1,261 research outputs found
Multi-electron SEFs for nuclear reactions involved in advanced stages of stellar evolution
Multi-electron screening effects encountered in laboratory astrophysical
reactions are investigated by considering the reactants Thomas-Fermi atoms. By
means of that model, previous studies are extended to derive the corresponding
screening enhancement factor (SEF), so that it takes into account ionization,
thermal, exchange and relativistic effects. The present study, by imposing a
very satisfactory constraint on the possible values of the screening energies
and the respective SEFs, corrects the current (and the future) experimental
values of the astrophysical factors associated with nuclear reactions involved
in advanced stages of stellar evolution.Comment: 13 RevTex pages+6 ps figures; Accepted for publication in Nuclear
Physics
A counterexample to parabolic dichotomies in holomorphic iteration
We give an example of a parabolic holomorphic self-map of the unit ball
whose canonical Kobayashi hyperbolic
semi-model is given by an elliptic automorphism of the disc , which can be chosen to be different from the identity. As a
consequence, in contrast to the one dimensional case, this provides a first
example of a holomorphic self-map of the unit ball which has points with zero
hyperbolic step and points with nonzero hyperbolic step, solving an open
question and showing that parabolic dynamics in the ball is
radically different from parabolic dynamics in the disc. The example is
obtained via a geometric method, embedding the ball as a domain
in the bidisc that is forward invariant
and absorbing for the map , where denotes the right half-plane. We also show that a complete
Kobayashi hyperbolic domain with such properties cannot be Gromov
hyperbolic w.r.t. the Kobayashi distance (hence, it cannot be biholomorphic to
) if an additional quantitative geometric condition is satisfied
Screening of Nuclear Reactions in the Sun and Solar Neutrinos
We quantitatively determine the effect and the uncertainty on solar neutrino
production arising from the screening process. We present predictions for the
solar neutrino fluxes and signals obtained with different screening models
available in the literature and by using our stellar evolution code. We explain
these numerical results in terms of simple laws relating the screening factors
with the neutrino fluxes. Futhermore we explore a wider range of models for
screening, obtained from the Mitler model by introducing and varying two
phenomenological parameters, taking into account effects not included in the
Mitler prescription. Screening implies, with respect to a no-screening case, a
central temperat reduction of 0.5%, a 2% (8%) increase of Beryllium
(Boron)-neutrino flux and a 2% (12%) increase of the Gallium (Chlorine) signal.
We also find that uncertainties due to the screening effect ar at the level of
1% for the predicted Beryllium-neutrino flux and Gallium signal, not exceeding
3% for the Boron-neutrino flux and the Chlorine signal.Comment: postscript file 11 pages + 4 figures compressed and uuencoded we have
replaced the previous paper with a uuencoded file (the text is the same) for
any problem please write to [email protected]
Abstract basins of attraction
Abstract basins appear naturally in different areas of several complex
variables. In this survey we want to describe three different topics in which
they play an important role, leading to interesting open problems
A survey on hospitalised community-acquired pneumonia in Italy.
Background and aim. Community Acquired Pneumonia (CAP) remains a major cause of disease and death. We evaluated the levels of care, the outcome and the characteristics of hospitalised patients with CAP in a primary hospital in Italy. We also investigated the value of both the Pneumonia Severity Index (PSI) and the modified Appropriateness Evaluation Protocol (AEP) for recognising both the outcome and the unnecessary admissions and stay of hospitalised patients with CAP. Methods. A retrospective review of all the charts of adult patients with CAP at Manerbio, Brescia, Italy between January 2001 and December 2002 was performed. Results. We evaluated 148 patients; their mean age (±SD) was 70 (±17) years; 34% were female. Most patients (87%) had at least a concomitant co-morbid disease. The overall survival rate at 30 days was 88%. All but one death occurred in the high-risk group of patients according to the PSI. On the contrary, the death rate of patients with inappropriate hospital admission according to the AEP was high. Patients with high PSI score had a significantly longer hospital length of stay than the low-risk group. However, a substantial part of the hospital stay did not show any justification into the charts. Conclusions. The PSI, but not the AEP, upon hospital admission, was useful for evaluating the outcome of patients with CAP. The PSI score and the modified AEP can be useful for assessing the appropriateness of hospitalisation for patients with CAP. There is the need for a practical and validated tool to support physicians in their decision making regarding the early and safe discharge of hospitalised patients with CAP
Differential cross sections for muonic atom scattering from hydrogenic molecules
The differential cross sections for low-energy muonic hydrogen atom
scattering from hydrogenic molecules are directly expressed by the
corresponding amplitudes for muonic atom scattering from hydrogen-isotope
nuclei. The energy and angular dependence of these three-body amplitudes is
thus taken naturally into account in scattering from molecules, without
involving any pseudopotentials. Effects of the internal motion of nuclei inside
the target molecules are included for every initial rotational-vibrational
state. These effects are very significant as the considered three-body
amplitudes often vary strongly within the energy interval eV.
The differential cross sections, calculated using the presented method, have
been successfully used for planning and interpreting many experiments in
low-energy muon physics. Studies of nuclear capture in and the
measurement of the Lamb shift in atoms created in H gaseous targets
are recent examples.Comment: 21 pages, 13 figures, submitted to Phys. Rev.
Observation of Long-Lived Muonic Hydrogen in the 2S State
The kinetic energy distribution of ground state muonic hydrogen atoms
mu-p(1S) is determined from time-of-flight spectra measured at 4, 16, and 64
hPa H2 room-temperature gas. A 0.9 keV-component is discovered and attributed
to radiationless deexcitation of long-lived mu-p(2S) atoms in collisions with
H2 molecules. The analysis reveals a relative population of about 1%, and a
pressure-dependent lifetime (e.g. (30.4 +21.4 -9.7) ns at 64 hPa) of the
long-lived mu-p(2S) population, equivalent to a 2S-quench rate in mu-p(2S) + H2
collisions of (4.4 +2.1 -1.8) 10^11 s^-1 at liquid hydrogen density.Comment: 4 pages, 2 figures, accepted for publication in Physical Review
Letter
Presynaptic control of corticostriatal synapses by endogenous GABA
Corticostriatal terminals have presynaptic GABAB receptors that limit glutamate release, but how these receptors are activated by endogenous GABA released by different types of striatal neurons is still unknown. To address this issue, we used single and paired whole-cell recordings combined with stimulation of corticostriatal fibers in rats and mice. In the presence of opioid, GABAA, and NK1 receptor antagonists, antidromic stimulation of a population of striatal projection neurons caused suppression of subsequently evoked EPSPs in projection neurons. These effects were larger at intervals of 500 ms than 1 or 2 s, and were fully blocked by the selective GABAB receptor antagonist CGP 52432. Bursts of spikes in individual projection neurons were not able to inhibit evoked EPSPs. Similarly, spikes in fast spiking interneurons and low-threshold spike interneurons failed to elicit detectable effects mediated by GABAB receptors. Conversely, spikes in individual neurogliaform interneurons suppressed evoked EPSPs, and these effects were blocked by CGP 52432. These results provide the first demonstration of how GABAB receptors are activated by endogenous GABA released by striatal neuronal types
On the realization of Symmetries in Quantum Mechanics
The aim of this paper is to give a simple, geometric proof of Wigner's
theorem on the realization of symmetries in quantum mechanics that clarifies
its relation to projective geometry. Although several proofs exist already, it
seems that the relevance of Wigner's theorem is not fully appreciated in
general. It is Wigner's theorem which allows the use of linear realizations of
symmetries and therefore guarantees that, in the end, quantum theory stays a
linear theory. In the present paper, we take a strictly geometrical point of
view in order to prove this theorem. It becomes apparent that Wigner's theorem
is nothing else but a corollary of the fundamental theorem of projective
geometry. In this sense, the proof presented here is simple, transparent and
therefore accessible even to elementary treatments in quantum mechanics.Comment: 8 page
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