21,716 research outputs found
Systematics of Charged Particle Production in Heavy-Ion Collisions with the PHOBOS Detector at RHIC
The multiplicity of charged particles produced in Au+Au collisions as a
function of energy, centrality, rapidity and azimuthal angle has been measured
with the PHOBOS detector at RHIC. These results contribute to our understanding
of the initial state of heavy ion collisions and provide a means to compare
basic features of particle production in nuclear collisions with more
elementary systems.Comment: 4 pages, 4 figures (in eps) talk given at XXXI International
Symposium on Multiparticle Dynamics, Sep. 1-7, 2001, Datong China URL
http://ismd31.ccnu.edu.cn
A New Characterization of Fine Scale Diffusion on the Cell Membrane
We use a large single particle tracking data set to analyze the short time
and small spatial scale motion of quantum dots labeling proteins in cell
membranes. Our analysis focuses on the jumps which are the changes in the
position of the quantum dots between frames in a movie of their motion.
Previously we have shown that the directions of the jumps are uniformly
distributed and the jump lengths can be characterized by a double power law
distribution.
Here we show that the jumps over a small number of time steps can be
described by scalings of a {\em single} double power law distribution. This
provides additional strong evidence that the double power law provides an
accurate description of the fine scale motion. This more extensive analysis
provides strong evidence that the double power law is a novel stable
distribution for the motion. This analysis provides strong evidence that an
earlier result that the motion can be modeled as diffusion in a space of
fractional dimension roughly 3/2 is correct. The form of the power law
distribution quantifies the excess of short jumps in the data and provides an
accurate characterization of the fine scale diffusion and, in fact, this
distribution gives an accurate description of the jump lengths up to a few
hundred nanometers. Our results complement of the usual mean squared
displacement analysis used to study diffusion at larger scales where the
proteins are more likely to strongly interact with larger membrane structures.Comment: 18 pages, 7 figure
Extracting joint weak values with local, single-particle measurements
Weak measurement is a new technique which allows one to describe the
evolution of postselected quantum systems. It appears to be useful for
resolving a variety of thorny quantum paradoxes, particularly when used to
study properties of pairs of particles. Unfortunately, such nonlocal or joint
observables often prove difficult to measure weakly in practice (for instance,
in optics -- a common testing ground for this technique -- strong photon-photon
interactions would be needed). Here we derive a general, experimentally
feasible, method for extracting these values from correlations between
single-particle observables.Comment: 6 page
Quantum Nonlocality in Two-Photon Experiments at Berkeley
We review some of our experiments performed over the past few years on
two-photon interference. These include a test of Bell's inequalities, a study
of the complementarity principle, an application of EPR correlations for
dispersion-free time-measurements, and an experiment to demonstrate the
superluminal nature of the tunneling process. The nonlocal character of the
quantum world is brought out clearly by these experiments. As we explain,
however, quantum nonlocality is not inconsistent with Einstein causality.Comment: 16 pages including 24 figure
Nonlinear optics with less than one photon
We demonstrate suppression and enhancement of spontaneous parametric down-
conversion via quantum interference with two weak fields from a local
oscillator (LO). Pairs of LO photons are observed to upconvert with high
efficiency for appropriate phase settings, exhibiting an effective nonlinearity
enhanced by at least 10 orders of magnitude. This constitutes a two-photon
switch, and promises to be useful for a variety of nonlinear optical effects at
the quantum level.Comment: 8 pages, 5 figure
Causality and Electromagnetic Transmissions Through Materials
There have been several experiments which hint at evidence for superluminal
transport of electromagnetic energy through a material slab. On the theoretical
side, it has appeared evident that acausal signals are indeed possible in
quantum electrodynamics. However, it is unlikely that superluminal signals can
be understood on the basis of a purely classical electrodynamic signals passing
through a material. The classical and quantum theories represent quite
different views, and it is the quantum view which may lead to violations of
Einstein causality.Comment: Plain TeX, No figures, 5 page
On the rational subset problem for groups
We use language theory to study the rational subset problem for groups and
monoids. We show that the decidability of this problem is preserved under graph
of groups constructions with finite edge groups. In particular, it passes
through free products amalgamated over finite subgroups and HNN extensions with
finite associated subgroups. We provide a simple proof of a result of
Grunschlag showing that the decidability of this problem is a virtual property.
We prove further that the problem is decidable for a direct product of a group
G with a monoid M if and only if membership is uniformly decidable for
G-automata subsets of M. It follows that a direct product of a free group with
any abelian group or commutative monoid has decidable rational subset
membership.Comment: 19 page
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