407 research outputs found
High Energy Neutrinos from Quasars
We review and clarify the assumptions of our basic model for neutrino
production in the cores of quasars, as well as those modifications to the model
subsequently made by other workers. We also present a revised estimate of the
neutrino background flux and spectrum obtained using more recent empirical
studies of quasars and their evolution. We compare our results with other
thoeretical calculations and experimental upper limits on the AGN neutrino
background flux. We also estimate possible neutrino fluxes from the jets of
blazars detected recently by the EGRET experiment on the Compton Gamma Ray
Observatory. We discuss the theoretical implications of these estimates.Comment: 14 pg., ps file (includes figures), To be published in Space Science
Review
Factors associated with pleurisy in pigs: a case-control analysis of slaughter pig data for England and Wales
UNLABELLED: A case-control investigation was undertaken to determine management and health related factors associated with pleurisy in slaughter pigs in England and Wales. METHODS: The British Pig Executive Pig Health Scheme database of abattoir pathology was used to identify 121 case (>10% prevalence of pleurisy on 3 or more assessment dates in the preceding 24 months) and 121 control units (≤5% prevalence of pleurisy on 3 or more assessment dates in the preceding 24 months). Farm data were collected by postal questionnaire. Data from respondents (70 cases and 51 controls) were analysed using simple logistic regression models with Bonferroni corrections. Limited multivariate analyses were also performed to check the robustness of the overall conclusions. RESULTS AND CONCLUSIONS: Management factors associated with increased odds of pleurisy included no all-in all-out pig flow (OR 9.3, 95% confidence interval [CI]: 3.3-29), rearing of pigs with an age difference of >1 month in the same airspace (OR 6.5 [2.8-17]) and repeated mixing (OR 2.2 [1.4-3.8]) or moving (OR 2.2 [1.5-3.4]) of pigs during the rearing phase. Those associated with decreased odds of pleurisy included filling wean-to-finish or grower-to-finish systems with piglets from ≤3 sources (OR 0.18 [0.07-0.41]) compared to farrow-to-finish systems, cleaning and disinfecting of grower (ORs 0.28 [0.13-0.61] and 0.29 [0.13-0.61]) and finisher (ORs 0.24 [0.11-0.51] and 0.2 [0.09-0.44]) accommodation between groups, and extended down time of grower and finisher accommodation (OR 0.84 [0.75-0.93] and 0.86 [0.77-0.94] respectively for each additional day of downtime). This study demonstrated the value of national-level abattoir pathology data collection systems for case control analyses and generated guidance for on-farm interventions to help reduce the prevalence of pleurisy in slaughter pigs.The authors thank the British Pig Executive (BPEX - a part of the UK's Agricultural and Horticultural Development Board) for funding this project and providing the BPHS data base. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
X-ray Absorption and Reflection in Active Galactic Nuclei
X-ray spectroscopy offers an opportunity to study the complex mixture of
emitting and absorbing components in the circumnuclear regions of active
galactic nuclei, and to learn about the accretion process that fuels AGN and
the feedback of material to their host galaxies. We describe the spectral
signatures that may be studied and review the X-ray spectra and spectral
variability of active galaxies, concentrating on progress from recent Chandra,
XMM-Newton and Suzaku data for local type 1 AGN. We describe the evidence for
absorption covering a wide range of column densities, ionization and dynamics,
and discuss the growing evidence for partial-covering absorption from data at
energies > 10 keV. Such absorption can also explain the observed X-ray spectral
curvature and variability in AGN at lower energies and is likely an important
factor in shaping the observed properties of this class of source.
Consideration of self-consistent models for local AGN indicates that X-ray
spectra likely comprise a combination of absorption and reflection effects from
material originating within a few light days of the black hole as well as on
larger scales. It is likely that AGN X-ray spectra may be strongly affected by
the presence of disk-wind outflows that are expected in systems with high
accretion rates, and we describe models that attempt to predict the effects of
radiative transfer through such winds, and discuss the prospects for new data
to test and address these ideas.Comment: Accepted for publication in the Astronomy and Astrophysics Review. 58
pages, 9 figures. V2 has fixed an error in footnote
Disturbance and the Dynamics of Coral Cover on the Great Barrier Reef (1995–2009)
Coral reef ecosystems worldwide are under pressure from chronic and acute stressors that threaten their continued existence. Most obvious among changes to reefs is loss of hard coral cover, but a precise multi-scale estimate of coral cover dynamics for the Great Barrier Reef (GBR) is currently lacking. Monitoring data collected annually from fixed sites at 47 reefs across 1300 km of the GBR indicate that overall regional coral cover was stable (averaging 29% and ranging from 23% to 33% cover across years) with no net decline between 1995 and 2009. Subregional trends (10–100 km) in hard coral were diverse with some being very dynamic and others changing little. Coral cover increased in six subregions and decreased in seven subregions. Persistent decline of corals occurred in one subregion for hard coral and Acroporidae and in four subregions in non-Acroporidae families. Change in Acroporidae accounted for 68% of change in hard coral. Crown-of-thorns starfish (Acanthaster planci) outbreaks and storm damage were responsible for more coral loss during this period than either bleaching or disease despite two mass bleaching events and an increase in the incidence of coral disease. While the limited data for the GBR prior to the 1980's suggests that coral cover was higher than in our survey, we found no evidence of consistent, system-wide decline in coral cover since 1995. Instead, fluctuations in coral cover at subregional scales (10–100 km), driven mostly by changes in fast-growing Acroporidae, occurred as a result of localized disturbance events and subsequent recovery
Phase Shift from a Coral to a Corallimorph-Dominated Reef Associated with a Shipwreck on Palmyra Atoll
Coral reefs can undergo relatively rapid changes in the dominant biota, a phenomenon referred to as phase shift. Various reasons have been proposed to explain this phenomenon including increased human disturbance, pollution, or changes in coral reef biota that serve a major ecological function such as depletion of grazers. However, pinpointing the actual factors potentially responsible can be problematic. Here we show a phase shift from coral to the corallimorpharian Rhodactis howesii associated with a long line vessel that wrecked in 1991 on an isolated atoll (Palmyra) in the central Pacific Ocean. We documented high densities of R. howesii near the ship that progressively decreased with distance from the ship whereas R. howesii were rare to absent in other parts of the atoll. We also confirmed high densities of R. howesii around several buoys recently installed on the atoll in 2001. This is the first time that a phase shift on a coral reef has been unambiguously associated with man-made structures. This association was made, in part, because of the remoteness of Palmyra and its recent history of minimal human habitation or impact. Phase shifts can have long-term negative ramification for coral reefs, and eradication of organisms responsible for phase shifts in marine ecosystems can be difficult, particularly if such organisms cover a large area. The extensive R. howesii invasion and subsequent loss of coral reef habitat at Palmyra also highlights the importance of rapid removal of shipwrecks on corals reefs to mitigate the potential of reef overgrowth by invasives
Search for a Technicolor omega_T Particle in Events with a Photon and a b-quark Jet at CDF
If the Technicolor omega_T particle exists, a likely decay mode is omega_T ->
gamma pi_T, followed by pi_T -> bb-bar, yielding the signature gamma bb-bar. We
have searched 85 pb^-1 of data collected by the CDF experiment at the Fermilab
Tevatron for events with a photon and two jets, where one of the jets must
contain a secondary vertex implying the presence of a b quark. We find no
excess of events above standard model expectations. We express the result of an
exclusion region in the M_omega_T - M_pi_T mass plane.Comment: 14 pages, 2 figures. Available from the CDF server (PS with figs):
http://www-cdf.fnal.gov/physics/pub98/cdf4674_omega_t_prl_4.ps
FERMILAB-PUB-98/321-
Measurement of the B0 anti-B0 oscillation frequency using l- D*+ pairs and lepton flavor tags
The oscillation frequency Delta-md of B0 anti-B0 mixing is measured using the
partially reconstructed semileptonic decay anti-B0 -> l- nubar D*+ X. The data
sample was collected with the CDF detector at the Fermilab Tevatron collider
during 1992 - 1995 by triggering on the existence of two lepton candidates in
an event, and corresponds to about 110 pb-1 of pbar p collisions at sqrt(s) =
1.8 TeV. We estimate the proper decay time of the anti-B0 meson from the
measured decay length and reconstructed momentum of the l- D*+ system. The
charge of the lepton in the final state identifies the flavor of the anti-B0
meson at its decay. The second lepton in the event is used to infer the flavor
of the anti-B0 meson at production. We measure the oscillation frequency to be
Delta-md = 0.516 +/- 0.099 +0.029 -0.035 ps-1, where the first uncertainty is
statistical and the second is systematic.Comment: 30 pages, 7 figures. Submitted to Physical Review
Search for New Particles Decaying to top-antitop in proton-antiproton collisions at squareroot(s)=1.8 TeV
We use 106 \ipb of data collected with the Collider Detector at Fermilab to
search for narrow-width, vector particles decaying to a top and an anti-top
quark. Model independent upper limits on the cross section for narrow, vector
resonances decaying to \ttbar are presented. At the 95% confidence level, we
exclude the existence of a leptophobic \zpr boson in a model of
topcolor-assisted technicolor with mass M_{\zpr} 480 \gev for natural
width = 0.012 M_{\zpr}, and M_{\zpr} 780 \gev for =
0.04 M_{\zpr}.Comment: The CDF Collaboration, submitted to PRL 25-Feb-200
Double Diffraction Dissociation at the Fermilab Tevatron Collider
We present results from a measurement of double diffraction dissociation in
collisions at the Fermilab Tevatron collider. The production cross
section for events with a central pseudorapidity gap of width
(overlapping ) is found to be [] at [630]
GeV. Our results are compared with previous measurements and with predictions
based on Regge theory and factorization.Comment: 10 pages, 4 figures, using RevTeX. Submitted to Physical Review
Letter
A Measurement of the Differential Dijet Mass Cross Section in p-pbar Collisions at sqrt{s}=1.8 TeV
We present a measurement of the cross section for production of two or more
jets as a function of dijet mass, based on an integrated luminosity of 86 pb^-1
collected with the Collider Detector at Fermilab. Our dijet mass spectrum is
described within errors by next-to-leading order QCD predictions using CTEQ4HJ
parton distributions, and is in good agreement with a similar measurement from
the D0 experiment.Comment: 18 pages including 2 figures and 3 tables. Submitted to Phys. Rev. D
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