1,608 research outputs found
Diffusion, Fragmentation and Coagulation Processes: Analytical and Numerical Results
We formulate dynamical rate equations for physical processes driven by a
combination of diffusive growth, size fragmentation and fragment coagulation.
Initially, we consider processes where coagulation is absent. In this case we
solve the rate equation exactly leading to size distributions of Bessel type
which fall off as for large -values. Moreover, we provide
explicit formulas for the expansion coefficients in terms of Airy functions.
Introducing the coagulation term, the full non-linear model is mapped exactly
onto a Riccati equation that enables us to derive various asymptotic solutions
for the distribution function. In particular, we find a standard exponential
decay, , for large , and observe a crossover from the Bessel
function for intermediate values of . These findings are checked by
numerical simulations and we find perfect agreement between the theoretical
predictions and numerical results.Comment: (28 pages, 6 figures, v2+v3 minor corrections
On the Early History of Current Algebra
The history of Current Algebra is reviewed up to the appearance of the
Adler-Weisberger sum rule. Particular emphasis is given to the role current
algebra played for the historical struggle in strong interaction physics of
elementary particles between the S-matrix approach based on dispersion
relations and field theory. The question whether there are fundamental
particles or all hadrons are bound or resonant states of one another played an
important role in this struggle and is thus also regarded.Comment: 17 page
Stabilizing single atom contacts by molecular bridge formation
Gold-molecule-gold junctions can be formed by carefully breaking a gold wire
in a solution containing dithiolated molecules. Surprisingly, there is little
understanding on the mechanical details of the bridge formation process and
specifically on the role that the dithiol molecules play themselves. We propose
that alkanedithiol molecules have already formed bridges between the gold
electrodes before the atomic gold-gold junction is broken. This leads to
stabilization of the single atomic gold junction, as observed experimentally.
Our data can be understood within a simple spring model.Comment: 14 pages, 3 figures, 1 tabl
Classical Open String Models in 4-Dim Minkowski Spacetime
Classical bosonic open string models in fourdimensional Minkowski spacetime
are discussed. A special attention is paid to the choice of edge conditions,
which can follow consistently from the action principle. We consider
lagrangians that can depend on second order derivatives of worldsheet
coordinates. A revised interpretation of the variational problem for such
theories is given. We derive a general form of a boundary term that can be
added to the open string action to control edge conditions and modify
conservation laws. An extended boundary problem for minimal surfaces is
examined. Following the treatment of this model in the geometric approach, we
obtain that classical open string states correspond to solutions of a complex
Liouville equation. In contrast to the Nambu-Goto case, the Liouville potential
is finite and constant at worldsheet boundaries. The phase part of the
potential defines topological sectors of solutions.Comment: 25 pages, LaTeX, preprint TPJU-28-93 (the previous version was
truncated by ftp...
Crossover from Electronic to Atomic Shell Structure in Alkali Metal Nanowires
After making a cold weld by pressing two clean metal surfaces together, upon
gradually separating the two pieces a metallic nanowire is formed, which
progressively thins down to a single atom before contact is lost. In previous
experiments [1,2] we have observed that the stability of such nanowires is
influenced by electronic shell filling effects, in analogy to shell effects in
metal clusters [3]. For sodium and potassium at larger diameters there is a
crossover to crystalline wires with shell-closings corresponding to the
completion of additional atomic layers. This observation completes the analogy
between shell effects observed for clusters and nanowires.Comment: 4 page
Colour-Dielectric Gauge Theory on a Transverse Lattice
We investigate in some detail consequences of the effective colour-dielectric
formulation of lattice gauge theory using the light-cone Hamiltonian formalism
with a transverse lattice. As a quantitative test of this approach, we have
performed extensive analytic and numerical calculations for 2+1-dimensional
pure gauge theory in the large N limit. Because of Eguchi-Kawai reduction, one
effectively studies a 1+1-dimensional gauge theory coupled to matter in the
adjoint representation. We study the structure of coupling constant space for
our effective potential by comparing with the physical results available from
conventional Euclidean lattice Monte Carlo simulations of this system. In
particular, we calculate and measure the scaling behaviour of the entire
low-lying glueball spectrum, glueball wavefunctions, string tension, asymptotic
density of states, and deconfining temperature. We employ a new hybrid
DLCQ/wavefunction basis in our calculations of the light-cone Hamiltonian
matrix elements, along with extrapolation in Tamm-Dancoff truncation,
significantly reducing numerical errors. Finally we discuss, in light of our
results, what further measurements and calculations could be made in order to
systematically remove lattice spacing dependence from our effective potential a
priori.Comment: 48 pages, Latex, uses macro boxedeps.tex, minor errors corrected in
revised versio
QCD corrections to decay-lepton polar and azimuthal angular distributions in e+e- -> t tbar in the soft-gluon approximation
QCD corrections to order alpha_s in the soft-gluon approximation to angular
distributions of decay charged leptons in the process e+e- -> t tbar followed
by semileptonic decay of t or tbar, are obtained in the e+e- centre-of-mass
frame. As compared to distributions in the top rest frame, these have the
advantage that they would allow direct comparison with experiment without the
need to reconstruct the top rest frame. The results also do not depend on the
choice of a spin quantization axis for t or tbar. Analytic expression for the
triple distribution in the polar angle of t and polar and azimuthal angles of
the lepton is obtained. Analytic expression is also derived for the
distribution in the charged-lepton polar angle. Numerical values are discussed
for total c.m. energies of 400 GeV, 800 GeV and 1500 GeV.Comment: 21 pages, Latex, 6 figures included in the submission. To appear in
Pramana - Journal of Physics; expanded version of hep-ph/0011321, v
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