395,755 research outputs found
The Wess-Zumino term and quantum tunneling
The significance of the Wess--Zumino term in spin tunneling is explored, and
a formula is established for the splitting of energy levels of a particle with
large fermionic spin as an applied magnetic field is switched on.Comment: Latex, 7 page
Calculation of Spin Tunneling Effects in the Presence of an Applied Magnetic Field
The tunneling splitting of the energy levels of a ferromagnetic particle in
the presence of an applied magnetic field - previously derived only for the
ground state with the path integral method - is obtained in a simple way from
Schroedinger theory. The origin of the factors entering the result is clearly
understood, in particular the effect of the asymmetry of the barriers of the
potential. The method should appeal particularly to experimentalists searching
for evidence of macroscopic spin tunneling.Comment: 10 pages, LaTex, 4 figures included using graphic
Sustained Acceleration of Over-dense Plasmas by Colliding Laser Pulses
We review recent PIC simulation results which show that double-sided
irradiaton of a thin overdense plasma slab by ultra-intense laser pulses from
both sides can lead to sustained comoving acceleration of surface electrons to
energies much higher than the conventional ponderomotive limit. The
acceleration stops only when the electrons drift transversely out of the laser
beam. We show results of parameter studies based on this concept and discuss
future laser experiments that can be used to test these computer results.Comment: 9 pages 6 figures. AIP Conference Proceedings for 2005 Varenna Conf.
on Superstrong Fields in Plasmas (AIP, NY 2006
A Unified Relay Framework with both D-F and C-F Relay Nodes
Decode-and-forward (D-F) and compress-and-forward (C-F) are two fundamentally
different relay strategies proposed by (Cover and El Gamal, 1979).
Individually, either of them has been successfully generalized to multi-relay
channels. In this paper, to allow each relay node the freedom of choosing
either of the two strategies, we propose a unified framework, where both the
D-F and C-F strategies can be employed simultaneously in the network. It turns
out that, to fully incorporate the advantages of both the best known D-F and
C-F strategies into a unified framework, the major challenge arises as follows:
For the D-F relay nodes to fully utilize the help of the C-F relay nodes,
decoding at the D-F relay nodes should not be conducted until all the blocks
have been finished; However, in the multi-level D-F strategy, the upstream
nodes have to decode prior to the downstream nodes in order to help, which
makes simultaneous decoding at all the D-F relay nodes after all the blocks
have been finished inapplicable. To tackle this problem, nested blocks combined
with backward decoding are used in our framework, so that the D-F relay nodes
at different levels can perform backward decoding at different frequencies. As
such, the upstream D-F relay nodes can decode before the downstream D-F relay
nodes, and the use of backward decoding at each D-F relay node ensures the full
exploitation of the help of both the other D-F relay nodes and the C-F relay
nodes. The achievable rates under our unified relay framework are found to
combine both the best known D-F and C-F achievable rates and include them as
special cases
Flow Decomposition for Multi-User Channels - Part I
A framework based on the idea of flow decomposition is proposed to
characterize the decode-forward region for general multi-source, multi-relay,
all-cast channels with independent input distributions. The region is difficult
to characterize directly when deadlocks occur between two relay nodes, in which
both relays benefit by decoding after each other. Rate-vectors in the
decode-forward region depend ambiguously on the outcomes of all deadlocks in
the channel. The region is characterized indirectly in two phases. The first
phase assumes relays can operate non-causally. It is shown that every
rate-vector in the decode-forward region corresponds to a set of flow
decompositions, which describe the messages decoded at each node with respect
to the messages forwarded by all the other nodes. The second phase imposes
causal restrictions on the relays. Given an arbitrary set of (possibly
non-causal) flow decompositions, necessary and sufficient conditions are
derived for the existence of an equivalent set of causal flow decompositions
that achieves the same rate-vector region
Federal Procurement and Equal Employment Opportunity
The paper contains a discussion about what results about the quality of an estimated model can be achieved, if no probabilitic assumptions are introduced. Several technical results that illustrate possibilities and difficulties are also given
Direct detection and solar capture of dark matter with momentum and velocity dependent elastic scattering
We explore the momentum and velocity dependent elastic scattering between the
dark matter (DM) particles and the nuclei in detectors and the Sun. In terms of
the non-relativistic effective theory, we phenomenologically discuss ten kinds
of momentum and velocity dependent DM-nucleus interactions and recalculate the
corresponding upper limits on the spin-independent DM-nucleon scattering cross
section from the current direct detection experiments. The DM solar capture
rate is calculated for each interaction. Our numerical results show that the
momentum and velocity dependent cases can give larger solar capture rate than
the usual contact interaction case for almost the whole parameter space. On the
other hand, we deduce the Super-Kamiokande's constraints on the solar capture
rate for eight typical DM annihilation channels. In contrast to the usual
contact interaction, the Super-Kamiokande and IceCube experiments can give more
stringent limits on the DM-nucleon elastic scattering cross section than the
current direct detection experiments for several momentum and velocity
dependent DM-nucleus interactions. In addition, we investigate the mediator
mass's effect on the DM elastic scattering cross section and solar capture
rate.Comment: 18 pages, 4 figures, 2 tables. minor changes and a reference added,
published in Nuclear Physics
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