346 research outputs found
Constraints on the Variation of the Fine Structure Constant from Big Bang Nucleosynthesis
We put bounds on the variation of the value of the fine structure constant
, at the time of Big Bang nucleosynthesis. We study carefully all light
elements up to Li. We correct a previous upper limit on estimated from He primordial abundance and we find interesting new
potential limits (depending on the value of the baryon-to-photon ratio) from
Li, whose production is governed to a large extent by Coulomb barriers. The
presently unclear observational situation concerning the primordial abundances
preclude a better limit than |\Delta \alpha/\alpha| \lsim 2\cdot 10^{-2}, two
orders of magnitude less restrictive than previous bounds. In fact, each of the
(mutually exclusive) scenarios of standard Big Bang nucleosynthesis proposed,
one based on a high value of the measured deuterium primordial abundance and
one based on a low value, may describe some aspects of data better if a change
in of this magnitude is assumed.Comment: 21 pages, eps figures embedded using epsfig macr
Gaps in the clinical management of influenza a century since the 1918 pandemic
This year marks the centennial of the devastating 1918 influenza A(H1N1) pandemic, which killed an estimated 50 million people worldwide. Prevention and control activities were limited in 1918 because global surveillance did not exist, influenza viruses were not yet discovered, and no influenza vaccines had been developed. Diagnostic tests for influenza were unavailable prior to isolation of influenza viruses in the 1930s, so spread of the pandemic virus was tracked by news reports of increased respiratory disease and related deaths. Establishment of the World Health Organizationâs Global Influenza Surveillance Network in 1952 has contributed substantially to coordinated surveillance, vaccine development, and influenza vaccine strain selection
Astrophysical thermonuclear functions
As theoretical knowledge and experimental verification of nuclear cross
sections increases it becomes possible to refine analytic representations for
nuclear reaction rates. In this paper mathematical/statistical techniques for
deriving closed-form representations of thermonuclear functions are summarized
and numerical results for them are given.The purpose of the paper is also to
compare numerical results for approximate and closed-form representations of
thermonuclear functions.Comment: 17 pages in LaTeX, 8 figures available on request from
[email protected]
Alpha scattering and capture reactions in the A = 7 system at low energies
Differential cross sections for He- scattering were measured in
the energy range up to 3 MeV. These data together with other available
experimental results for He and H scattering were
analyzed in the framework of the optical model using double-folded potentials.
The optical potentials obtained were used to calculate the astrophysical
S-factors of the capture reactions HeBe and
HLi, and the branching ratios for the transitions into
the two final Be and Li bound states, respectively. For
HeBe excellent agreement between calculated and
experimental data is obtained. For HLi a value
has been found which is a factor of about 1.5 larger than the adopted value.
For both capture reactions a similar branching ratio of has been obtained.Comment: submitted to Phys.Rev.C, 34 pages, figures available from one of the
authors, LaTeX with RevTeX, IK-TUW-Preprint 930540
Doing today's work superbly well - Treating ebola with current tools
The Ebola outbreak that is ravaging West Africa is a daily staple of the lay press and of scholarly medical publications. Ebola evokes fear among both the public and clinicians. It also evokes a sort of therapeutic nihilism â after all, if there is no treatment, what can be done? And without an Ebola-specific antiviral medication, of what use are infectious-disease clinicians? Without oxygen, let alone mechanical ventilators, how can acute and critical care clinicians possibly contribute
Reaction rate for two--neutron capture by He
Recent investigations suggest that the neutrino--heated hot bubble between
the nascent neutron star and the overlying stellar mantle of a type--II
supernova may be the site of the r--process. In the preceding --process
building up the elements to , the He(2n,)He--
and He(,n)Be--reactions bridging the instability gap at
and could be of relevance. We suggest a mechanism for
He(2n,)He and calculate the reaction rate within the
+n+n approach. The value obtained is about a factor 1.6 smaller than
the one obtained recently in the simpler direct--capture model, but is at least
three order of magnitude enhanced compared to the previously adopted value. Our
calculation confirms the result of the direct--capture calculation that under
representative conditions in the --process the reaction path proceeding
through He is negligible compared to He(n,)Be.Comment: 13 pages, 4 postscript figures, to appear in "Zeitschrift f. Physik
A", changed internet address and filename, the uuencoded postscript file
including the figures is available at
ftp://is1.kph.tuwien.ac.at/pub/ohu/twoneutron.u
Astrophysical Reaction Rates for B(p,)Be and B(p,)Be From a Direct Model
The reactions B(p,)Be and B(p,)Be
are studied at thermonuclear energies using DWBA calculations. For both
reactions, transitions to the ground states and first excited states are
investigated. In the case of B(p,)Be, a resonance at
keV can be consistently described in the potential model, thereby
allowing the extension of the astrophysical -factor data to very low
energies. Strong interference with a resonance at about keV
require a Breit-Wigner description of that resonance and the introduction of an
interference term for the reaction B(p,)Be. Two
isospin resonances (at keV and keV)
observed in the B+p reactions necessitate Breit-Wigner resonance and
interference terms to fit the data of the B(p,)Be
reaction. -factors and thermonuclear reaction rates are given for each
reaction. The present calculation is the first consistent parametrization for
the transition to the ground states and first excited states at low energies.Comment: 27 pages, 5 Postscript figures, uses RevTex and aps.sty; preprint
also available at http://quasar.physik.unibas.ch/ Phys. Rev. C, in pres
Direct and sequential radiative three-body reaction rates at low temperatures
We investigate the low-temperature reaction rates for radiative capture
processes of three particles. We compare direct and sequential capture
mechanisms and rates using realistic phenomenological parametrizations of the
corresponding photodissociation cross sections.Energy conservation prohibits
sequential capture for energies smaller than that of the intermediate two-body
structure. A finite width or a finite temperature allows this capture
mechanism. We study generic effects of positions and widths of two- and
three-body resonances for very low temperatures. We focus on nuclear reactions
relevant for astrophysics, and we illustrate with realistic estimates for the
-- and -- radiative capture
processes. The direct capture mechanism leads to reaction rates which for
temperatures smaller than 0.1 GK can be several orders of magnitude larger than
those of the NACRE compilation.Comment: To be published in European Physical Journal
Current constraints on Cosmological Parameters from Microwave Background Anisotropies
We compare the latest observations of Cosmic Microwave Background (CMB)
Anisotropies with the theoretical predictions of the standard scenario of
structure formation. Assuming a primordial power spectrum of adiabatic
perturbations we found that the total energy density is constrained to be
while the energy density in baryon and Cold Dark
Matter (CDM) are and ,
(all at 68% C.L.) respectively. The primordial spectrum is consistent with
scale invariance, () and the age of the universe is
Gyrs. Adding informations from Large Scale Structure and
Supernovae, we found a strong evidence for a cosmological constant
and a value of the Hubble parameter
. Restricting this combined analysis to flat universes, we put
constraints on possible 'extensions' of the standard scenario. A gravity waves
contribution to the quadrupole anisotropy is limited to be (95%
c.l.). A constant equation of state for the dark energy component is bound to
be (95% c.l.). We constrain the effective relativistic degrees
of freedom and the neutrino chemical potential and (massless neutrinos).Comment: The status of cosmological parameters before WMAP. In press on Phys.
Rev. D., Rapid Communication, 6 pages, 5 figure
Ebola virus disease: an update on post-exposure prophylaxis
The massive outbreak of Ebola virus disease in west Africa between 2013 and 2016 resulted in intense efforts to evaluate the efficacy of several specific countermeasures developed through years of preclinical work, including the first clinical trials for therapeutics and vaccines. In this Review, we discuss how the experience and data generated from that outbreak have helped to advance the understanding of the use of these countermeasures for post-exposure prophylaxis against Ebola virus infection. In future outbreaks, post-exposure prophylaxis could play an important part in reducing community transmission of Ebola virus by providing more immediate protection than does immunisation as well as providing additional protection for health-care workers who are inadvertently exposed over the course of their work. We propose provisional guidance for use of post-exposure prophylaxis in Ebola virus disease and identify the priorities for future preparedness and further research
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