15,905 research outputs found
Detection of K+ mesons in segmented electromagnetic calorimeters
The combination of the CrystalBall and TAPS electromagnetic calorimeters were installed in the MAMI A2 hall in 2003. Here they are able to detect the reaction products from photo-induced reactions in combination with the Glasgow photon tagger. In the last two years the MAMI facility was upgraded from 885 MeV to 1.5 GeV, the A2 photon tagger underwent a similar upgrade crossing the threshold for strangeness photoproduction. For the CrystalBall this created a new challenge, to identify K+ mesons above the large background from other charged hadrons, in a situation where the detector setup does not benefit from a magnetic field to help separate particle species. These proceedings outline a novel technique which uses the decay products of the K+ as a strangeness tag
Building an alternative economic network? Consumer cooperation in Scotland from the 1870s to the 1960s
Peer reviewedPostprin
Pre-Big Bang Scenario on Self-T-Dual Bouncing Branes
We consider a new class of 5-dimensional dilatonic actions which are
invariant under T-duality transformations along three compact coordinates,
provided that an appropriate potential is chosen. We show that the invariance
remains when we add a boundary term corresponding to a moving 3-brane, and we
study the effects of the T-duality symmetry on the brane cosmological
equations. We find that T-duality transformations in the bulk induce scale
factor duality on the brane, together with a change of sign of the pressure of
the brane cosmological matter. However, in a remarkable analogy with the
Pre-Big Bang scenario, the cosmological equations are unchanged. Finally, we
propose a model where the dual phases are connected through a scattering of the
brane induced by an effective potential. We show how this model can realise a
smooth, non-singular transition between a pre-Big Bang superinflationary
Universe and a post-Big Bang accelerating Universe.Comment: 18 pages, minor typos corrected, Sec. 2 expanded with more details on
the self-T-dual background, Sec.4 and 5 revised accordingly. Version to
appear on JCA
Magnetar giant flare high-energy emission
High energy ( keV) emission has been detected persisting for several
tens of seconds after the initial spike of magnetar giant flares. It has been
conjectured that this emission might arise via inverse Compton scattering in a
highly extended corona generated by super-Eddington outflows high up in the
magnetosphere. In this paper we undertake a detailed examination of this model.
We investigate the properties of the required scatterers, and whether the
mechanism is consistent with the degree of pulsed emission observed in the tail
of the giant flare. We conclude that the mechanism is consistent with current
data, although the origin of the scattering population remains an open
question. We propose an alternative picture in which the emission is closer to
that star and is dominated by synchrotron radiation. The observations
of the December 2004 flare modestly favor this latter picture. We assess the
prospects for the Fermi Gamma-Ray Space Telescope to detect and characterize a
similar high energy component in a future giant flare. Such a detection should
help to resolve some of the outstanding issues.Comment: 20 pages, 14 figure
From brain to earth and climate systems: Small-world interaction networks or not?
We consider recent reports on small-world topologies of interaction networks
derived from the dynamics of spatially extended systems that are investigated
in diverse scientific fields such as neurosciences, geophysics, or meteorology.
With numerical simulations that mimic typical experimental situations we have
identified an important constraint when characterizing such networks:
indications of a small-world topology can be expected solely due to the spatial
sampling of the system along with commonly used time series analysis based
approaches to network characterization
Mean-field solution of the small-world network model
The small-world network model is a simple model of the structure of social
networks, which simultaneously possesses characteristics of both regular
lattices and random graphs. The model consists of a one-dimensional lattice
with a low density of shortcuts added between randomly selected pairs of
points. These shortcuts greatly reduce the typical path length between any two
points on the lattice. We present a mean-field solution for the average path
length and for the distribution of path lengths in the model. This solution is
exact in the limit of large system size and either large or small number of
shortcuts.Comment: 14 pages, 2 postscript figure
Neutron star glitches have a substantial minimum size
Glitches are sudden spin-up events that punctuate the steady spin down of
pulsars and are thought to be due to the presence of a superfluid component
within neutron stars. The precise glitch mechanism and its trigger, however,
remain unknown. The size of glitches is a key diagnostic for models of the
underlying physics. While the largest glitches have long been taken into
account by theoretical models, it has always been assumed that the minimum size
lay below the detectability limit of the measurements. In this paper we define
general glitch detectability limits and use them on 29 years of daily
observations of the Crab pulsar, carried out at Jodrell Bank Observatory. We
find that all glitches lie well above the detectability limits and by using an
automated method to search for small events we are able to uncover the full
glitch size distribution, with no biases. Contrary to the prediction of most
models, the distribution presents a rapid decrease of the number of glitches
below ~0.05 Hz. This substantial minimum size indicates that a glitch must
involve the motion of at least several billion superfluid vortices and provides
an extra observable which can greatly help the identification of the trigger
mechanism. Our study also shows that glitches are clearly separated from all
the other rotation irregularities. This supports the idea that the origin of
glitches is different to that of timing noise, which comprises the unmodelled
random fluctuations in the rotation rates of pulsars.Comment: 8 pages; 4 figures. Accepted for publication in MNRA
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