990 research outputs found
2. Survey Of Urban Blood Donors And Rural Populations
A CAJM survey on the prevalence of Australia Antigen in the blood of Zimbabweans (formerly Rhodesians)Consequently a more comprehensive survey was undertaken to determine the general prevalence of the antigen in Rhodesia by testing African and European blood donors as examples of urban populations and, in contrast, certain African tribal groups as examples of rural populations. The results are presented here together with the findings in cases of hepatitis and leprosy which have come to hand subsequent to the pilot survey
Dressing Up the Kink
Many quantum field theoretical models possess non-trivial solutions which are
stable for topological reasons. We construct a self-consistent example for a
self-interacting scalar field--the quantum (or dressed) kink--using a two
particle irreducible effective action in the Hartree approximation. This new
solution includes quantum fluctuations determined self-consistently and
nonperturbatively at the 1-loop resummed level and allowed to backreact on the
classical mean-field profile. This dressed kink is static under the familiar
Hartree equations for the time evolution of quantum fields. Because the quantum
fluctuation spectrum is lower lying in the presence of the defect, the quantum
kink has a lower rest energy than its classical counterpart. However its energy
is higher than well-known strict 1-loop results, where backreaction and
fluctuation self-interactions are omitted. We also show that the quantum kink
exists at finite temperature and that its profile broadens as temperature is
increased until it eventually disappears.Comment: 13 pages, latex, 3 eps figures; revised with yet additional
references, minor rewordin
Can invasions occur without change? A comparison of G-matrices and selection in the peach-potato aphid, Myzus persicae
Most evolutionary research on biological invasions has focused on changes seen between the native and invaded range for a particular species. However, it is likely that species that live in human-modified habitats in their native range might have evolved specific adaptations to those environments, which increase the likelihood of establishment and spread in similar human-altered environments. From a quantitative genetic perspective, this hypothesis suggests that both native and introduced populations should reside at or near the same adaptive peak. Therefore, we should observe no overall changes in the G (genetic variance–covariance) matrices between native and introduced ranges, and stabilizing selection on fitness-related traits in all populations. We tested these predictions comparing three populations of the worldwide pest Myzus persicae from the Middle East (native range) and the UK and Chile (separately introduced ranges). In general, our results provide mixed support for this idea, but further comparisons of other species are needed. In particular, we found that there has been some limited evolution in the studied traits, with the Middle East population differing from the UK and Chilean populations. This was reflected in the structure of the G-matrices, in which Chile differed from both UK and Middle East populations. Furthermore, the amount of genetic variation was massively reduced in Chile in comparison with UK and Middle East populations. Finally, we found no detectable selection on any trait in the three populations, but clones from the introduced ranges started to reproduce later, were smaller, had smaller offspring, and had lower reproductive fitness than clones from the native range
Breakdown of correspondence in chaotic systems: Ehrenfest versus localization times
Breakdown of quantum-classical correspondence is studied on an experimentally
realizable example of one-dimensional periodically driven system. Two relevant
time scales are identified in this system: the short Ehrenfest time t_h and the
typically much longer localization time scale T_L. It is shown that
surprisingly weak modification of the Hamiltonian may eliminate the more
dramatic symptoms of localization without effecting the more subtle but
ubiquitous and rapid loss of correspondence at t_h.Comment: 4 pages, 5 figures, replaced with a version submitted to PR
A Way to Reopen the Window for Electroweak Baryogenesis
We reanalyse the sphaleron bound of electroweak baryogenesis when allowing
deviations to the Friedmann equation. These modifications are well motivated in
the context of brane cosmology where they appear without being in conflict with
major experimental constraints on four-dimensional gravity. While suppressed at
the time of nucleosynthesis, these corrections can dominate at the time of the
electroweak phase transition and in certain cases provide the amount of
expansion needed to freeze out the baryon asymmetry without requiring a
strongly first order phase transition. The sphaleron bound is substantially
weakened and can even disappear so that the constraints on the higgs and stop
masses do not apply anymore. Such modification of cosmology at early times
therefore reopens the parameter space allowing electroweak baryogenesis which
had been reduced substantially given the new bound on the higgs mass imposed by
LEP. In contrast with previous attempts to turn around the sphaleron bound
using alternative cosmologies, we are still considering that the electroweak
phase transition takes place in a radiation dominated universe. The universe is
expanding fast because of the modification of the Friedmann equation itself
without the need for a scalar field and therefore evading the problem of the
decay of this scalar field after the completion of the phase transition and the
risk that its release of entropy dilutes the baryon asymmetry produced at the
transition.Comment: 19 pages, 3 figures; v2: minor changes, remark added at end of
section 5 and in caption of figure 1; v3: references added, version to be
publishe
Lectures on conformal field theory and Kac-Moody algebras
This is an introduction to the basic ideas and to a few further selected
topics in conformal quantum field theory and in the theory of Kac-Moody
algebras.Comment: 59 pages, LaTeX2e, extended version of lectures given at the Graduate
Course on Conformal Field Theory and Integrable Models (Budapest, August
1996), to appear in Springer Lecture Notes in Physic
Whatever happened to curriculum theory? Critical realism and curriculum change
In the face of what has been characterised as a ‘crisis’ in curriculum – an apparent decline of some aspects of curriculum studies combined with the emergence of new types of national curriculum which downgrade knowledge – some writers have been arguing for the use of realist theory to address these issues. This paper offers a contribution to this debate, drawing upon critical realism, and especially upon the social theory of Margaret Archer. The paper first outlines the supposed crisis in curriculum, before providing an overview of some of the key tenets of critical realism. The paper concludes by speculating on how critical realism may offer new ways of thinking to inform policy and practice in a key curricular problematic. This is the issue of curriculum change
Measurement of the Charged Multiplicities in b, c and Light Quark Events from Z0 Decays
Average charged multiplicities have been measured separately in , and
light quark () events from decays measured in the SLD experiment.
Impact parameters of charged tracks were used to select enriched samples of
and light quark events, and reconstructed charmed mesons were used to select
quark events. We measured the charged multiplicities:
,
, from
which we derived the differences between the total average charged
multiplicities of or quark events and light quark events: and . We compared
these measurements with those at lower center-of-mass energies and with
perturbative QCD predictions. These combined results are in agreement with the
QCD expectations and disfavor the hypothesis of flavor-independent
fragmentation.Comment: 19 pages LaTex, 4 EPS figures, to appear in Physics Letters
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