Neutrinos from the Sun: experimental results confronted with solar models

For standard neutrinos, recent solar neutrino results together with the assumption of a nuclearly powered Sun imply severe constraints on the individual components of the total neutrino flux: \Phi_{Be}<0.7*10^9cm^-2 s^-1, \Phi_{CNO}< 0.6*10^9 cm^-2 s^-1, and $64*10^9 cm^-2 s^-1< \Phi_{pp+pep} < 65*10^9 cm^-2 s^-1 (at 1 \sigma level). The bound on \Phi_{Be} is in strong disagreement with the standard solar model prediction \Phi_{Be}^{SSM}\approx 5*10^9 cm^-2 s^-1. We study a large variety of non-standard solar models with low inner temperature, finding that the temperature profiles T(m) follow the homology relationship: T(m)=kT^{SSM}(m), so that they are specified just by the central temperature T_c. There is no value of T_c which can account for all the available experimental results. Even if we only consider the Gallium and Kamiokande results, they remain incompatible. Lowering the cross section p+Be7 \to \gamma+B8 is not a remedy. The shift of the nuclear fusion chain towards the pp-I termination could be induced by a hypothetical low energy resonance in the He3+He3 reaction. This mechanism gives a somehow better, but still bad fit to the combined experimental data. We also discuss what can be learnt from new generation experiments about the properties of neutrinos and of the Sun.