1,138 research outputs found
Dynamical derivation of Bode's law
In a planetary or satellite system, idealized as n small bodies in initially
coplanar, concentric orbits around a large central body, obeying Newtonian
point-particle mechanics, resonant perturbations will cause dynamical evolution
of the orbital radii except under highly specific mutual relationships, here
derived analytically apparently for the first time. In particular, the most
stable situation is achieved (in this idealized model) only when each planetary
orbit is roughly twice as far from the Sun as the preceding one, as observed
empirically already by Titius (1766) and Bode (1778) and used in both the
discoveries of Uranus (1781) and the Asteroid Belt (1801). ETC.Comment: 27 page
Evolutionary Convergence on Highly-Conserved 3′ Intron Structures in Intron-Poor Eukaryotes and Insights into the Ancestral Eukaryotic Genome
The presence of spliceosomal introns in eukaryotes raises a range of questions about genomic evolution. Along with the fundamental mysteries of introns' initial proliferation and persistence, the evolutionary forces acting on intron sequences remain largely mysterious. Intron number varies across species from a few introns per genome to several introns per gene, and the elements of intron sequences directly implicated in splicing vary from degenerate to strict consensus motifs. We report a 50-species comparative genomic study of intron sequences across most eukaryotic groups. We find two broad and striking patterns. First, we find that some highly intron-poor lineages have undergone evolutionary convergence to strong 3′ consensus intron structures. This finding holds for both branch point sequence and distance between the branch point and the 3′ splice site. Interestingly, this difference appears to exist within the genomes of green alga of the genus Ostreococcus, which exhibit highly constrained intron sequences through most of the intron-poor genome, but not in one much more intron-dense genomic region. Second, we find evidence that ancestral genomes contained highly variable branch point sequences, similar to more complex modern intron-rich eukaryotic lineages. In addition, ancestral structures are likely to have included polyT tails similar to those in metazoans and plants, which we found in a variety of protist lineages. Intriguingly, intron structure evolution appears to be quite different across lineages experiencing different types of genome reduction: whereas lineages with very few introns tend towards highly regular intronic sequences, lineages with very short introns tend towards highly degenerate sequences. Together, these results attest to the complex nature of ancestral eukaryotic splicing, the qualitatively different evolutionary forces acting on intron structures across modern lineages, and the impressive evolutionary malleability of eukaryotic gene structures
Interferometry of Direct Photons in Central 280Pb+208Pb Collisions at 158A GeV
Two-particle correlations of direct photons were measured in central
208Pb+208Pb collisions at 158 AGeV. The invariant interferometric radii were
extracted for 100<K_T<300 MeV/c and compared to radii extracted from charged
pion correlations. The yield of soft direct photons, K_T<300 MeV/c, was
extracted from the correlation strength and compared to theoretical
calculations.Comment: 5 pages, 4 figure
Systematics of Inclusive Photon Production in 158 AGeV Pb Induced Reactions on Ni, Nb, and Pb Targets
The multiplicity of inclusive photons has been measured on an event-by-event
basis for 158 AGeV Pb induced reactions on Ni, Nb, and Pb targets. The
systematics of the pseudorapidity densities at midrapidity (rho_max) and the
width of the pseudorapidity distributions have been studied for varying
centralities for these collisions. A power law fit to the photon yield as a
function of the number of participating nucleons gives a value of 1.13+-0.03
for the exponent. The mean transverse momentum, , of photons determined
from the ratio of the measured electromagnetic transverse energy and photon
multiplicity, remains almost constant with increasing rho_max. Results are
compared with model predictions.Comment: 16 pages including 4 figure
Men??s circumcision status and women??s risk of HIV acquisition in Zimbabwe and Uganda
OBJECTIVE: To assess whether male circumcision of the primary sex partner is associated with women's risk of HIV.
DESIGN: Data were analyzed from 4417 Ugandan and Zimbabwean women participating in a prospective study of hormonal contraception and HIV acquisition. Most were recruited from family planning clinics; some in Uganda were referred from higher-risk settings such as sexually transmitted disease clinics.
METHODS: Using Cox proportional hazards models, time to HIV acquisition was compared for women with circumcised or uncircumcised primary partners. Possible misclassification of male circumcision was assessed using sensitivity analysis.
RESULTS: At baseline, 74% reported uncircumcised primary partners, 22% had circumcised partners and 4% had partners of unknown circumcision status. Median follow-up was 23 months, during which 210 women acquired HIV (167, 34, and 9 women whose primary partners were uncircumcised, circumcised, or of unknown circumcision status, respectively). Although unadjusted analyses indicated that women with circumcised partners had lower HIV risk than those with uncircumcised partners, the protective effect disappeared after adjustment for other risk factors [hazard ratio (HR), 1.03; 95% confidence interval (CI), 0.69-1.53]. Subgroup analyses suggested a non-significant protective effect of male circumcision on HIV acquisition among Ugandan women referred from higher-risk settings: adjusted HR 0.16 (95% CI, 0.02-1.25) but little effect in Ugandans (HR, 1.33; 95% CI, 0.72-2.47) or Zimbabweans (HR, 1.12; 95% CI, 0.65-1.91) from family planning clinics.
CONCLUSIONS: After adjustment, male circumcision was not significantly associated with women's HIV risk. The potential protection offered by male circumcision for women recruited from high-risk settings warrants further investigation
Scaling of Particle and Transverse Energy Production in 208Pb+208Pb collisions at 158 A GeV
Transverse energy, charged particle pseudorapidity distributions and photon
transverse momentum spectra have been studied as a function of the number of
participants (N_{part}) and the number of binary nucleon-nucleon collisions
(N_{coll}) in 158 A GeV Pb+Pb collisions over a wide impact parameter range. A
scaling of the transverse energy pseudorapidity density at midrapidity as
N_{part}^{1.08 \pm 0.06} and N_{coll}^{0.83 \pm 0.05} is observed. For the
charged particle pseudorapidity density at midrapidity we find a scaling as
N_{part}^{1.07 \pm 0.04} and N_{coll}^{0.82 \pm 0.03}. This faster than linear
scaling with N_{part} indicates a violation of the naive Wounded Nucleon Model.Comment: 13 pages, 16 figures, submitted to European Physical Journal C
(revised results for scaling exponents
Event-by-Event Fluctuations in Particle Multiplicities and Transverse Energy Produced in 158.A GeV Pb+Pb collisions
Event-by-event fluctuations in the multiplicities of charged particles and
photons, and the total transverse energy in 158 GeV Pb+Pb collisions
are studied for a wide range of centralities. For narrow centrality bins the
multiplicity and transverse energy distributions are found to be near perfect
Gaussians. The effect of detector acceptance on the multiplicity fluctuations
has been studied and demonstrated to follow statistical considerations. The
centrality dependence of the charged particle multiplicity fluctuations in the
measured data has been found to agree reasonably well with those obtained from
a participant model. However for photons the multiplicity fluctuations has been
found to be lower compared to those obtained from a participant model. The
multiplicity and transverse energy fluctuations have also been compared to
those obtained from the VENUS event generator.Comment: To appear in Physical Review C; changes : more detailed discussion on
errors and few figures modifie
Centrality Dependence of Neutral Pion Production in 158 A GeV Pb + Pb Collisions
The production of neutral pions in 158AGeV Pb+Pb collisions has been studied
in the WA98 experiment at the CERN SPS. Transverse momentum spectra are studied
for the range 0.3 GeV/c < mT-m0 < 4.0 GeV/c. The results for central collisions
are compared to various models. The centrality dependence of the neutral pion
spectral shape and yield is investigated. An invariance of the spectral shape
and a simple scaling of the yield with the number of participating nucleons is
observed for centralities with greater than about 30 participating nucleons
which is most naturally explained by assuming an equilibrated system.Comment: 5 pages, Latex, including 3 eps figures, submitted to Phys.Rev.Lett;
updated pQCD comparison due to new input from the author, updated references,
corrected plotting error in figure
Determination of the Michel Parameters and the tau Neutrino Helicity in tau Decay
Using the CLEO II detector at the storage ring CESR, we have
determined the Michel parameters , , and in decay as well as the tau neutrino helicity parameter
in decay. From a data sample of
tau pairs produced at , using events of
the topology and , and the determined sign of , the combined
result of the three samples is: , , , and
. The results are in agreement with
the Standard Model V-A interaction.Comment: 18 page postscript file, postscript file also available through
http://w4.lns.cornell.edu/public/CLN
Study of 3-prong Hadronic Decays with Charged Kaons
Using a sample of 4.7/fb integrated luminosity accumulated with the CLEO-II
detector at the Cornell Electron Storage Ring (CESR), we have measured the
branching fractions of the tau lepton into and relative to and relative to . The relative branching fractions are: (5.16+-0.20+-0.50)*,
(1.52+-0.14+-0.29)*, (2.54+-0.44+-0.39)* and at 95%
C.L., respectively. Coupled with additional experimental information, we use
our results to extract information on the structure of three-prong tau decays
to charged kaons.Comment: 16 pages postscript file also available through
http://w4.lns.cornell.edu/public/CLN
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