521 research outputs found

### Systematic quantum corrections to screening in thermonuclear fusion

We develop a series expansion of the plasma screening length away from the
classical limit in powers of $\hbar^{2}$. It is shown that the leading order
quantum correction increases the screening length in solar conditions by
approximately 2% while it decreases the fusion rate by approximately $0.34%$.
We also calculate the next higher order quantum correction which turns out to
be approximately 0.05%

### Non-Markovian effects in the solar neutrino problem

The solar core, because of its density and temperature, is not a
weakly-interacting or a high-temperature plasma. Collective effects have time
scales comparable to the average time between collisions, and the microfield
distribution influences the particle dynamics. In this conditions ion and
electron diffusion is a non-Markovian process, memory effects are present and
the equilibrium statistical distribution function differs from the Maxwellian
one. We show that, even if the deviations from the standard velocity
distribution that are compatible with our present knowledge of the solar
interior are small, they are sufficient to sensibly modify the sub-barrier
nuclear reaction rates. The consequent changes of the neutrino fluxes are
comparable to the flux deficits that constitute the solar neutrino problem.Comment: 4 pages, to appear in the Proceedings of Nuclei in the Cosmos

### Chemical and mechanical instabilities in high energy heavy-ion collisions

We investigate the possible thermodynamic instability in a warm and dense nuclear medium where a phase transition from nucleonic matter to resonance-dominated Delta-matter can take place. Such a phase transition is characterized by both mechanical instability (fluctuations on the baryon density) and by chemical-diffusive instability (fuctuations onthe isospin concentration) in asymmetric nuclear matter. Similarly to the liquid-gas phase transition, the nucleonic and the Delta-matter phase have a different isospin density in the mixed phase. In the liquid-gas phase transition, the process of producing a larger neutron excess in
the gas phase is referred to as isospin fractionation. A similar effects can occur in the nucleon-Delta matter phase transition due essentially to a Delta- excess in the Delta-matter phase in asymmetric nuclear matter. In this context, we study the hadronic equation of state by means of an effective quantum relativistic mean field model with the inclusion of the full octet of baryons, the Delta-isobar
degrees of freedom, and the lightest pseudoscalar and vector mesons. Finally, we will investigate the presence of thermodynamic instabilities in a hot and dense nuclear medium where phases with different values of antibaryon-baryon ratios and strangeness content may coexist. Such a physical regime could be in principle investigated in the future high-energy compressed nuclear
matter experiments where will make it possible to create compressed baryonic matter with a high net baryon density

### Quantum thermodynamic instabilities in compact stars

We study the existence of thermodynamic instabilities in the nuclear equation of state relative to the high density regime reached in the central core of compact stars. In the framework of a relativistic mean-field theory, we analyze the asymmetric nuclear properties in beta-equilibrium, including hyperons and Delta-isobar degrees of freedom. We investigate a finite density phase transition characterized by pure hadronic matter with the presence of mechanical instability (relative to the fluctuation of baryon number) and of chemical-dffusive instability (relative to the fluctuation of electric charge concentration). We find that, in the presence of thermodynamic instabilities, two hadronic phases with dfferent values of electric charge content may coexist, with several phenomenological consequences in the physics of compact stars

### Quantum uncertainty in weakly non-ideal astrophysical plasma

Galitskii and Yakimets showed that in dense or low temperature plasma, due to quantum uncertainty effect, the particle distribution function over momenta acquires a power-like tail even under conditions of thermodynamic equilibrium. We show that in weakly non-ideal plasmas, like the solar interior, both non-extensivity and quantum uncertainty should be taken into account to derive equilibrium ion distribution functions and to estimate nuclear reaction rates and solar neutrino fluxes. The order of magnitude of the deviation from the standard Maxwell-Boltzmann distribution can be derived microscopically by considering the presence of random electrical microfield in the stellar plasma. We show that such a nonextensive statistical effect can be very relevant in many nuclear astrophysical problems

### Ellipsoidal nested sampling, expression of the model uncertainty and measurement

The measurand value, the conclusions, and the decisions inferred from measurements may depend on the models used to explain and to analyze the results. In this paper, the problems of identifying the most appropriate model and of assessing the model contribution to the uncertainty are formulated and solved in terms of Bayesian model selection and model averaging. As computational cost of this approach increases with the dimensionality of the problem, a numerical strategy, based on multimodal ellipsoidal nested sampling, to integrate over the nuisance parameters and to compute the measurand post-data distribution is outlined. In order to illustrate the numerical strategy, by use of MATHEMATICA an elementary example concerning a bimodal, two-dimensional distribution has also been studied

### Evidence for narrow resonant structures at $W \approx 1.68$ and $W \approx 1.72$ GeV in real Compton scattering off the proton

First measurement of the beam asymmetry $\Sigma$ for Compton scattering off
the proton in the energy range $E_{\gamma}=0.85 - 1.25$ GeV is presented. The
data reveals two narrow structures at $E_{\gamma}= 1.036$ and
$E_{\gamma}=1.119$ GeV. They may signal narrow resonances with masses near
$1.68$ and $1.72$ GeV, or they may be generated by the sub-threshold $K\Lambda$
and $\omega p$ production. Their decisive identification requires additional
theoretical and experimental efforts.Comment: Published versio

### Power-law quantum distributions in protoneutron stars

We investigate the bulk properties of protoneutron stars in the framework of a relativistic mean field theory based on nonextensive statistical mechanics, originally proposed by C. Tsallis and characterized by power-law quantum distributions. We study the relevance of nonextensive statistical effects on the ÎČ-stable equation of state at fixed entropy per baryon, for nucleonic and hyperonic matter. We concentrate our analysis in the maximum heating and entropy per baryon s = 2 stage and T â 40 Ă· 80 MeV. This is the phase, at high temperature and high baryon density, in which the presence of nonextensive effects may alter more sensibly the thermodynamical and mechanical properties of the protoneutron star. We show that nonextensive power-law effects could play a crucial role in the structure and in the evolution of the protoneutron stars also for small deviations from the standard Boltzmann-Gibbs statistics

### Nonlinear statistical effects in relativistic mean field theory

We investigate the relativistic mean field theory of nuclear matter at finite
temperature and baryon density taking into account of nonlinear statistical
effects, characterized by power-law quantum distributions. The analysis is
performed by requiring the Gibbs conditions on the global conservation of
baryon number and electric charge fraction. We show that such nonlinear
statistical effects play a crucial role in the equation of state and in the
formation of mixed phase also for small deviations from the standard
Boltzmann-Gibbs statistics.Comment: 9 pages, 5 figures. arXiv admin note: substantial text overlap with
arXiv:1005.4643 and arXiv:0912.460

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