43 research outputs found
Depletion of a Bose-Einstein condensate by laser-iduced dipole-dipole interactions
We study a gaseous Bose-Einstein condensate with laser-induced dipole-dipole
interactions using the Hartree-Fock-Bogoliubov theory within the Popov
approximation. The dipolar interactions introduce long-range atom-atom
correlations, which manifest themselves as increased depletion at momenta
similar to that of the laser wavelength, as well as a "roton" dip in the
excitation spectrum. Surprisingly, the roton dip and the corresponding peak in
the depletion are enhanced by raising the temperature above absolute zero.Comment: 10 pages, 6 figure
Sequence of Potentials Interpolating between the U(5) and E(5) Symmetries
It is proved that the potentials of the form (with being
integer) provide a ``bridge'' between the U(5) symmetry of the Bohr Hamiltonian
with a harmonic oscillator potential (occuring for ) and the E(5) model of
Iachello (Bohr Hamiltonian with an infinite well potential, materialized for
infinite ). Parameter-free (up to overall scale factors) predictions for
spectra and B(E2) transition rates are given for the potentials ,
, , corresponding to ratios of 2.093, 2.135,
2.157 respectively, compared to the ratios 2.000 of U(5) and 2.199 of
E(5). Hints about nuclei showing this behaviour, as well as about potentials
``bridging'' the E(5) symmetry with O(6) are briefly discussed. A note about
the appearance of Bessel functions in the framework of E(n) symmetries is given
as a by-product.Comment: LaTeX, 17 pages, 9 postscript figure
On the breaking of a plasma wave in a thermal plasma: I. The structure of the density singularity
The structure of the singularity that is formed in a relativistically large
amplitude plasma wave close to the wavebreaking limit is found by using a
simple waterbag electron distribution function. The electron density
distribution in the breaking wave has a typical "peakon" form. The maximum
value of the electric field in a thermal breaking plasma is obtained and
compared to the cold plasma limit. The results of computer simulations for
different initial electron distribution functions are in agreement with the
theoretical conclusions.Comment: 21 pages, 12 figure
Constraints on the Electrical Charge Asymmetry of the Universe
We use the isotropy of the Cosmic Microwave Background to place stringent
constraints on a possible electrical charge asymmetry of the universe. We find
the excess charge per baryon to be in the case of a uniform
distribution of charge, where is the charge of the electron. If the charge
asymmetry is inhomogeneous, the constraints will depend on the spectral index,
, of the induced magnetic field and range from
() to (). If one could further
assume that the charge asymmetries of individual particle species are not
anti-correlated so as to cancel, this would imply, for photons, ; for neutrinos, ; and for heavy (light) dark
matter particles ().Comment: New version to appear in JCA
Circular String-Instabilities in Curved Spacetime
We investigate the connection between curved spacetime and the emergence of
string-instabilities, following the approach developed by Loust\'{o} and
S\'{a}nchez for de Sitter and black hole spacetimes. We analyse the linearised
equations determining the comoving physical (transverse) perturbations on
circular strings embedded in Schwarzschild, Reissner-Nordstr\"{o}m and de
Sitter backgrounds. In all 3 cases we find that the "radial" perturbations grow
infinitely for (ring-collapse), while the "angular"
perturbations are bounded in this limit. For we find that
the perturbations in both physical directions (perpendicular to the string
world-sheet in 4 dimensions) blow up in the case of de Sitter space. This
confirms results recently obtained by Loust\'{o} and S\'{a}nchez who considered
perturbations around the string center of mass.Comment: 24 pages Latex + 2 figures (not included). Observatoire de Paris,
Meudon No. 9305
Strings Propagating in the 2+1 Dimensional Black Hole Anti de Sitter Spacetime
We study the string propagation in the 2+1 black hole anti de Sitter
background (2+1 BH-ADS). We find the first and second order fluctuations around
the string center of mass and obtain the expression for the string mass. The
string motion is stable, all fluctuations oscillate with real frequencies and
are bounded, even at We compare with the string motion in the ordinary
black hole anti de Sitter spacetime, and in the black string background, where
string instabilities develop and the fluctuations blow up at We find the
exact general solution for the circular string motion in all these backgrounds,
it is given closely and completely in terms of elliptic functions. For the
non-rotating black hole backgrounds the circular strings have a maximal bounded
size they contract and collapse into No indefinitely growing
strings, neither multi-string solutions are present in these backgrounds. In
rotating spacetimes, both the 2+1 BH-ADS and the ordinary Kerr-ADS, the
presence of angular momentum prevents the string from collapsing into
The circular string motion is also completely solved in the black hole de
Sitter spacetime and in the black string background (dual of the 2+1 BH-ADS
spacetime), in which expanding unbounded strings and multi-string solutions
appear.Comment: Latex, 54 pages + 2 tables and 4 figures (not included). PARIS-DEMIRM
94/01
Large expert-curated database for benchmarking document similarity detection in biomedical literature search
Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency–Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research