26,670 research outputs found
The galactic antiproton spectrum at high energies: background expectation vs. exotic contributions
A new generation of upcoming space-based experiments will soon start to probe
the spectrum of cosmic ray antiparticles with an unprecedented accuracy and, in
particular, will open up a window to energies much higher than those accessible
so far. It is thus timely to carefully investigate the expected antiparticle
fluxes at high energies. Here, we perform such an analysis for the case of
antiprotons. We consider both standard sources as the collision of other cosmic
rays with interstellar matter, as well as exotic contributions from dark matter
annihilations in the galactic halo. Up to energies well above 100 GeV, we find
that the background flux in antiprotons is almost uniquely determined by the
existing low-energy data on various cosmic ray species; for even higher
energies, however, the uncertainties in the parameters of the underlying
propagation model eventually become significant. We also show that if the dark
matter is composed of particles with masses at the TeV scale, which is
naturally expected in extra-dimensional models as well as in certain parameter
regions of supersymmetric models, the annihilation flux can become comparable
to - or even dominate - the antiproton background at the high energies
considered here.Comment: 17 pages revtex4, 7 figures; minor changes (to match the published
version
Team incentives in public organisations; an experimental study
Using a simple production game, we investigate whether public firms perform better when they increase the power of their workersââ¬â¢ incentive schemes. In a laboratory experiment, subjects choose between a ‘public firm’ and a ‘private firm’ with team and individual incentives, respectively. When exposed to individual incentives, workers in the public firm increase effort in one parametrisation, but show a decrease in another. One reason for the latter observation is that reciprocators self-select in the public firm, rendering cooperation profitable.
Viscous fingering in miscible, immiscible and reactive fluids
With the Lattice Boltzmann method (using the BGK approximation) we
investigate the dynamics of Hele-Shaw flow under conditions corresponding to
various experimental systems. We discuss the onset of the instability
(dispersion relation), the static properties (characterization of the
interface) and the dynamic properties (growth of the mixing zone) of simulated
Hele-Shaw systems. We examine the role of reactive processes (between the two
fluids) and we show that they have a sharpening effect on the interface similar
to the effect of surface tension.Comment: 6 pages with 2 figure, to be published in J.Mod.Phys
Atmospheric Calorimetry above 10 eV: Shooting Lasers at the Pierre Auger Cosmic-Ray Observatory
The Pierre Auger Cosmic-Ray Observatory uses the earth's atmosphere as a
calorimeter to measure extensive air-showers created by particles of
astrophysical origin. Some of these particles carry joules of energy. At these
extreme energies, test beams are not available in the conventional sense. Yet
understanding the energy response of the observatory is important. For example,
the propagation distance of the highest energy cosmic-rays through the cosmic
microwave background radiation (CMBR) is predicted to be strong function of
energy. This paper will discuss recently reported results from the observatory
and the use of calibrated pulsed UV laser "test-beams" that simulate the
optical signatures of ultra-high energy cosmic rays. The status of the much
larger 200,000 km companion detector planned for the northern hemisphere
will also be outlined.Comment: 6 pages, 11 figures XIII International Conference on Calorimetry in
High Energy Physic
A Three-Point Cosmic Ray Anisotropy Method
The two-point angular correlation function is a traditional method used to
search for deviations from expectations of isotropy. In this paper we develop
and explore a statistically descriptive three-point method with the intended
application being the search for deviations from isotropy in the highest energy
cosmic rays. We compare the sensitivity of a two-point method and a
"shape-strength" method for a variety of Monte-Carlo simulated anisotropic
signals. Studies are done with anisotropic source signals diluted by an
isotropic background. Type I and II errors for rejecting the hypothesis of
isotropic cosmic ray arrival directions are evaluated for four different event
sample sizes: 27, 40, 60 and 80 events, consistent with near term data
expectations from the Pierre Auger Observatory. In all cases the ability to
reject the isotropic hypothesis improves with event size and with the fraction
of anisotropic signal. While ~40 event data sets should be sufficient for
reliable identification of anisotropy in cases of rather extreme (highly
anisotropic) data, much larger data sets are suggested for reliable
identification of more subtle anisotropies. The shape-strength method
consistently performs better than the two point method and can be easily
adapted to an arbitrary experimental exposure on the celestial sphere.Comment: Fixed PDF erro
Higgs mechanism in a light front formulation
We give a simple derivation of the Higgs mechanism in an abelian light front
field theory. It is based on a finite volume quantization with antiperiodic
scalar fields and a periodic gauge field. An infinite set of degenerate vacua
in the form of coherent states of the scalar field that minimize the light
front energy, is constructed. The corresponding effective Hamiltonian descibes
a massive vector field whose third component is generated by the would-be
Goldstone boson. This mechanism, understood here quantum mechanically in the
form analogous to the space-like quantization, is derived without gauge fixing
as well as in the unitary and the light cone gauge.Comment: 9 page
Disordered free fermions and the Cardy Ostlund fixed line at low temperature
Using functional RG, we reexamine the glass phase of the 2D random-field Sine
Gordon model. It is described by a line of fixed points (FP) with a
super-roughening amplitude as
temperature is varied. A speculation is that this line is identical to the
one found in disordered free-fermion models via exact results from ``nearly
conformal'' field theory. This however predicts , contradicting
numerics. We point out that this result may be related to failure of
dimensional reduction, and that a functional RG method incorporating higher
harmonics and non-analytic operators predicts a non-zero which
compares reasonably with numerics.Comment: 8 pages, 3 figures, only material adde
ESR evidence for disordered magnetic phase from ultra-small carbon nanotubes embedded in zeolite nanochannels
A multi-frequency electron spin resonance (ESR) study provides evidence for
the occurrence of low temperature ferromagnetic/spin-glass behavior in aligned
arrays of sub-nanometer single walled carbon nanotubes confined in zeolite
nano-channels, owing to sp2-type non-bonding carbon associated localized states
with density of ~3 x 1019 /g. Features related to the much anticipated
conduction ESR are not detected. In the paramagnetic phase, the ESR linewidth
is found to be weakly dependent on microwave frequency.Comment: Accepted to be published in EuroPhysics Letter
Viscous spreading of an inertial wave beam in a rotating fluid
We report experimental measurements of inertial waves generated by an
oscillating cylinder in a rotating fluid. The two-dimensional wave takes place
in a stationary cross-shaped wavepacket. Velocity and vorticity fields in a
vertical plane normal to the wavemaker are measured by a corotating Particule
Image Velocimetry system. The viscous spreading of the wave beam and the
associated decay of the velocity and vorticity envelopes are characterized.
They are found in good agreement with the similarity solution of a linear
viscous theory, derived under a quasi-parallel assumption similar to the
classical analysis of Thomas and Stevenson [J. Fluid Mech. 54 (3), 495-506
(1972)] for internal waves
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