24,922 research outputs found
Pomeron Vertices in Perturbative QCD in Diffractive Scattering
We analyse the momentum space triple Pomeron vertex in perturbative QCD. In
addition to the standard form of this vertex which is used in the context of
total cross-sections at high energies and in the QCD reggeon field theory,
there exists an alternative form which has to be used in the study of high-mass
diffraction. We review and analyse the relation between these two versions. We
discuss some implications for the BK-equation. In the second part of our paper
we extend this analysis to the Pomeron-Odderon-Odderon vertex.Comment: 23 pages, 5 figures, Late
Decoherence in a system of many two--level atoms
I show that the decoherence in a system of degenerate two--level atoms
interacting with a bosonic heat bath is for any number of atoms governed by
a generalized Hamming distance (called ``decoherence metric'') between the
superposed quantum states, with a time--dependent metric tensor that is
specific for the heat bath.The decoherence metric allows for the complete
characterization of the decoherence of all possible superpositions of
many-particle states, and can be applied to minimize the over-all decoherence
in a quantum memory. For qubits which are far apart, the decoherence is given
by a function describing single-qubit decoherence times the standard Hamming
distance. I apply the theory to cold atoms in an optical lattice interacting
with black body radiation.Comment: replaced with published versio
On Free Quotients of Complete Intersection Calabi-Yau Manifolds
In order to find novel examples of non-simply connected Calabi-Yau
threefolds, free quotients of complete intersections in products of projective
spaces are classified by means of a computer search. More precisely, all
automorphisms of the product of projective spaces that descend to a free action
on the Calabi-Yau manifold are identified.Comment: 39 pages, 3 tables, LaTe
Drift velocity and gain in argon- and xenon-based mixtures
We present measurements of drift velocities and gains in gas mixtures based
on Ar and Xe, with CO2, CH4, and N2 as quenchers, and compare them with
calculations. In particular, we show the dependence of Ar- and Xe-CO2 drift
velocities and gains on the amount of nitrogen contamination in the gas, which
in real experiments may build up through leaks. A quantification of the Penning
mechanism which contributes to the Townsend coefficients of a given gas mixture
is proposed.Comment: 11 pages, 7 figures, accepted for publication in Nucl.Instrum.Meth.
A. Data files available at http://www-alice.gsi.de/tr
Solitonic-exchange mechanism of surface~diffusion
We study surface diffusion in the framework of a generalized
Frenkel-Kontorova model with a nonconvex transverse degree of freedom. The
model describes a lattice of atoms with a given concentration interacting by
Morse-type forces, the lattice being subjected to a two-dimensional substrate
potential which is periodic in one direction and nonconvex (Morse) in the
transverse direction. The results are used to describe the complicated
exchange-mediated diffusion mechanism recently observed in MD simulations [J.E.
Black and Zeng-Ju Tian, Phys. Rev. Lett. {\bf 71}, 2445-2448(1993)].Comment: 22 Revtex pages, 9 figures to appear in Phys. Rev.
Spin exchange interaction with tunable range between graphene quantum dots
We study the spin exchange between two electrons localized in separate
quantum dots in graphene. The electronic states in the conduction band are
coupled indirectly by tunneling to a common continuum of delocalized states in
the valence band. As a model, we use a two-impurity Anderson Hamiltonian which
we subsequently transform into an effective spin Hamiltonian by way of a
two-stage Schrieffer-Wolff transformation. We then compare our result to that
from a Coqblin-Schrieffer approach as well as to fourth order perturbation
theory.Comment: 8 pages, 3 figure
Centrality dependence of global variables in relativistic heavy ion collisions: Final data analysis in the framework of a statistical model
The global variables like the transverse energy at midrapidity, the charged
particle multiplicity at midrapidity and the total multiplicity of charged
particles are evaluated in the single-freeze-out statistical model for
different centrality bins at RHIC at and 200 GeV. Full
description of decays of hadron resonances is applied in these estimations. The
geometric parameters of the model are obtained from the fit to the final data
on the spectra. The predicted values of the global variables agree
qualitatively well with the experimental data. The centrality independence of
the total number of charged particles per participant pair has been also
reproduced.Comment: Revtex, 12 figures (included), 16 pages. This is the revised final
version accepted for publication in Physical Review C. The main difference
with the first version is that the geometric parameters of the model have
been fitted again with the use of the newer estimates of the statistical
parameters reported in Refs. [20,21] for the case of GeV.
Also because of the editorial reasons the title has been slightly change
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