1,067 research outputs found
Bose-Einstein condensation in the presence of a uniform field and a point-like impurity
The behavior of an ideal -dimensional boson gas in the presence of a
uniform gravitational field is analyzed. It is explicitly shown that,
contrarily to an old standing folklore, the three-dimensional gas does not
undergo Bose-Einstein condensation at finite temperature. On the other hand,
Bose-Einstein condensation occurs at for if there is a
point-like impurity at the bottom of the vessel containing the gas.Comment: 14 pages, REVTEX. Revised version, accepted for publication in Phys.
Rev.
Mother Moose: Generating Extra Dimensions from Simple Groups at Large N
We show that there exists a correspondence between four dimensional gauge
theories with simple groups and higher dimensional gauge theories at large N.
As an example, we show that a four dimensional {N}=2 supersymmetric SU(N) gauge
theory, on the Higgs branch, has the same correlators as a five dimensional
SU(N) gauge theory in the limit of large N provided the couplings are
appropriately rescaled. We show that our results can be applied to the AdS/CFT
correspondence to derive correlators of five or more dimensional gauge theories
from solutions of five dimensional supergravity in the large t'Hooft coupling
limit.Comment: 12 pages, references adde
Perturbative Computation of the Gluonic Effective Action via Polyaokov's World-Line Path Integral
The Polyakov world-line path integral describing the propagation of gluon
field quanta is constructed by employing the background gauge fixing method and
is subsequently applied to analytically compute the divergent terms of the one
(gluonic) loop effective action to fourth order in perturbation theory. The
merits of the proposed approach is that, to a given order, it reduces to
performing two integrations, one over a set of Grassmann and one over a set of
Feynman-type parameters through which one manages to accomodate all Feynman
diagrams entering the computation at once.Comment: 21 page
Towards a Gravitational Analog to S-duality in Non-abelian Gauge Theories
For non-abelian non-supersymmetric gauge theories, generic dual theories have
been constructed. In these theories the couplings appear inverted. However,
they do not possess a Yang-Mills structure but rather are a kind of non-linear
sigma model. It is shown that for a topological gravitational model an analog
to this duality exists.Comment: LaTeX, 14 pages, no figures, minor correction
Selective Constraints on Amino Acids Estimated by a Mechanistic Codon Substitution Model with Multiple Nucleotide Changes
Empirical substitution matrices represent the average tendencies of
substitutions over various protein families by sacrificing gene-level
resolution. We develop a codon-based model, in which mutational tendencies of
codon, a genetic code, and the strength of selective constraints against amino
acid replacements can be tailored to a given gene. First, selective constraints
averaged over proteins are estimated by maximizing the likelihood of each 1-PAM
matrix of empirical amino acid (JTT, WAG, and LG) and codon (KHG) substitution
matrices. Then, selective constraints specific to given proteins are
approximated as a linear function of those estimated from the empirical
substitution matrices.
Akaike information criterion (AIC) values indicate that a model allowing
multiple nucleotide changes fits the empirical substitution matrices
significantly better. Also, the ML estimates of transition-transversion bias
obtained from these empirical matrices are not so large as previously
estimated. The selective constraints are characteristic of proteins rather than
species. However, their relative strengths among amino acid pairs can be
approximated not to depend very much on protein families but amino acid pairs,
because the present model, in which selective constraints are approximated to
be a linear function of those estimated from the JTT/WAG/LG/KHG matrices, can
provide a good fit to other empirical substitution matrices including cpREV for
chloroplast proteins and mtREV for vertebrate mitochondrial proteins.
The present codon-based model with the ML estimates of selective constraints
and with adjustable mutation rates of nucleotide would be useful as a simple
substitution model in ML and Bayesian inferences of molecular phylogenetic
trees, and enables us to obtain biologically meaningful information at both
nucleotide and amino acid levels from codon and protein sequences.Comment: Table 9 in this article includes corrections for errata in the Table
9 published in 10.1371/journal.pone.0017244. Supporting information is
attached at the end of the article, and a computer-readable dataset of the ML
estimates of selective constraints is available from
10.1371/journal.pone.001724
Activated Magnetospheres of Magnetars
Like the solar corona, the external magnetic field of magnetars is twisted by
surface motions of the star. The twist energy is dissipated over time. We
discuss the theory of this activity and its observational status. (1) Theory
predicts that the magnetosphere tends to untwist in a peculiar way: a bundle of
electric currents (the "j-bundle") is formed with a sharp boundary, which
shrinks toward the magnetic dipole axis. Recent observations of shrinking hot
spots on magnetars are consistent with this behavior. (2) Continual discharge
fills the j-bundle with electron-positron plasma, maintaining a nonthermal
corona around the neutron star. The corona outside a few stellar radii strongly
interacts with the stellar radiation and forms a "radiatively locked" outflow
with a high e+- multiplicity. The locked plasma annihilates near the apexes of
the closed magnetic field lines. (3) New radiative-transfer simulations suggest
a simple mechanism that shapes the observed X-ray spectrum from 0.1 keV to 1
MeV: part of the thermal X-rays emitted by the neutron star are reflected from
the outer corona and then upscattered by the inner relativistic outflow in the
j-bundle, producing a beam of hard X-rays.Comment: 23 pages, 7 figures; review chapter in the proceedings of ICREA
Workshop on the High-Energy Emission from Pulsars and Their Systems, Sant
Cugat, Spain, April 201
Region of Excessive Flux of PeV Cosmic Rays in the Direction Toward Pulsars PSR J1840+5640 and LAT PSR J1836+5925
An analysis of arrival directions of extensive air showers (EAS) registered
with the EAS MSU and EAS-1000 prototype arrays has revealed a region of
excessive flux of PeV cosmic rays in the direction toward pulsars PSR
J1840+5640 and LAT PSR J1836+5925 at significance level up to 4.5sigma. The
first of the pulsars was discovered almost 30 years ago and is a well-studied
old radio pulsar located at the distance of 1.7pc from the Solar system. The
second pulsar belongs to a new type of pulsars, discovered by the space
gamma-ray observatory Fermi, pulsations of which are not observed in optical
and radio wavelengths but only in the gamma-ray range of energies
(gamma-ray-only pulsars). In our opinion, the existence of the region of
excessive flux of cosmic rays registered with two different arrays provides a
strong evidence that isolated pulsars can give a noticeable contribution to the
flux of Galactic cosmic rays in the PeV energy range.Comment: 14 pages; v.2: a few remarks to match a version accepted for
Astronomy Letters added. They can be found by redefining the \NEW command in
the preamble of the LaTeX fil
BICEP3: a 95 GHz refracting telescope for degree-scale CMB polarization
BICEP3 is a 550 mm-aperture refracting telescope for polarimetry of radiation
in the cosmic microwave background at 95 GHz. It adopts the methodology of
BICEP1, BICEP2 and the Keck Array experiments - it possesses sufficient
resolution to search for signatures of the inflation-induced cosmic
gravitational-wave background while utilizing a compact design for ease of
construction and to facilitate the characterization and mitigation of
systematics. However, BICEP3 represents a significant breakthrough in
per-receiver sensitivity, with a focal plane area 5 larger than a
BICEP2/Keck Array receiver and faster optics ( vs. ).
Large-aperture infrared-reflective metal-mesh filters and infrared-absorptive
cold alumina filters and lenses were developed and implemented for its optics.
The camera consists of 1280 dual-polarization pixels; each is a pair of
orthogonal antenna arrays coupled to transition-edge sensor bolometers and read
out by multiplexed SQUIDs. Upon deployment at the South Pole during the 2014-15
season, BICEP3 will have survey speed comparable to Keck Array 150 GHz (2013),
and will significantly enhance spectral separation of primordial B-mode power
from that of possible galactic dust contamination in the BICEP2 observation
patch.Comment: 12 pages, 5 figures. Presented at SPIE Astronomical Telescopes and
Instrumentation 2014: Millimeter, Submillimeter, and Far-Infrared Detectors
and Instrumentation for Astronomy VII. To be published in Proceedings of SPIE
Volume 915
The concept of solidarity: emerging from the theoretical shadows?
The concept of solidarity has been relatively neglected by social scientists since Durkheim's pioneering work in the late 19th century. The discipline of politics has been guilty of overlooking this 'subjective' element of community life, but recent works by Stjernø and Brunkhorst reflect a growing awareness of the theoretical significance of the concept. Whereas early liberal attempts to theorise solidarity took the nation state to be the appropriate community for its realisation, the emergence of globalisation raises the possibility of human solidarity developing in the global community. Traditional forms of solidarity have been dissipated by the social changes accompanying globalisation, but they were often locked into the defence of particular interests. New forms may be emerging to rekindle the broader vision of human solidarity. Recent work by writers such as Habermas, Honneth, Rorty and Touraine focuses on widening and deepening democratic participation and/or the articulation of our ethical obligations in various ways. It is argued here that these perspectives need to be supplemented by a radical humanist approach grounded in a normative theory of human self-realisation
BICEP2 / Keck Array V: Measurements of B-mode Polarization at Degree Angular Scales and 150 GHz by the Keck Array
The Keck Array is a system of cosmic microwave background (CMB) polarimeters,
each similar to the BICEP2 experiment. In this paper we report results from the
2012 and 2013 observing seasons, during which the Keck Array consisted of five
receivers all operating in the same (150 GHz) frequency band and observing
field as BICEP2. We again find an excess of B-mode power over the
lensed-CDM expectation of in the range
and confirm that this is not due to systematics using jackknife tests and
simulations based on detailed calibration measurements. In map difference and
spectral difference tests these new data are shown to be consistent with
BICEP2. Finally, we combine the maps from the two experiments to produce final
Q and U maps which have a depth of 57 nK deg (3.4 K arcmin) over an
effective area of 400 deg for an equivalent survey weight of 250,000
K. The final BB band powers have noise uncertainty a factor of 2.3
times better than the previous results, and a significance of detection of
excess power of .Comment: 13 pages, 9 figure
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