transmitted to eight (groups of) neck motoneurons, which activate the eight corresponding muscle
groups. These signals may be either excitatory or inhibitory, depending on the direction of head
acceleration. McCollum and Boyle have observed that in the cat the relevant network of neurons
possesses octahedral symmetry, a structure that they deduce from the known innervation patterns
(connections) from canals to muscles. We rederive the octahedral symmetry from mathematical
features of the probable network architecture, and model the movement of the head in response to
the activation patterns of the muscles concerned. We assume that connections between neck muscles
can be modeled by a “coupled cell network,” a system of coupled ODEs whose variables correspond
to the eight muscles, and that this network also has octahedral symmetry. The network and its
symmetries imply that these ODEs must be equivariant under a suitable action of the octahedral
group. It is observed that muscle motoneurons form natural “push-pull pairs” in which, for given
movements of the head, one neuron produces an excitatory signal, whereas the other produces an
inhibitory signal. By incorporating this feature into the mathematics in a natural way, we are led
to a model in which the octahedral group acts by signed permutations on muscle motoneurons.
We show that with the appropriate group actions, there are six possible spatiotemporal patterns of
time-periodic states that can arise by Hopf bifurcation from an equilibrium representing an immobile
head. Here we use results of Ashwin and Podvigina. Counting conjugate states, whose physiological
interpretations can have significantly different features, there are 15 patterns of periodic oscillation,
not counting left-right reflections or time-reversals as being different. We interpret these patterns
as motions of the head, and note that all six types of pattern appear to correspond to natural head
motions
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