857 research outputs found
Differential nature of inelastic collisions facilitating runaway electron generation in weakly-ionized plasmas
We report extention of the Dreicer generation theory to situation where the
small energy exchange no more predominates. In weakly-ionized plamsas, the
Dreicer mechanism can be severely underestimated due to the broken assumption
of dominant small energy exchange. This Letter numerically demonstrates that
the differential nature of inelastic collisions facilitates the Dreicer
generation by developing the new Fokker-Planck-Boltzmann operator of
electron-hydrogen atom collisions based on experimental data. This work is
envisaged to predict runaway electron generations in future fusion reactors
Partonic description of a supersymmetric p-brane
We consider supersymmetric extensions of a recently proposed partonic
description of a bosonic p-brane which reformulates the Nambu-Goto action as an
interacting multi-particle action with Filippov-Lie algebra gauge symmetry. We
construct a worldline supersymmetric action by postulating, among others, a
p-form fermion. Demanding a local worldline supersymmetry rather than the full
worldvolume supersymmetry, we circumvent a known no-go theorem against the
construction of a Ramond-Neveu-Schwarz supersymmetric action for a p-brane of
p>1. We also derive a spacetime supersymmetric Green-Schwarz extension from the
preexisting kappa-symmetric action.Comment: 1+16 pages, no figure; References added and Concluding section
expanded. Final version to appear in JHE
Acceleration of Solar Wind Ions by Nearby Interplanetary Shocks: Comparison of Monte Carlo Simulations with Ulysses Observations
The most stringent test of theoretical models of the first-order Fermi
mechanism at collisionless astrophysical shocks is a comparison of the
theoretical predictions with observational data on particle populations. Such
comparisons have yielded good agreement between observations at the
quasi-parallel portion of the Earth's bow shock and three theoretical
approaches, including Monte Carlo kinetic simulations. This paper extends such
model testing to the realm of oblique interplanetary shocks: here observations
of proton and alpha particle distributions made by the SWICS ion mass
spectrometer on Ulysses at nearby interplanetary shocks are compared with test
particle Monte Carlo simulation predictions of accelerated populations. The
plasma parameters used in the simulation are obtained from measurements of
solar wind particles and the magnetic field upstream of individual shocks. Good
agreement between downstream spectral measurements and the simulation
predictions are obtained for two shocks by allowing the the ratio of the
mean-free scattering length to the ionic gyroradius, to vary in an optimization
of the fit to the data. Generally small values of this ratio are obtained,
corresponding to the case of strong scattering. The acceleration process
appears to be roughly independent of the mass or charge of the species.Comment: 26 pages, 6 figures, AASTeX format, to appear in the Astrophysical
Journal, February 20, 199
Pointlike structure for super p-branes
We present an efficient method to understand the p-brane dynamics in a
unified framework. For this purpose, we reformulate the action for super
p-branes in the form appropriate to incorporate the pointlike (parton)
structure of higher dimensional p-branes and intend to interpret the p-brane
dynamics as the collective dynamics of superparticles. In order to examine such
a parton picture of super p-branes, we consider various superparticle
configurations that can be reduced from super p-branes, especially, a
supermembrane, and study the partonic structure of classical p-brane solutions.Comment: 22 pages, corrected typos, to appear in Phys. Rev. D58, 085018 (1998
Ballistic Josephson junctions in edge-contacted graphene
Hybrid graphene-superconductor devices have attracted much attention since
the early days of graphene research. So far, these studies have been limited to
the case of diffusive transport through graphene with poorly defined and modest
quality graphene-superconductor interfaces, usually combined with small
critical magnetic fields of the superconducting electrodes. Here we report
graphene based Josephson junctions with one-dimensional edge contacts of
Molybdenum Rhenium. The contacts exhibit a well defined, transparent interface
to the graphene, have a critical magnetic field of 8 Tesla at 4 Kelvin and the
graphene has a high quality due to its encapsulation in hexagonal boron
nitride. This allows us to study and exploit graphene Josephson junctions in a
new regime, characterized by ballistic transport. We find that the critical
current oscillates with the carrier density due to phase coherent interference
of the electrons and holes that carry the supercurrent caused by the formation
of a Fabry-P\'{e}rot cavity. Furthermore, relatively large supercurrents are
observed over unprecedented long distances of up to 1.5 m. Finally, in the
quantum Hall regime we observe broken symmetry states while the contacts remain
superconducting. These achievements open up new avenues to exploit the Dirac
nature of graphene in interaction with the superconducting state.Comment: Updated version after peer review. Includes supplementary material
and ancillary file with source code for tight binding simulation
Nonabelian Phenomena on D-branes
A remarkable feature of D-branes is the appearance of a nonabelian gauge
theory in the description of several (nearly) coincident branes. This
nonabelian structure plays an important role in realizing various geometric
effects with D-branes. In particular, the branes' transverse displacements are
described by matrix-valued scalar fields and so noncommutative geometry
naturally appears in this framework. I review the action governing this
nonabelian theory, as well as various related physical phenomena such as the
dielectric effect, giant gravitons and fuzzy funnels.Comment: Lecture at Leuven workshop on ``The quantum structure of spacetime
and the geometrical nature of fundamental interactions'' (September 13-19,
2002); ref.'s adde
Notes on Giant Gravitons on PP-waves
We investigate the giant gravitons in the maximally supersymmetric IIB
pp-wave from several viewpoints: (i) the dynamics of D3-branes, (ii) the
world-sheet description and (iii) the correlation functions in the dual N=4
Yang-Mills theory. In particular, we derive the BPS equation of a D3-brane with
magnetic flux, which is equivalent to multiple D-strings, and discuss the
behavior of solutions in the presence of RR-flux. We find solutions which
represent the excitations of the giant gravitons in that system.Comment: 25 pages, Latex, typos corrected, (minor) factors in
eq.(2.11),(2.12),(2.16),(2.17) corrected, a footnote added, to appear in JHE
Identification of Radiopure Titanium for the LZ Dark Matter Experiment and Future Rare Event Searches
The LUX-ZEPLIN (LZ) experiment will search for dark matter particle
interactions with a detector containing a total of 10 tonnes of liquid xenon
within a double-vessel cryostat. The large mass and proximity of the cryostat
to the active detector volume demand the use of material with extremely low
intrinsic radioactivity. We report on the radioassay campaign conducted to
identify suitable metals, the determination of factors limiting radiopure
production, and the selection of titanium for construction of the LZ cryostat
and other detector components. This titanium has been measured with activities
of U~1.6~mBq/kg, U~0.09~mBq/kg,
Th~~mBq/kg, Th~~mBq/kg, K~0.54~mBq/kg, and Co~0.02~mBq/kg (68\% CL).
Such low intrinsic activities, which are some of the lowest ever reported for
titanium, enable its use for future dark matter and other rare event searches.
Monte Carlo simulations have been performed to assess the expected background
contribution from the LZ cryostat with this radioactivity. In 1,000 days of
WIMP search exposure of a 5.6-tonne fiducial mass, the cryostat will contribute
only a mean background of (stat)(sys) counts.Comment: 13 pages, 3 figures, accepted for publication in Astroparticle
Physic
5D action for longitudinal five branes on a pp-wave
String modes in a pp-wave background are generically massive, and the
worldvolume description of the branes is to be given by `massive' gauge
theories. In this paper, we present a five dimensional super Yang-Mills action
with the Kahler-Chern-Simons term plus the Myers term as a low energy
worldvolume description of the longitudinal five branes in a maximally
supersymmetric pp-wave background. We derive the action from the M-theory
matrix model on the pp-wave. We utilize the previously found 4/32 BPS solution
of rotating five branes with stacks of membranes, but, to obtain the static
configuration, we reformulate the matrix model in a rotating coordinate system
which provides the inertial frame for the branes. Expanding the matrix model
around the solution, we first obtain a non-commutative field theory action
naturally equipped with the full sixteen dynamical supersymmetries. In the
commutative limit, we show only four supersymmetries survive, resulting in a
novel five dimensional "N=1/2" theory.Comment: 1+19 pages, No figure, JHEP style; a reference added, to appear in
JHE
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