5,213 research outputs found
Quantum-State Controlled Penning Ionization Reactions between Ultracold Alkali and Metastable Helium Atoms
In an ultracold, optically trapped mixture of Rb and metastable
triplet He atoms we have studied trap loss for different spin-state
combinations, for which interspecies Penning ionization is the main two-body
loss process. We observe long trapping lifetimes for the purely quartet
spin-state combination, indicating strong suppression of Penning ionization
loss by at least two orders of magnitude. For the other spin-mixtures we
observe short lifetimes that depend linearly on the doublet character of the
entrance channel. We compare the extracted loss rate coefficient with recent
predictions of multichannel quantum-defect theory for reactive collisions
involving a strong exothermic loss channel and find near-universal loss for
doublet scattering. Our work demonstrates control of reactive collisions by
internal atomic state preparation.Comment: 5 pages, 5 figures + Supplemental Material
Efficient production of an 87Rb F = 2, mF = 2 Bose-Einstein condensate in a hybrid trap
We have realized Bose-Einstein condensation (BEC) of 87Rb in the F=2, m_F=2
hyperfine substate in a hybrid trap, consisting of a quadrupole magnetic field
and a single optical dipole beam. The symmetry axis of the quadrupole magnetic
trap coincides with the optical beam axis, which gives stronger axial
confinement than previous hybrid traps. After loading 2x10^6 atoms at 14 muK
from a quadrupole magnetic trap into the hybrid trap, we perform efficient
forced evaporation and reach the onset of BEC at a temperature of 0.5 muK and
with 4x10^5 atoms. We also obtain thermal clouds of 1x10^6 atoms below 1 muK in
a pure single beam optical dipole trap, by ramping down the magnetic field
gradient after evaporative cooling in the hybrid trap.Comment: 8 pages, 8 figures, improved on basis of referee comment
Finite mixtures of matrix-variate Poisson-log normal distributions for three-way count data
Three-way data structures, characterized by three entities, the units, the
variables and the occasions, are frequent in biological studies. In RNA
sequencing, three-way data structures are obtained when high-throughput
transcriptome sequencing data are collected for n genes across p conditions at
r occasions. Matrix-variate distributions offer a natural way to model
three-way data and mixtures of matrix-variate distributions can be used to
cluster three-way data. Clustering of gene expression data is carried out as
means to discovering gene co-expression networks. In this work, a mixture of
matrix-variate Poisson-log normal distributions is proposed for clustering read
counts from RNA sequencing. By considering the matrix-variate structure, full
information on the conditions and occasions of the RNA sequencing dataset is
simultaneously considered, and the number of covariance parameters to be
estimated is reduced. A Markov chain Monte Carlo expectation-maximization
algorithm is used for parameter estimation and information criteria are used
for model selection. The models are applied to both real and simulated data,
giving favourable clustering results
New Central American and Mexican Enaphalodes Haldeman, 1847 (Coleoptera: Cerambycidae) with taxonomic notes and a key to species
A review of Enaphalodes Haldeman, 1847 is presented. Descriptions of four new species of Enaphalodes are included: E. antonkozlovi, sp. nov. from Costa Rica, E. bingkirki, sp. nov. from Nicaragua, E. monzoni, sp. nov. from Guatemala, and E. cunninghami, sp. nov. from Mexico. Enaphalodes senex (Bates, 1884) is revalidated and it is newly recorded from Nicaragua and Guatemala. A key to the 15 currently recognized species of Enaphalodes is included
Semantic variation operators for multidimensional genetic programming
Multidimensional genetic programming represents candidate solutions as sets
of programs, and thereby provides an interesting framework for exploiting
building block identification. Towards this goal, we investigate the use of
machine learning as a way to bias which components of programs are promoted,
and propose two semantic operators to choose where useful building blocks are
placed during crossover. A forward stagewise crossover operator we propose
leads to significant improvements on a set of regression problems, and produces
state-of-the-art results in a large benchmark study. We discuss this
architecture and others in terms of their propensity for allowing heuristic
search to utilize information during the evolutionary process. Finally, we look
at the collinearity and complexity of the data representations that result from
these architectures, with a view towards disentangling factors of variation in
application.Comment: 9 pages, 8 figures, GECCO 201
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