10,405 research outputs found
The origin of the positron excess in cosmic rays
We show that the positron excess measured by the PAMELA experiment in the
region between 10 and 100 GeV may well be a natural consequence of the standard
scenario for the origin of Galactic cosmic rays. The 'excess' arises because of
positrons created as secondary products of hadronic interactions inside the
sources, but the crucial physical ingredient which leads to a natural
explanation of the positron flux is the fact that the secondary production
takes place in the same region where cosmic rays are being accelerated.
Therefore secondary positrons (and electrons) participate in the acceleration
process and turn out to have a very flat spectrum, which is responsible, after
propagation in the Galaxy, for the observed positron 'excess'. This effect
cannot be avoided though its strength depends on the values of the
environmental parameters during the late stages of evolution of supernova
remnants.Comment: 4 Pages, 2 figures. Some references and discussion adde
Teleportation on a quantum dot array
We present a model of quantum teleportation protocol based on a double
quantum dot array. The unknown qubit is encoded using a pair of quantum dots,
coupled by tunneling, with one excess electron. It is shown how to create
maximally entangled states with this kind of qubits using an adiabatically
increasing Coulomb repulsion between different pairs. This entangled states are
exploited to perform teleportation again using an adiabatic coupling between
them and the incoming unknown state. Finally, a sudden separation of Bob's
qubit enables a time evolution of Alice's state providing a modified version of
standard Bell measurement. Substituting the four quantum dots entangled state
with a chain of coupled DQD's, a quantum channel with high fidelity arises from
this scheme allowing the transmission over long distances.Comment: 4 pages, 2 figure
Asymptotics of Harish-Chandra expansions, bounded hypergeometric functions associated with root systems, and applications
A series expansion for Heckman-Opdam hypergeometric functions
is obtained for all
As a consequence, estimates for away from the walls of a Weyl
chamber are established. We also characterize the bounded hypergeometric
functions and thus prove an analogue of the celebrated theorem of Helgason and
Johnson on the bounded spherical functions on a Riemannian symmetric space of
the noncompact type. The -theory for the hypergeometric Fourier transform
is developed for . In particular, an inversion formula is proved when
Regulation of the Neuron-specific Ras GTPase-activating Protein, synGAP, by Ca2+/Calmodulin-dependent Protein Kinase II
synGAP is a neuron-specific Ras GTPase-activating protein found in high concentration in the postsynaptic density fraction from mammalian forebrain. Proteins in the postsynaptic density, including synGAP, are part of a signaling complex attached to the cytoplasmic tail of the N-methyl-D-aspartate-type glutamate receptor. synGAP can be phosphorylated by a second prominent component of the complex, Ca2+/calmodulin-dependent protein kinase II. Here we show that phosphorylation of synGAP by Ca2+/calmodulin-dependent protein kinase II increases its Ras GTPase-activating activity by 70-95%. We identify four major sites of phosphorylation, serines 1123, 1058, 750/751/756, and 764/765. These sites together with other minor phosphorylation sites in the carboxyl tail of synGAP control stimulation of GTPase-activating activity. When three of these sites and four other serines in the carboxyl tail are mutated, stimulation of GAP activity after phosphorylation is reduced to 21 ± 5% compared with 70-95% for the wild type protein. We used phosphosite-specific antibodies to show that, as predicted, phosphorylation of serines 765 and 1123 is increased in cultured cortical neurons after exposure of the neurons to the agonist N-methyl-D-aspartate
Flavor stability analysis of dense supernova neutrinos with flavor-dependent angular distributions
Numerical simulations of the supernova (SN) neutrino self-induced flavor
conversions, associated with the neutrino-neutrino interactions in the deepest
stellar regions, have been typically carried out assuming the "bulb-model". In
this approximation, neutrinos are taken to be emitted half-isotropically by a
common neutrinosphere. In the recent Ref. \cite{Mirizzi:2011tu} we have removed
this assumption by introducing flavor-dependent angular distributions for SN
neutrinos, as suggested by core-collapse simulations. We have found that in
this case a novel multi-angle instability in the self-induced flavor
transitions can arise. In this work we perform an extensive study of this
effect, carrying out a linearized flavor stability analysis for different SN
neutrino energy fluxes and angular distributions, in both normal and inverted
neutrino mass hierarchy. We confirm that spectra of different nu species which
cross in angular space (where F_{\nu_e}=F_{\nu_x} and
F_{\bar\nu_e}=F_{\bar\nu_x}) present a significant enhancement of the flavor
instability, and a shift of the onset of the flavor conversions at smaller
radii with respect to the case of an isotropic neutrino emission. We also
illustrate how a qualitative (and sometimes quantitative) understanding of the
dynamics of these systems follows from a stability analysis.Comment: (v2: revised version. 10 pages, 10 eps figures. References updated.
Figures imrproved. Matches the version published in PRD.
Three-dimensional evolution of magnetic and velocity shear driven instabilities in a compressible magnetized jet
The problem of three-dimensional combined magnetic and velocity shear driven
instabilities of a compressible magnetized jet modeled with a plane
neutral/current double vortex sheet in the framework of the resistive
magnetohydrodynamics is addressed. The resulting dynamics given by the
stream+current sheet interaction is analyzed and the effects of a variable
geometry of the basic fields are considered. Depending on the basic asymptotic
magnetic field configuration, a selection rule of the linear instability modes
can be obtained. Hence, the system follows a two-stage path developing either
through a fully three-dimensional dynamics with a rapid evolution of kink modes
leading to a final turbulent state, or rather through a driving two-dimensional
instability pattern that develops on parallel planes on which a
reconnection+coalescence process takes place.Comment: 33 pages, 15 figures, accepted for publication in Physics of Plasma
- …