2,231 research outputs found
Narrow Pentaquark States in a Quark Model with Antisymmetrized Molecular Dynamics
The exotic baryon is studied with microscopic
calculations in a quark model by using a method of antisymmetrized molecular
dynamics(AMD). We predict narrow states, ,
, and , which nearly degenerate in a
low-energy region of the system. We discuss decay widths and
estimate them to be for the , and
MeV for the state.Comment: Talk given at International Workshop on PENTAQUARK04, Spring-8,
Hyogo, Japan, 20-23 Jul 200
The emission of energetic electrons from the complex streamer corona adjacent to leader stepping
We here propose a model to capture the complexity of the streamer corona
adjacent to leader stepping and relate it to the production of energetic
electrons serving as a source of X-rays and -rays, manifesting in
terrestrial gamma-ray flashes (TGFs). During its stepping, the leader tip is
accompanied by a corona consisting of multitudinous streamers perturbing the
air in its vicinity and leaving residual charge behind. We explore the relative
importance of air perturbations and preionization on the production of
energetic run-away electrons by 2.5D cylindrical Monte Carlo particle
simulations of streamers in ambient fields of 16 kV cm and 50 kV
cm at ground pressure. We explore preionization levels between
m and m, channel widths between 0.5 and 1.5 times the
original streamer widths and air perturbation levels between 0\% and 50\% of
ambient air. We observe that streamers in preionized and perturbed air
accelerate more efficiently than in non-ionized and uniform air with air
perturbation dominating the streamer acceleration. We find that in unperturbed
air preionization levels of m are sufficient to explain
run-away electron rates measured in conjunction with terrestrial gamma-ray
flashes. In perturbed air, the production rate of runaway electrons varies from
s to s with maximum electron energies from
some hundreds of eV up to some hundreds of keV in fields above and below the
breakdown strength. In the presented simulations the number of runaway
electrons matches with the number of energetic electrons measured in alignment
with the observations of terrestrial gamma-ray flashes. Conclusively, the
complexity of the streamer zone ahead of leader tips allows explaining the
emission of energetic electrons and photons from streamer discharges.Comment: 29 pages, 11 figures, 2 table
Fermi Liquids and the Luttinger Integral
The Luttinger Theorem, which relates the electron density to the volume of
the Fermi surface in an itinerant electron system, is taken to be one of the
essential features of a Fermi liquid. The microscopic derivation of this result
depends on the vanishing of a certain integral, the Luttinger integral , which is also the basis of the Friedel sum rule for impurity models,
relating the impurity occupation number to the scattering phase shift of the
conduction electrons. It is known that non-zero values of with
, occur in impurity models in phases with non-analytic low
energy scattering, classified as singular Fermi liquids. Here we show the same
values, , occur in an impurity model in phases with regular
low energy Fermi liquid behavior. Consequently the Luttinger integral can be
taken to characterize these phases, and the quantum critical points separating
them interpreted as topological.Comment: 5 pages 7 figure
Axial vector tetraquark with S=+2
Possibility of an axial vector isoscalar tetraquark with
is discussed. If the pentaquark has the
configuration, the isoscalar
(-meson) state with is expected to
exist in the mass region lower than or close to the mass of .
Within a flux-tube quark model, a possible resonant state of
is suggested to appear around 1.4 GeV with the
width MeV. We propose that the -meson is a
good candidate for the tetraquark search, which would be observed in the
decay channel.Comment: 20 pages, 5 figures, submitted to Phys.Rev.
Ion dynamics and acceleration in relativistic shocks
Ab-initio numerical study of collisionless shocks in electron-ion
unmagnetized plasmas is performed with fully relativistic particle in cell
simulations. The main properties of the shock are shown, focusing on the
implications for particle acceleration. Results from previous works with a
distinct numerical framework are recovered, including the shock structure and
the overall acceleration features. Particle tracking is then used to analyze in
detail the particle dynamics and the acceleration process. We observe an energy
growth in time that can be reproduced by a Fermi-like mechanism with a reduced
number of scatterings, in which the time between collisions increases as the
particle gains energy, and the average acceleration efficiency is not ideal.
The in depth analysis of the underlying physics is relevant to understand the
generation of high energy cosmic rays, the impact on the astrophysical shock
dynamics, and the consequent emission of radiation.Comment: 5 pages, 3 figure
Anti-prion activity of an RNA aptamer and its structural basis.
Prion proteins (PrPs) cause prion diseases, such as bovine spongiform encephalopathy. The conversion of a normal cellular form (PrP(C)) of PrP into an abnormal form (PrP(Sc)) is thought to be associated with the pathogenesis. An RNA aptamer that tightly binds to and stabilizes PrP(C) is expected to block this conversion and to thereby prevent prion diseases. Here, we show that an RNA aptamer comprising only 12 residues, r(GGAGGAGGAGGA) (R12), reduces the PrP(Sc) level in mouse neuronal cells persistently infected with the transmissible spongiform encephalopathy agent. Nuclear magnetic resonance analysis revealed that R12, folded into a unique quadruplex structure, forms a dimer and that each monomer simultaneously binds to two portions of the N-terminal half of PrP(C), resulting in tight binding. Electrostatic and stacking interactions contribute to the affinity of each portion. Our results demonstrate the therapeutic potential of an RNA aptamer as to prion diseases
Acceleration in perpendicular relativistic shocks for plasmas consisting of leptons and hadrons
We investigate the acceleration of light particles in perpendicular shocks
for plasmas consisting of a mixture of leptonic and hadronic particles.
Starting from the full set of conservation equations for the mixed plasma
constituents, we generalize the magneto-hydrodynamical jump conditions for a
multi-component plasma, including information about the specific adiabatic
constants for the different species. The impact of deviations from the standard
model of an ideal gas is compared in theory and particle-in-cell simulations,
showing that the standard-MHD model is a good approximation. The simulations of
shocks in electron-positron-ion plasmas are for the first time
multi-dimensional, transverse effects are small in this configuration and 1D
simulations are a good representation if the initial magnetization is chosen
high. 1D runs with a mass ratio of 1836 are performed, which identify the
Larmor frequency \omega_{ci} as the dominant frequency that determines the
shock physics in mixed component plasmas. The maximum energy in the non-thermal
tail of the particle spectra evolves in time according to a power-law
proportional to t^\alpha with \alpha in the range 1/3 < \alpha < 1, depending
on the initial parameters. A connection is made with transport theoretical
models by Drury (1983) and Gargate & Spitkovsky (2011), which predict an
acceleration time proportional to \gamma and the theory for small wavelength
scattering by Kirk & Reville (2010), which predicts a behavior rather as
proportional to \gamma^2. Furthermore, we compare different magnetic field
orientations with B_0 inside and out of the plane, observing qualitatively
different particle spectra than in pure electron-ion shocks
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