9,237 research outputs found
New Gauged N=8, D=4 Supergravities
New gaugings of four dimensional N=8 supergravity are constructed, including
one which has a Minkowski space vacuum that preserves N=2 supersymmetry and in
which the gauge group is broken to . Previous gaugings used the
form of the ungauged action which is invariant under a rigid symmetry
and promoted a 28-dimensional subgroup ( or the
non-semi-simple contraction ) to a local gauge group. Here, a
dual form of the ungauged action is used which is invariant under
instead of and new theories are obtained by gauging 28-dimensional
subgroups of . The gauge groups are non-semi-simple and are different
real forms of the groups, denoted , and the new
theories have a rigid SU(2) symmetry. The five dimensional gauged N=8
supergravities are dimensionally reduced to D=4. The gauge
theories reduce, after a duality transformation, to the
gauging while the gauge theory reduces to the gauge
theory. The new theories are related to the old ones via an analytic
continuation. The non-semi-simple gaugings can be dualised to forms with
different gauge groups.Comment: 33 pages. Reference adde
Singular Contractions of W-algebras
Many -algebras (e.g. the algebras) are consistent for all values of
the central charge except for a discrete set of exceptional values. We show
that such algebras can be contracted to new consistent degenerate algebras at
these exceptional values of the central charge.Comment: 10 pages, phyzzx.tex, QMW-92-7.(minor spelling and acknowledgement
corrections
Euclidean Supersymmetry, Twisting and Topological Sigma Models
We discuss two dimensional N-extended supersymmetry in Euclidean signature
and its R-symmetry. For N=2, the R-symmetry is SO(2)\times SO(1,1), so that
only an A-twist is possible. To formulate a B-twist, or to construct Euclidean
N=2 models with H-flux so that the target geometry is generalised Kahler, it is
necessary to work with a complexification of the sigma models. These issues are
related to the obstructions to the existence of non-trivial twisted chiral
superfields in Euclidean superspace.Comment: 8 page
Can aggregate quarry silt lagoons provide resources for wading birds?
Wading birds have declined across Europe as the intensification of lowland agriculture has resulted in the loss and degradation of wetland areas. Lowland aggregate extraction sites that incorporate areas of fine, waste sediments deposited in silt lagoons have the potential to be restored for wader conservation. We set out to determine the potential value of silt lagoons to wading birds by comparing the water quality, sediment profiles, aquatic invertebrate abundance and diversity (prey availability) and wader site use at five sites representing various stages of active aggregate extraction and restoration for conservation purposes. Wader counts were conducted monthly over a twelve month period using replicated scan samples, and the invertebrate communities studied during the breeding and autumn migration season (June–September). Water quality variables were similar between sites, but sediments from active quarries were dominated by moderately sorted fine sands in comparison to the coarser sediment profiles of restored areas. June and September there was no significant difference in invertebrate diversity between sites, however richness was significantly lower on quarry sites and total abundance a factor of ten higher at restored sites than on silt lagoons, with the dominant taxa similar across all sites. Waders used all sites; albeit at lower abundance and richness on silt lagoons and two species were recorded breeding on active silting sites. We conclude that the fine, uniform sediments of modern silt lagoons limited invertebrate diversity and abundance, diminishing the value of silt lagoons to waders. Simple low-cost intervention measures increasing substrate heterogeneity and creating temporary ponds could increase invertebrate richness and abundance, and enhance the conservation potential of these sites
Timelike Hopf Duality and Type IIA^* String Solutions
The usual T-duality that relates the type IIA and IIB theories compactified
on circles of inversely-related radii does not operate if the dimensional
reduction is performed on the time direction rather than a spatial one. This
observation led to the recent proposal that there might exist two further
ten-dimensional theories, namely type IIA^* and type IIB^*, related to type IIB
and type IIA respectively by a timelike dimensional reduction. In this paper we
explore such dimensional reductions in cases where time is the coordinate of a
non-trivial U(1) fibre bundle. We focus in particular on situations where there
is an odd-dimensional anti-de Sitter spacetime AdS_{2n+1}, which can be
described as a U(1) bundle over \widetilde{CP}^n, a non-compact version of CP^n
corresponding to the coset manifold SU(n,1)/U(n). In particular, we study the
AdS_5\times S^5 and AdS_7\times S^4 solutions of type IIB supergravity and
eleven-dimensional supergravity. Applying a timelike Hopf T-duality
transformation to the former provides a new solution of the type IIA^* theory,
of the form \widetilde{CP}^2\times S^1\times S^5. We show how the Hopf-reduced
solutions provide further examples of ``supersymmetry without supersymmetry.''
We also present a detailed discussion of the geometrical structure of the
Hopf-fibred metric on AdS_{2n+1}, and its relation to the horospherical metric
that arises in the AdS/CFT correspondence.Comment: Latex, 26 page
BIOMECHANICS OF BICYCLE PEDALLING
Five pedalling conditions were investigated for three test subjects using a six load component pedal dynamometer. EMG electrodes simultaneously monitored the activity of eight leg muscles. Both pedal dynamometer and EMG data were digitized by a minicomputer. A five-bar linkage model of the leg-bicycle system was used to calculate the joint moments of the leg.
Data analysis entailed generating plots of joint moments due to pedal load only and acceleration only. Total moments were produced by superimposing the two moment histories . The separate moment histories, together with the pedal forces and IEMG results, enable a detailed picture of the biomechanics of bicycle pedalling
JOINT MOMENTS AND PEDALLING RATES IN BICYCLING
Joint moments are of interest because they bear some relation to muscular effort and hence rider performance. The general objective o f this study is to explore the relation between joint moments and cadence. Joint moments are computed by modelling the leg-bicycle system as a five-bar linkage constrained to plane motion. Using dynamometer pedal force data and potentiometer crank and pedal position data, system equations are solved on a computer to produce moments at the ankle, knee, and hip joints . Cadence and pedal forces are varied inversely to maintain constant power. Results indicate that average joint moments vary considerably with changes in cadence. Both hip and knee joints show an average moment which is minimum near 105 RPM for cruising cycling. It appears that an optimum RPM can be determined from a mechanical approach for any given power level and bicycle-rider geometry
MEASUREMENT OF RIDER INDUCED LOADS DURING SIMULATED BICYCLING
Research related to bicycling has broadened in scope over the last decade. Prior to about 1975, the majority of bicycling related research was dedicated to topics surrounding the physiology of human performance. These early efforts served to stimulate interest in bicycling research with the result that more recent research has explored a diversity of topics ranging from fundamentals of muscle mechanics to measurement of foot/pedal loads. Despite the both broadened and intensified research activity, one topic, which has recei ved no attention to the authors I knowledge, is measurement of the complete loading induced by the rider on the bicycle frame. The importance of this topic lies in the applicability of the results to two areas, design analysis of bicycle components including the frame and biomechanical analysis of the pedalling process. The concern in the present article is with the biomechanical analysis
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