112 research outputs found
Has the character of gastric cancer changed? A descriptive study of a IO-year period
Over the 10-year period January 1976 - December 1985, 446 patients with histologically verified adenocarcinoma of the stomach were treated at Tygerberg Hospital. Coloured patients made up 63,4% of the study population and a significant increase in the annual proportion of this group was observed. Coloured men comprised 47,6% of the total group. The mean age of white and coloured patients differed significantly (68,9 v. 56,5; P < 0,001). The symptom complex was essentially similar in the two racial groups and in general the character of the symptoms had no bearing on the prevalence of resection. Although antral tumours were most common in whites and in coloureds, there was a significant increase in tumours located in the fundus in whites. The resection rate remained unchanged over the 10-year period. Only 4 cases of early gastric cancer were detected during this period without any signs of an increased yield of early lesions over time. This audit revealed no beneficial changes over time, which is in stark contrast with reports from Japan regarding the proportion of curable lesions
Marginally trapped tubes and dynamical horizons
We investigate the generic behaviour of marginally trapped tubes (roughly
time-evolved apparent horizons) using simple, spherically symmetric examples of
dust and scalar field collapse/accretion onto pre-existing black holes. We find
that given appropriate physical conditions the evolution of the marginally
trapped tube may be either null, timelike, or spacelike and further that the
marginally trapped two-sphere cross-sections may either expand or contract in
area. Spacelike expansions occur when the matter falling into a black hole
satisfies , where is the area of the horizon while
and are respectively the density and pressure of the matter.
Timelike evolutions occur when is greater than this cut-off and so
would be expected to be more common for large black holes. Physically they
correspond to horizon "jumps" as extreme conditions force the formation of new
horizons outside of the old.Comment: 31 pages, many figures. Final Version to appear in CQG: improvements
include more complete references, a discussion of those references,
Penrose-Carter diagrams for several of the spacetimes, and improved numerics
for the scalar field
Litho- and chemostratigraphy of the Flatreef PGE deposit, northern Bushveld Complex
The Flatreef is a world-class platinum-group element (PGE) deposit recently discovered down-dip from existing mining and exploration operations on the northern limb of the Bushveld Complex. Current indicated resources stand at 42 Moz PGE (346 Mt with 3.8 g/t Pt+Pd+Rh+Au, 0.32% Ni and 0.16% Cu) which, in the case of Pt, is equivalent to ~ 10 years global annual production, making it one of the largest PGE deposits on earth. The grade and thickness of the Flatreef mineralised interval is highly unusual, with some drill core intersections containing up to 4.5 g/t Pt+Pd+Rh+Au over 90 m in drill core. Here, we document the down-dip and along-strike litho- and chemostratigraphy of the Flatreef and its footwall and hanging wall rocks, based on a diamond drill core database totalling > 720 km. At the base of the sequence intersected in the drill cores are up to 700-m-thick sills of ultramafic rocks (dunite, harzburgite, pyroxenite) emplaced into pelitic, dolomitic, and locally quartzitic and evaporitic rocks belonging to the Duitschland Formation of the Transvaal Supergroup. Next is an approximately 100–200-m sequence of low-grade-sulphide-mineralised, layered mafic-ultramafic rocks containing abundant sedimentary xenoliths and, in places, several chromite seams or stringers. This is overlain by a ~ 100-m-thick sequence of well-mineralised mafic-ultramafic rocks (the Flatreef sensu strictu), overlain by a laterally persistent mottled compositional analogies at the base of > 1 km of homogenous Main Zone gabbronorite. Based on stratigraphic, lithological and compositional alanalogies to the layered rocks in the eastern and western Bushveld Complex, we correlate the Flatreef and its chromite bearing footwall rocks with the Upper Critical Zone, notably the interval between the UG2 chromitite and the Bastard Reef as found elsewhere in the Bushveld Complex. This includes recognition of a Merensky Reef correlative. The ultramafic rocks below the main chromitite seam (UG2 correlative) in the Flatreef footwall are correlated with the Lower Critical and Lower zones. However, compared to the western and eastern Bushveld limbs, the studied sequence is strongly enriched in sulphide and PGE, many of the rocks show elevated CaO, K2O, Rb and Zr contents, and lateral continuity of layers between drill cores is less pronounced than elsewhere in the Bushveld, whereas ultramafic units are locally considerably thickened. These compositional and lithological traits are interpreted to result from a range of processes which include contamination with calcsilicate and hornfels, intrusion of granitic magmas, and the influence of multiple structural events such as pre- to syn-emplacement regional-scale open folding and growth faults. Evidence for the existence of potholes also exists. In the shallow, up-dip portions of the project area, the entire magmatic sequence below the Main Zone becomes increasingly contaminated to the extent that individual units are somewhat more difficult to correlate between drill cores. This package represents the Platreef as exposed in outcrop and shallow bore holes across much of the northern limb of the Bushveld Complex. The new data presented here thus indicate that the Platreef is a relatively more contaminated up-dip extension of parts of the Critical and Lower zones
Horizon energy and angular momentum from a Hamiltonian perspective
Classical black holes and event horizons are highly non-local objects,
defined in terms of the causal past of future null infinity. Alternative,
(quasi)local definitions are often used in mathematical, quantum, and numerical
relativity. These include apparent, trapping, isolated, and dynamical horizons,
all of which are closely associated to two-surfaces of zero outward null
expansion. In this paper we show that three-surfaces which can be foliated with
such two-surfaces are suitable boundaries in both a quasilocal action and a
phase space formulation of general relativity. The resulting formalism provides
expressions for the quasilocal energy and angular momentum associated with the
horizon. The values of the energy and angular momentum are in agreement with
those derived from the isolated and dynamical horizon frameworks.Comment: 39 pages, 3 figures, Final Version : content essentially unchanged
but many small improvements made in response to referees, a few references
adde
Statistical gamma-ray decay studies at iThemba LABS
Abstract. A program to study the γ -ray decay from the region of high-level density has been established
at iThemba LABS, where a high-resolution gamma-ray detector array is used in conjunction with silicon
particle-telescopes. Results from two recent projects are presented: 1) The 74Ge(α, α
γ ) reaction was used
to investigate the Pygmy Dipole Resonance. The results were compared to (γ,γ
) data and indicate that the
dipole states split into mixed isospin and relatively pure isovector excitations. 2) Data from the 95Mo(d,p)
reaction were used to develop a novel method for the determination of spins for low-lying discrete levels
utilizing statistical γ -ray decay in the vicinity of the neutron separation energy. These results provide insight
into the competition of (γ ,n) and (γ,γ
) reactions and highlights the need to correct for angular momentum
barrier effect
Nonperturbative renormalization group approach to frustrated magnets
This article is devoted to the study of the critical properties of classical
XY and Heisenberg frustrated magnets in three dimensions. We first analyze the
experimental and numerical situations. We show that the unusual behaviors
encountered in these systems, typically nonuniversal scaling, are hardly
compatible with the hypothesis of a second order phase transition. We then
review the various perturbative and early nonperturbative approaches used to
investigate these systems. We argue that none of them provides a completely
satisfactory description of the three-dimensional critical behavior. We then
recall the principles of the nonperturbative approach - the effective average
action method - that we have used to investigate the physics of frustrated
magnets. First, we recall the treatment of the unfrustrated - O(N) - case with
this method. This allows to introduce its technical aspects. Then, we show how
this method unables to clarify most of the problems encountered in the previous
theoretical descriptions of frustrated magnets. Firstly, we get an explanation
of the long-standing mismatch between different perturbative approaches which
consists in a nonperturbative mechanism of annihilation of fixed points between
two and three dimensions. Secondly, we get a coherent picture of the physics of
frustrated magnets in qualitative and (semi-) quantitative agreement with the
numerical and experimental results. The central feature that emerges from our
approach is the existence of scaling behaviors without fixed or pseudo-fixed
point and that relies on a slowing-down of the renormalization group flow in a
whole region in the coupling constants space. This phenomenon allows to explain
the occurence of generic weak first order behaviors and to understand the
absence of universality in the critical behavior of frustrated magnets.Comment: 58 pages, 15 PS figure
The Chiral MagnetoHydroDynamics of QCD fluid at RHIC and LHC
The experimental results on heavy ion collisions at RHIC and LHC indicate
that QCD plasma behaves as a nearly perfect fluid described by relativistic
hydrodynamics. Hydrodynamics is an effective low-energy Theory Of Everything
stating that the response of a system to external perturbations is dictated by
conservation laws that are a consequence of the symmetries of the underlying
theory. In the case of QCD fluid produced in heavy ion collisions, this theory
possesses anomalies, so some of the apparent classical symmetries are broken by
quantum effects. Even though the anomalies appear as a result of UV
regularization and so look like a short distance phenomenon, it has been
realized recently that they also affect the large distance, macroscopic
behavior in hydrodynamics. One of the manifestations of anomalies in
relativistic hydrodynamics is the Chiral Magnetic Effect (CME). At this
conference, a number of evidences for CME have been presented, including i) the
disappearance of charge asymmetry fluctuations in the low-energy RHIC data
where the energy density is thought to be below the critical one for
deconfinement; ii) the observation of charge asymmetry fluctuations in Pb-Pb
collisions at the LHC. Here I give a three-page summary of some of the recent
theoretical and experimental developments and of the future tests that may
allow to establish (or to refute) the CME as the origin of the observed charge
asymmetry fluctuations.Comment: 4 pages, talk at Quark Matter 2011 Conference, Annecy, France, 23-28
May 201
A Chiral Magnetic Effect from AdS/CFT with Flavor
For (3+1)-dimensional fermions, a net axial charge and external magnetic
field can lead to a current parallel to the magnetic field. This is the chiral
magnetic effect. We use gauge-gravity duality to study the chiral magnetic
effect in large-Nc, strongly-coupled N=4 supersymmetric SU(Nc) Yang-Mills
theory coupled to a number Nf << Nc of N=2 hypermultiplets in the Nc
representation of SU(Nc), i.e. flavor fields. Specifically, we introduce an
external magnetic field and a time-dependent phase for the mass of the flavor
fields, which is equivalent to an axial chemical potential for the flavor
fermions, and we compute holographically the resulting chiral magnetic current.
For massless flavors we find that the current takes the value determined by the
axial anomaly. For massive flavors the current appears only in the presence of
a condensate of pseudo-scalar mesons, and has a smaller value than for massless
flavors, dropping to zero for sufficiently large mass or magnetic field. The
axial symmetry in our system is part of the R-symmetry, and the states we study
involve a net flow of axial charge to the adjoint sector from an external
source coupled to the flavors. We compute the time rate of change of axial
charge and of energy both in field theory and from holography, with perfect
agreement. In contrast to previous holographic models of the chiral magnetic
effect, in our system the vector current is conserved and gauge-invariant
without any special counterterms.Comment: 54 pages, 18 eps files in 6 figure
Holographic Gravitational Anomaly and Chiral Vortical Effect
We analyze a holographic model with a pure gauge and a mixed
gauge-gravitational Chern-Simons term in the action. These are the holographic
implementations of the usual chiral and the mixed gauge-gravitational anomalies
in four dimensional field theories with chiral fermions. We discuss the
holographic renormalization and show that the gauge-gravitational Chern-Simons
term does not induce new divergences. In order to cancel contributions from the
extrinsic curvature at a boundary at finite distance a new type of counterterm
has to be added however. This counterterm can also serve to make the Dirichlet
problem well defined in case the gauge field strength vanishes on the boundary.
A charged asymptotically AdS black hole is a solution to the theory and as an
application we compute the chiral magnetic and chiral vortical conductivities
via Kubo formulas. We find that the characteristic term proportional to T^2 is
present also at strong coupling and that its numerical value is not
renormalized compared to the weak coupling result.Comment: 27 pages, no figure
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