42,607 research outputs found
Combinatorial Hopf algebras, noncommutative Hall-Littlewood functions, and permutation tableaux
We introduce a new family of noncommutative analogues of the Hall-Littlewood
symmetric functions. Our construction relies upon Tevlin's bases and simple
q-deformations of the classical combinatorial Hopf algebras. We connect our new
Hall-Littlewood functions to permutation tableaux, and also give an exact
formula for the q-enumeration of permutation tableaux of a fixed shape. This
gives an explicit formula for: the steady state probability of each state in
the partially asymmetric exclusion process (PASEP); the polynomial enumerating
permutations with a fixed set of weak excedances according to crossings; the
polynomial enumerating permutations with a fixed set of descent bottoms
according to occurrences of the generalized pattern 2-31.Comment: 37 pages, 4 figures, new references adde
Transport of positrons in the interstellar medium
This work investigates some aspects of the transport of low-energy positrons
in the interstellar medium (ISM). We consider resonance interactions with
magnetohydrodynamic waves above the resonance threshold. Below the threshold,
collisions take over and deflect positrons in their motion parallel to
magnetic-field lines. Using Monte-Carlo simulations, we model the propagation
and energy losses of positrons in the different phases of the ISM until they
annihilate. We suggest that positrons produced in the disk by an old population
of stars, with initial kinetic energies below 1 MeV, and propagating in the
spiral magnetic field of the disk, can probably not penetrate the Galactic
bulge.Comment: 4 pages, 3 figures, accepted for publication in the proceeding of the
6th INTEGRAL worksho
Sigma and omega meson propagation in a dense nuclear medium
The propagation of the scalar () and vector () mesons in
nuclear matter is studied in detail using the Walecka model over a wide range
of densities and including the effects of a finite width through the
inclusion of a two-pion loop. We calculate the dispersion relation and spectral
functions of the and (transverse and longitudinal) mesons,
including the effect of - mixing in matter. It is shown that
the mixing effect is quite important in the propagation of the (longitudinal)
and mesons above normal nuclear matter density. We find that
there is a two-peak structure in the spectral function of the channel,
caused by - mixing.Comment: 17 pages including 6 ps files, submitted to Phys. Lett. B.
Acknowledgement is revise
Tip-sample interactions in atomic force microscopy: I. Modulating adhesion between silicon nitride and glass
An adhesive interaction between a silicon nitride AFM tip and glass substrate in water is described. This adhesion is in the range 5-40 nN, of which a large component is likely to be due to hydrogen bonding between the silanol groups on both surfaces. The interaction can be modulated by a variety of buffers commonly used in biochemical and biological research, including sodium phosphate, tris(hydroxymethyl)aminomethane, glycine, and N-2-hydroxyethyl-piperazine N'-2-ethanesulfonic acid. Using these buffers it appears that there are effects of ion concentration, ion type and pH on the measured adhesion. Of the conditions examined, phosphate was most effective at reducing adhesion and could be used at concentrations as low as 10 mM at neutral pH. The results demonstrate that the chemical interactions between tip and sample can be modulated, and provide a basis for designing conditions for imaging and manipulating biological molecules and structures
Security Estimates for Quadratic Field Based Cryptosystems
We describe implementations for solving the discrete logarithm problem in the
class group of an imaginary quadratic field and in the infrastructure of a real
quadratic field. The algorithms used incorporate improvements over
previously-used algorithms, and extensive numerical results are presented
demonstrating their efficiency. This data is used as the basis for
extrapolations, used to provide recommendations for parameter sizes providing
approximately the same level of security as block ciphers with
and -bit symmetric keys
Supernova Constraints on MeV Dark Sectors from e+ e- Annihilations
Theories with dark forces and dark sectors are of interest for dark matter
models. In this paper we find the region in parameter space that is constrained
by supernova cooling constraints when the models include dark sector particles
with masses around 100 MeV or less. We include only interactions with electrons
and positrons. The constraint is important for small mixing parameters.Comment: 18 pages, 3 figure
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