13,950 research outputs found
QCD and e+e- --> Baryon + anti-Baryon
We discuss the QCD constraints on e+e- --> baryon-anti-baryon close to
threshold, in light of the puzzling experimental data which indicate that close
to threshold sigma (e+e- --> n n-bar) > sigma (e+e- --> p p-bar). We focus on
the process e+e- --> Delta Delta-bar, which is particularly simple from the
theoretical point of view. In this case it is possible to make exact QCD
predictions for the relative yields of the four members of the Delta multiplet,
modulo one crucial dynamical assumption.Comment: extended discussion of quark loop suppression in baryons in the
large-N_c limit; updated ref
Study of the performance of the NA62 Small-Angle Calorimeter at the DANE Linac
The measurement of with 10% precision by the
NA62 experiment requires extreme background suppression. The Small Angle
Calorimeter aims to provide an efficient veto for photons flying at angles down
to zero with respect to the kaon flight direction. The initial prototype was
upgraded and tested at the Beam Test Facility of the DANE Linac at
Frascati. The energy resolution and the efficiency were measured and are
presented.Comment: 5 pages, 7 figure
Random Time Forward Starting Options
We introduce a natural generalization of the forward-starting options, first
discussed by M. Rubinstein. The main feature of the contract presented here is
that the strike-determination time is not fixed ex-ante, but allowed to be
random, usually related to the occurrence of some event, either of financial
nature or not. We will call these options {\bf Random Time Forward Starting
(RTFS)}. We show that, under an appropriate "martingale preserving" hypothesis,
we can exhibit arbitrage free prices, which can be explicitly computed in many
classical market models, at least under independence between the random time
and the assets' prices. Practical implementations of the pricing methodologies
are also provided. Finally a credit value adjustment formula for these OTC
options is computed for the unilateral counterparty credit risk.Comment: 19 pages, 1 figur
The BeppoSAX WFC X-ray source catalogue
We present the catalogue of X-ray sources detected by the two Wide Field
Cameras (WFCs) in complete observations on board BeppoSAX during its 6 years of
operational lifetime, between April 1996 and April 2002. The BeppoSAX WFCs were
coded mask instruments sensitive in the 2-28 keV energy band with a 40x40
square degree fields of view, pointing in opposite directions and
perpendicularly to the BeppoSAX Narrow Field Instruments (NFI). The WFCs were
usually operated simultaneously to NFI observations, each lasting up to several
days. WFCs observed thus the entire sky several times with a typical
sensitivity of 2 to 10 mCrab. A systematic analysis of all WFC observations in
the BeppoSAX archive has been carried out using the latest post-mission release
of the WFC analysis software and calibrations. The catalogue includes 253
distinct sources, obtained from a total sample of 8253 WFC detections. We
describe the basic statistical properties of the sample and present a six-year
history of two celestial calibration X-ray sources.Comment: 15 pages, 11 figures, Catalogue, Accepted for publication on A&
GRB 081029: Understanding Multiple Afterglow Components
We present an analysis of the unusual optical light curve of the gamma-ray
burst GRB~081029, which occurred at a redshift of z = 3.8479$. We combine X-ray
and optical observations from the Swift X-Ray Telescope and the Swift
UltraViolet/Optical Telescope with optical and infrared data obtained using the
REM and ROTSE telescopes to construct a detailed data set extending from 86 s
to approximately 100,000 s after the BAT trigger. Our data also cover a wide
energy range, from 10 keV to 0.77 eV (1.24 Angstrom to 16,000 Angstrom). The
X-ray afterglow shows a shallow initial decay followed by a rapid decay
starting at about 18,000s. The optical and infrared afterglow, however, shows
an uncharacteristic rise at about 5000 s that does not correspond to any
feature in the X-ray light curve. Our data are not consistent with synchrotron
radiation from a single-component jet interacting with an external medium. We
do, however, find that the observed light curve can be explained using
multi-component model for the jet.Comment: 4 pages, 3 figures, to appear in the AIP Conference Proceedings for
the Gamma-Ray Burst 2010 Conference, Annapolis, MD, USA, November 201
Estimating Age-Dependent Extinction: Contrasting Evidence from Fossils and Phylogenies.
The estimation of diversification rates is one of the most vividly debated topics in modern systematics, with considerable controversy surrounding the power of phylogenetic and fossil-based approaches in estimating extinction. Van Valen's seminal work from 1973 proposed the "Law of constant extinction," which states that the probability of extinction of taxa is not dependent on their age. This assumption of age-independent extinction has prevailed for decades with its assessment based on survivorship curves, which, however, do not directly account for the incompleteness of the fossil record, and have rarely been applied at the species level. Here, we present a Bayesian framework to estimate extinction rates from the fossil record accounting for age-dependent extinction (ADE). Our approach, unlike previous implementations, explicitly models unobserved species and accounts for the effects of fossil preservation on the observed longevity of sampled lineages. We assess the performance and robustness of our method through extensive simulations and apply it to a fossil data set of terrestrial Carnivora spanning the past 40 myr. We find strong evidence of ADE, as we detect the extinction rate to be highest in young species and declining with increasing species age. For comparison, we apply a recently developed analogous ADE model to a dated phylogeny of extant Carnivora. Although the phylogeny-based analysis also infers ADE, it indicates that the extinction rate, instead, increases with increasing taxon age. The estimated mean species longevity also differs substantially, with the fossil-based analyses estimating 2.0 myr, in contrast to 9.8 myr derived from the phylogeny-based inference. Scrutinizing these discrepancies, we find that both fossil and phylogeny-based ADE models are prone to high error rates when speciation and extinction rates increase or decrease through time. However, analyses of simulated and empirical data show that fossil-based inferences are more robust. This study shows that an accurate estimation of ADE from incomplete fossil data is possible when the effects of preservation are jointly modeled, thus allowing for a reassessment of Van Valen's model as a general rule in macroevolution
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