57,929 research outputs found
Temporal Feedback for Tweet Search with Non-Parametric Density Estimation
This paper investigates the temporal cluster hypothesis: in search tasks where time plays an important role, do relevant documents tend to cluster together in time? We explore this question in the context of tweet search and temporal feedback: starting with an initial set of results from a baseline retrieval model, we estimate the temporal density of relevant documents, which is then used for result reranking. Our contributions lie in a method to characterize this temporal density function using kernel density estimation, with and without human relevance judgments, and an approach to integrating this information into a standard retrieval model. Experiments on TREC datasets confirm that our temporal feedback formulation improves search effectiveness, thus providing support for our hypothesis. Our approach outperforms both a standard baseline and previous temporal retrieval models. Temporal feedback improves over standard lexical feedback (with and without human judgments), illustrating that temporal relevance signals exist independently of document content
Osculating and neighbour-avoiding polygons on the square lattice
We study two simple modifications of self-avoiding polygons. Osculating
polygons are a super-set in which we allow the perimeter of the polygon to
touch at a vertex. Neighbour-avoiding polygons are only allowed to have nearest
neighbour vertices provided these are joined by the associated edge and thus
form a sub-set of self-avoiding polygons. We use the finite lattice method to
count the number of osculating polygons and neighbour-avoiding polygons on the
square lattice. We also calculate their radius of gyration and the first
area-weighted moment. Analysis of the series confirms exact predictions for the
critical exponents and the universality of various amplitude combinations. For
both cases we have found exact solutions for the number of convex and
almost-convex polygons.Comment: 14 pages, 5 figure
A logarithmic generalization of tensor product theory for modules for a vertex operator algebra
We describe a logarithmic tensor product theory for certain module categories
for a ``conformal vertex algebra.'' In this theory, which is a natural,
although intricate, generalization of earlier work of Huang and Lepowsky, we do
not require the module categories to be semisimple, and we accommodate modules
with generalized weight spaces. The corresponding intertwining operators
contain logarithms of the variables.Comment: 39 pages. Misprints corrected. Final versio
A Bose-Einstein Condensate in a Uniform Light-induced Vector Potential
We use a two-photon dressing field to create an effective vector gauge
potential for Bose-condensed Rb atoms in the F=1 hyperfine ground state. The
dressed states in this Raman field are spin and momentum superpositions, and we
adiabatically load the atoms into the lowest energy dressed state. The
effective Hamiltonian of these neutral atoms is like that of charged particles
in a uniform magnetic vector potential, whose magnitude is set by the strength
and detuning of Raman coupling. The spin and momentum decomposition of the
dressed states reveals the strength of the effective vector potential, and our
measurements agree quantitatively with a simple single-particle model. While
the uniform effective vector potential described here corresponds to zero
magnetic field, our technique can be extended to non-uniform vector potentials,
giving non-zero effective magnetic fields.Comment: 5 pages, submitted to Physical Review Letter
Twisted and Nontwisted Bifurcations Induced by Diffusion
We discuss a diffusively perturbed predator-prey system. Freedman and
Wolkowicz showed that the corresponding ODE can have a periodic solution that
bifurcates from a homoclinic loop. When the diffusion coefficients are large,
this solution represents a stable, spatially homogeneous time-periodic solution
of the PDE. We show that when the diffusion coefficients become small, the
spatially homogeneous periodic solution becomes unstable and bifurcates into
spatially nonhomogeneous periodic solutions.
The nature of the bifurcation is determined by the twistedness of an
equilibrium/homoclinic bifurcation that occurs as the diffusion coefficients
decrease. In the nontwisted case two spatially nonhomogeneous simple periodic
solutions of equal period are generated, while in the twisted case a unique
spatially nonhomogeneous double periodic solution is generated through
period-doubling.
Key Words: Reaction-diffusion equations; predator-prey systems; homoclinic
bifurcations; periodic solutions.Comment: 42 pages in a tar.gz file. Use ``latex2e twisted.tex'' on the tex
files. Hard copy of figures available on request from
[email protected]
Perimeter Generating Functions For The Mean-Squared Radius Of Gyration Of Convex Polygons
We have derived long series expansions for the perimeter generating functions
of the radius of gyration of various polygons with a convexity constraint.
Using the series we numerically find simple (algebraic) exact solutions for the
generating functions. In all cases the size exponent .Comment: 8 pages, 1 figur
Growth of primordial black holes in a universe containing a massless scalar field
The evolution of primordial black holes in a flat Friedmann universe with a
massless scalar field is investigated in fully general relativistic numerical
relativity. A primordial black hole is expected to form with a scale comparable
to the cosmological apparent horizon, in which case it may go through an
initial phase with significant accretion. However, if it is very close to the
cosmological apparent horizon size, the accretion is suppressed due to general
relativistic effects. In any case, it soon gets smaller than the cosmological
horizon and thereafter it can be approximated as an isolated vacuum solution
with decaying mass accretion. In this situation the dynamical and inhomogeneous
scalar field is typically equivalent to a perfect fluid with a stiff equation
of state . The black hole mass never increases by more than a factor of
two, despite recent claims that primordial black holes might grow substantially
through accreting quintessence. It is found that the gravitational memory
scenario, proposed for primordial black holes in Brans-Dicke and scalar-tensor
theories of gravity, is highly unphysical.Comment: 24 pages, accepted for publication in Physical Review
Scattering of relativistic particles with Aharonov-Bohm-Coulomb interaction in two dimensions
The Aharonov-Bohm-Coulomb potentials in two dimensions may describe the
interaction between two particles carrying electric charge and magnetic flux,
say, Chern--Simons solitons, or so called anyons. The scattering problem for
such two-body systems is extended to the relativistic case, and the scattering
amplitude is obtained as a partial wave series. The electric charge and
magnetic flux is (, ) for one particle and (, ) for the
other. When , and takes on integer
or half integer values, the partial wave series is summed up approximately to
give a closed form. The results exhibit some nonperturbative features and
cannot be obtained from perturbative quantum electrodynamics at the tree level.Comment: revtex, 11 pages, no figur
A synthetic electric force acting on neutral atoms
Electromagnetism is a simple example of a gauge theory where the underlying
potentials -- the vector and scalar potentials -- are defined only up to a
gauge choice. The vector potential generates magnetic fields through its
spatial variation and electric fields through its time-dependence. We
experimentally produce a synthetic gauge field that emerges only at low energy
in a rubidium Bose-Einstein condensate: the neutral atoms behave as charged
particles do in the presence of a homogeneous effective vector potential. We
have generated a synthetic electric field through the time dependence of an
effective vector potential, a physical consequence even though the vector
potential is spatially uniform
Dust Distribution in Gas Disks. A Model for the Ring Around HR 4796A
There have been several model analyses of the near and mid IR flux from the
circumstellar ring around HR4796A. In the vicinity of a young star, the
possibility that the dust ring is embedded within a residual protostellar gas
disk cannot be ruled out. In a gas-rich environment, larger sizes () are needed for the particles to survive the radiative blow out. The total
dust mass required to account for the IR flux is . The
combined influence of gas and stellar radiation may also account for the
observed sharp inner boundary and rapidly fading outer boundary of the ring.
The pressure gradient induced by a small (10%) amplitude variation in the
surface density distribution of a low-mass gaseous disk would be sufficient to
modify the rotation speed of the gas.Comment: proof read version, 26 pages, LaTex, 11 figures. To appear in The
Astronomical Journal June 200
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