Abstract:
An improved standard solar model has been used to calculate the fluxes of standard solar neutrinos. It includes premain sequence evolution, element diffusion, partial ionization effects, and all the possible nuclear reactions between the main elements. It uses updated values for the initial solar element abundances, the solar age, the solar luminosity, the nuclear reaction rates and the radiative opacities. Neither nuclear equilibrium, nor complete ionization are assumed. The calculated solar neutrino fluxes are compared with published results from the four solar neutrino experiments. The calculated $^8$B solar neutrino flux is consistent, within the theoretical and experimental uncertainties, with the solar neutrino observations at Homestake and Kamiokande. The observations suggest that the $^7$Be solar neutrino flux is much smaller than that predicted. However, conclusive evidence for the suppression of the $^7$Be solar neutrino flux will require experiments like BOREXINO and HELLAZ. If the $^7$Be solar neutrino flux is suppressed, it still can be due either to standard physics and astrophysics or neutrino properties beyond the standard electroweak model. Only future neutrino experiments, such as SNO, Superkamiokande, BOREXINO and HELLAZ, will be able to show that the solar neutrino problem is a consequence of neutrino properties beyond the standard electroweak model.

Abstract:
The predictions of an improved standard solar model are compared with the observations of the four solar neutrino experiments. The improved model includes premain sequence evolution, element diffusion, partial ionization effects, and all the possible nuclear reactions between the main elements. It uses updated values for the initial solar element abundances, the solar age, the solar luminosity, the nuclear reaction rates and the radiative opacities. Neither nuclear equilibrium, nor complete ionization are assumed. The calculated $^8$B solar neutrino flux is consistent, within the theoretical and experimental uncertainties, with the solar neutrino flux measured by Kamiokande. The results from the $^{37}$Cl and $^{71}$Ga radiochemical experiments seem to suggest that the terrestrial $^7$Be solar neutrino flux is much smaller than that predicted. However, the present terrestrial ``defecit'' of $^7$Be solar neutrinos may be due to the use of inaccurate theoretical neutrino absorption cross sections near threshold for extracting solar neutrino fluxes from production rates. Conclusive evidence for a real deficit of $^7$Be solar neutrinos will require experiments such as BOREXINO or HELLAZ. A real defecit of $^7$Be solar neutrinos can be due to either astrophysical reasons or neutrino properties beyond the standard electroweak model. Only future neutrino experiments, such as SNO, Superkamiokande, BOREXINO and HELLAZ, will be able to provide conclusive evidence that the solar neutrino problem is a consequence of neutrino properties beyond the standard electroweak model. Earlier indications may be provided by long baseline neutrino oscillation experiments.

Abstract:
The standard solar model (SSM) yield a $^8$B solar neutrino flux which is consistent within the theoretical and experimental uncertainties with that observed at Super-Kamiokande. The combined results from the Super- Kamiokande and the Chlorine solar neutrino experiments do not provide a solid evidence for neutrino properties beyond the minimal standard electroweak model. The results from the Gallium experiments and independently the combined results from Super-Kamiokande and the Chlorine experiment imply that the $^7$Be solar neutrino flux is strongly suppressed compared with that predicted by the SSM. This conclusion, however, is valid only if the neutrino absorption cross sections near threshold in Gallium and Chlorine do not differ significantly from their theoretical estimates. Such a departure has not been ruled out by the Chromium source experiments in Gallium. Even if the $^7$Be solar neutrino flux is suppressed compared with that predicted by the SSM, still it can be due to astrophysical effects not included in the simplistic SSM. Such effects include spatial and/or temporal variations in the temperature in the solar core induced by the convective layer through g-modes or by rotational mixing in the solar core, and dense plasma effects which may strongly enhance p-capture by $^7$Be relative to e-capture. The new generation of solar observations, which already look non stop deep into the sun, like Super-Kamiokande through neutrinos, and SOHO and GONG through acoustic waves, may be able to point at the correct solution; astrophysical solutions if they detect unexpected temporal and/or spatial behaviour, or particle physics solutions if Super-Kamiokande detects characteristic spectral distortion or temporal variations (e.g., the day-night effect) of the $^8$B solar neutrino flux . If

Abstract:
After a short survey of the physics of solar neutrinos, giving an overview of hydrogen burning reactions, predictions of standard solar models and results of solar neutrino experiments, we discuss the solar-model-independent indications in favour of non-standard neutrino properties. The experimental results look to be in contradiction with each other, even disregarding some experiment: unless electron neutrinos disappear in their trip from the sun to the earth, the fluxes of intermediate energy neutrinos (those from 7Be electron capture and from the CNO cycle) result to be unphysically negative, or anyway extremely reduced with respect to standard solar model predictions. Next we review extensively non-standard solar models built as attempts to solve the solar neutrino puzzle. The dependence of the central solar temperature on chemical composition, opacity, age and on the values of the astrophysical S-factors for hydrogen-burning reactions is carefully investigated. Also, possible modifications of the branching among the various pp-chains in view of nuclear physics uncertainties are examined. Assuming standard neutrinos, all solar models examined fail in reconciling theory with experiments, even when the physical and chemical inputs are radically changed with respect to present knowledge and even if some of the experimental results are discarded.

Abstract:
Recent solar neutrino results together with the assumption of a stationary Sun imply severe constraints on the individual components of the total neutrino flux : $\Phi_{Be} \leq 0.7 \cdot 10^{9} cm^{-2} s^{-1}, \Phi_{CNO} \leq 0.6 \cdot 10^{9} cm^{-2} s^{-1}$ and $64 \cdot 10^{9} cm^{-2} s^{-1} \leq \Phi_{pp+pep} \leq 65 \cdot 10^{9} cm^{-2} s^{-1}$ (at 1$ \sigma$ level), the constraint on $\nu_{Be}$ being in strong disagreement with $\Phi_{Be}^{SSM} = 5 \cdot 10^{9} cm^{-2} s^{-1}$. We study a large variety of non-standard solar models with low inner temperatures, finding that the temperature profiles T(m) follow the homology relationship: T(m)=k$T(m)^{SSM}$, 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 and also if we restrict to consider just Gallium and Kamiokande results the fit is poor. Finally we discuss what can be learned from new generation experiments, planned for the detection of monochromatic solar neutrinos, about the properties of neutrinos and of the Sun.

Abstract:
Solar neutrino experiments have yet to see directly the transition region between matter-enhanced and vacuum oscillations. The transition region is particularly sensitive to models of non-standard neutrino interactions and propagation. We examine several such non-standard models, which predict a lower-energy transition region and a flatter survival probability for the ^{8}B solar neutrinos than the standard large-mixing angle (LMA) model. We find that while some of the non-standard models provide a better fit to the solar neutrino data set, the large measured value of \theta_{13} and the size of the experimental uncertainties lead to a low statistical significance for these fits. We have also examined whether simple changes to the solar density profile can lead to a flatter ^{8}B survival probability than the LMA prediction, but find that this is not the case for reasonable changes. We conclude that the data in this critical region is still too poor to determine whether any of these models, or LMA, is the best description of the data.

Abstract:
The sound speeds of solar models that include element diffusion agree with helioseismological measurements to a rms discrepancy of better than 0.2% throughout almost the entire sun. Models that do not include diffusion, or in which the interior of the sun is assumed to be significantly mixed, are effectively ruled out by helioseismology. Standard solar models predict the measured properties of the sun more accurately than is required for applications involving solar neutrinos.

Abstract:
It has been speculated that quantum gravity might induce a "foamy" space-time structure at small scales, randomly perturbing the propagation phases of free-streaming particles (such as kaons, neutrons, or neutrinos). Particle interferometry might then reveal non-standard decoherence effects, in addition to standard ones (due to, e.g., finite source size and detector resolution.) In this work we discuss the phenomenology of such non-standard effects in the propagation of electron neutrinos in the Sun and in the long-baseline reactor experiment KamLAND, which jointly provide us with the best available probes of decoherence at neutrino energies E ~ few MeV. In the solar neutrino case, by means of a perturbative approach, decoherence is shown to modify the standard (adiabatic) propagation in matter through a calculable damping factor. By assuming a power-law dependence of decoherence effects in the energy domain (E^n with n = 0,+/-1,+/-2), theoretical predictions for two-family neutrino mixing are compared with the data and discussed. We find that neither solar nor KamLAND data show evidence in favor of non-standard decoherence effects, whose characteristic parameter gamma_0 can thus be significantly constrained. In the "Lorentz-invariant" case n=-1, we obtain the upper limit gamma_0<0.78 x 10^-26 GeV at 95% C.L. In the specific case n=-2, the constraints can also be interpreted as bounds on possible matter density fluctuations in the Sun, which we improve by a factor of ~ 2 with respect to previous analyses.

Abstract:
It has been speculated that quantum gravity might induce a "foamy" space-time structure at small scales, randomly perturbing the propagation phases of free-streaming particles (such as kaons, neutrons, or neutrinos). Particle interferometry might then reveal non-standard decoherence effects, in addition to standard ones (due to, e.g., finite source size and detector resolution.) In this work we discuss the phenomenology of such non-standard effects in the propagation of electron neutrinos in the Sun and in the long-baseline reactor experiment KamLAND, which jointly provide us with the best available probes of decoherence at neutrino energies E ~ few MeV. In the solar neutrino case, by means of a perturbative approach, decoherence is shown to modify the standard (adiabatic) propagation in matter through a calculable damping factor. By assuming a power-law dependence of decoherence effects in the energy domain (E^n with n = 0,+/-1,+/-2), theoretical predictions for two-family neutrino mixing are compared with the data and discussed. We find that neither solar nor KamLAND data show evidence in favor of non-standard decoherence effects, whose characteristic parameter gamma_0 can thus be significantly constrained. In the "Lorentz-invariant" case n=-1, we obtain the upper limit gamma_0<0.78 x 10^-26 GeV at 95% C.L. In the specific case n=-2, the constraints can also be interpreted as bounds on possible matter density fluctuations in the Sun, which we improve by a factor of ~ 2 with respect to previous analyses.

Abstract:
We extract information on the fluxes of Be and CNO neutrinos directly from solar neutrino experiments, with minimal assumptions about solar models. Next we compare these results with solar models, both standard and non standard ones. Finally we discuss the expectations for Borexino, both in the case of standard and non standard neutrinos.