Piero Quarati

Generalized statistics and solar neutrinos

Abstract The generalized Tsallis statistics produces a distribution function appropriate to describe the interior solar plasma, thought as a stellar polytrope, showing a tail depleted with respect to the Maxwell-Boltzmann distribution and reduces to zero at energies greater than about 20 k B T . The Tsallis statistics can theoretically support the distribution suggested in the past by Clayton and collaborators, which shows also a depleted tail, to explain the solar neutrino counting rate.

Non-extensive statistics, fluctuations and correlations in high-energy nuclear collisions

Abstract. Starting from the experimental evidence that high-energy nucleus–nucleus collisions cannot be described in terms of superpositions of elementary nucleon–nucleon interactions, we analyze the possibility that memory effects and long-range forces imply a non-extensive statistical regime during high-energy heavy-ion collisions. The relevance of these statistical effects and their compatibility with the available experimental data are discussed. In particular, we show that theoretical estimates obtained in the framework of the generalized non-extensive thermostatistics, can reproduce the shape of the pion transverse mass spectrum and explain the different physical origin of the transverse momentum correlation function of the pions emitted during the central Pb + Pb and during the p +p collisions at 158 GeV.

Laboratory for underground nuclear astrophysics(LUNA)

A compact high-current 50 kV ion accelerator facility including a windowless gas target system, a beam calorimeter, and detector telescopes in close geometry has been built and tested. The data acquisition and analysis involved a multiparameter system and a Monte Carlo program. The LUNA facility, presently installed at the Gran Sasso underground laboratory, is a pilot project focused initially on cross section measurements of the ^3He(^3He, 2p)^4He reaction within the thermal energy region of the sun. To achieve this goal, the experimental sensitivity has been improved by more than four orders of magnitude over that of previous work.

Non extensive resonant reaction rates in astrophysical plasmas

Abstract.We study two different physical scenarios of thermonuclear reactions in stellar plasmas proceeding through a narrow resonance at low energy or through the low-energy wing of a wide resonance at high energy. Correspondingly, we derive two approximate analytical formulae in order to calculate thermonuclear resonant reaction rates inside very coupled and non-ideal astrophysical plasmas in which non-extensive effects are likely to arise. Our results are presented as simple first-order corrective factors that generalize the well-known classical rates obtained in the framework of Maxwell-Boltzmann statistical mechanics. As a possible application of our results, we calculate the dependence of the total corrective factor with respect to the energy at which the resonance is located, in an extremely dense and non-ideal carbon plasma.

Atomic effects in the determination of nuclear cross sections of astrophysical interest

Abstract We discuss the relevance of electron screening on nuclear reactions between ions and neutral atoms from a few to a few-hundred keV. We present a general method of calculation of the atomic effects and discuss quantitatively the limiting cases of low and high velocity of the nuclei as compared to typical electron velocities. Results for several low-energy fusion reactions and proton capture processes are presented and recent experimental data on the d 3 He reaction are discussed. Concerning this latter reaction, the reported rise of the astrophysical factor at keV energies cannot be explained in terms of electron screening and should be connected with a nuclear-structure effect.

Kinetic model for q-deformed bosons and fermions

Abstract We have studied the kinetics of q -deformed bosons and fermions, within a semiclassical approach. This investigation is realized by introducing a generalized exclusion-inclusion principle, intrinsically connected with the quantum q -algebra by means of the creation and annihilation operator matrix elements. In this framework, we have derived a nonlinear Fokker-Planck equation for q -deformed bosons and fermions which can be seen as a time evolution equation, appropriate to consider nonequilibrium or near-equilibrium systems in a semiclassical approximation. The steady state of this equation reproduces in a simple mode the q -oscillators equilibrium statistics.

Nonextensive statistics in stellar plasma and solar neutrinos

Abstract Nonextensive and quantum uncertainty effects (related to the quasiparticles composing the stellar core) have strong influence on the nuclear rates and, of course, affect solar neutrino fluxes. Both effects do coexist and are due to the frequent collisions among the ions. The weakly nonextensive nature of the solar core is confirmed. The range of predictions for the neutrino fluxes is enlarged and the solar neutrino problem becomes less dramatic.

Kinetic approach to fractional exclusion statistics

Abstract We show that the kinetic approach to statistical mechanics permits an efficient treatment of fractional exclusion statistics. By using the exclusion-inclusion principle recently proposed [Phys. Rev. E 49 (1994) 5103] as a generalization of the Pauli exclusion principle, we derive a variety of different statistical distributions interpolating between bosons and fermions. The Haldane exclusion principle and the Haldane-Wu fractional exclusion statistics are obtained in a natural way as particular cases. The thermodynamic properties of the statistical systems obeying the generalized exclusion-inclusion principle are discussed.

Anomalous diffusion modifies solar neutrino fluxes

Density and temperature conditions in the solar core suggest that the microscopic diffusion of electrons and ions could be nonstandard: Diffusion and friction coefficients are energy dependent, collisions are not two-body processes and retain memory beyond the single scattering event. A direct consequence of nonstandard diffusion is that the equilibrium energy distribution of particles departs from the Maxwellian one (tails go to zero more slowly or faster than exponentially) modifying the reaction rates. This effect is qualitatively different from temperature and/or composition modification: Small changes in the number of particles in the distribution tails can strongly modify the rates without affecting bulk properties, such as the sound speed or hydrostatic equilibrium, which depend on the mean values from the distribution. This mechanism can considerably increase the range of predictions for the neutrino fluxes allowed by the current experimental values (cross-sections and solar properties) and can be used to reduce the discrepancy between these predictions and the solar neutrino experiments.

Constraints for solar neutrinos fluxes

Abstract Constraints on the neutrino fluxes Φp, ΦB, ΦCNO and ΦBe are derived from the available experimental results. Physical implications are discussed by means of non-extensive statistics which takes into account many-body effects and long-range interactions. The different reduction factors for 7Be and 8B fluxes with respect to their standard solar model values are physically understood, with proper shapes of the shape of the electron and proton distribution functions.