The physics of nuclear reactions in stellar plasma is reviewed with special emphasis on the importance of the velocity distribution of ions. Then the properties density and temperature of the weak-coupled solar plasma are analysed, showing that the ion velocities should deviate from the Maxwellian distribution and could be better described by a w eakly-nonexstensive jq , 1j 0:02 Tsallis' distribution. We discuss concrete physical frameworks for calculating this deviation: the introduction of higher-order corrections to the di usion and friction coe cients in the Fokker-Planck equation, the in uence of the electric-micro eld stochastic distribution on the particle dynamics, a v elocity correlation function with long-time memory arising from the coupling of the collective and individual degrees of freedom. Finally, w e study the e ects of such deviations on stellar nuclear rates, on the solar neutrino uxes, and on the pp neutrino energy spectrum, and analyse the consequences for the solar neutrino problem.

Asymptotically entropy of chaotic systems increases linearly and the sensitivity to initial conditions is exponential with time: these two types of behavior are related. Such relationship is analogous to and, under specific conditions, has been shown to coincide with the Pesin identity. Numerical evidence supports the proposal that the statistical formalism can be extended to the edge of chaos by using a specific generalization of the exponential and of the Boltzmann-Gibbs entropy. We extend this picture and a Pesin-like identity to a wide class of deformed entropies and exponentials using the logistic map as a test case. The physical criterion of finite-entropy growth strongly restricts the suitable entropies. The nature and characteristics of this generalization are clarified. * )

The Super-Kamiokande best global fit, which includes data from SNO, Gallium and Chlorine experiments, results in a hep neutrino contribution to the signals that, even after oscillation, is greater than the SSM prediction. The solar hep neutrino flux that would yield this contribution is four times larger than the one predicted by the SSM. Recent detailed calculations exclude that the astrophysical factor S hep (0) could be wrong by such a large factor. Given the reliability of the temperature and densities profiles inside the Sun, this experimental result indicates that plasma effects are important for this reaction. We show that a slight enhancement of the high-energy tail, enhancement that is of the order of the deviations from the Maxwell-Boltzmann distribution expected in the solar core plasma, produces an increment of the hep rate of the magnitude required.We verified that the other neutrino fluxes remain compatible with experimental signals and SSM predictions. Better measurements of the high-energy tail of the neutrino spectrum would improve our understanding of reaction rates in the solar plasma. * Electronic address: massimo.coraddu@ca.infn.it † Electronic address: marcello.lissia@ca.infn.it ‡ Electronic address: giuseppe.mezzorani@ca.infn.it § Electronic address: quarati@polito.it

In fusion reactions, the Coulomb barrier selects particles from the high-momentum part of the distribution. Therefore, small variations of the high-momentum tail of the velocity distribution can produce strong effects on fusion rates. In plasmas several potential mechanisms exist that can produce deviations from the standard Maxwell-Boltzmann distribution. Quantum broadening of the energy-momentum dispersion relation of the plasma quasi-particles modifies the high-momentum tail and could explain the fusion-rate enhancement observed in low-energy nuclear reaction experiments.

Results of numerical simulations of fusion rate d(d,p)t, for low-energy deuteron beam, colliding with deuterated metallic matrix (Raiola [1,2]) confirm analytical estimates given in Ref.[3] (M. Coraddu et al., this issue), taking into account quantum tails in the momentum distribution function of target particles, and predict an enhanced astrophysical factor in the 1 keV region in qualitative agreement with experiments.