Not really. It is more correct to say that in our paper we review all the major discoveries in neutrino physics of the last few years, we discuss in detail their compatibility and complementarity, and we draw conclusions which will be useful in planning the research in coming years.

Neutrinos are elementary particles very similar to electrons, but they have no electric charge and an extremely tiny mass. In fact, the Standard Model of Particle Theory postulates that their mass is exactly zero, and until a few years ago there was no evidence against this assumption. Since their interactions with matter are extremely weak, they are very difficult to detect, and the task usually requires very large detectors and many years of data taking. Neutrinos are known to exist in three different types, called "flavors": electron neutrino (nu_e), muon neutrino (nu_mu), and tau neutrino (nu_tau). There are a number of processes, both in nature and man-produced, where neutrinos are produced: among them, we highlight nuclear fusion reactions in the core of the Sun (solar neutrinos, nu_e only), interaction of cosmic rays with the upper layers of the Earth’s atmosphere (atmospheric neutrinos, both nu_e and nu_mu), nuclear fission reactions in nuclear power plants (reactor neutrinos, nu_e only), and a number of particle interaction processes at accelerator facilities (accelerator neutrinos, both nu_e and nu_mu).

Over the last few decades, many experiments have been designed and built with the purpose of measuring the neutrino flux coming from different sources, but until a few years ago, the experimental scenario was still inconclusive, mainly because different experiments gave contradictory results. This was mostly due to the fact that, given the very weak interactions of neutrinos with matter, it is extremely difficult to build a reliable detector and to calibrate it properly. It is only during the last 10 years that clear and unambiguous experimental results have become available, first for atmospheric neutrinos: (the Super-Kamiokande experiment), and then for solar neutrinos: (the Super-Kamiokande and SNO experiments). These experiments have provided conclusive evidence that neither solar nor atmospheric neutrino data can be reconciled with the theoretical expectations predicted by the Standard Model, hence a "New Physics" model is needed. Very recently, these results have also been confirmed by reactors (KamLAND) and accelerator (K2K) experiments, which make use of man-produced neutrinos. It is worth mentioning that, as a coronation of many decades of research, in 2002, Raymond Davis Jr. of The University of Pennsylvania and Masatoshi Koshiba of the University of Tokyo were awarded the Nobel Prize in physics (shared with Riccardo Giacconi) "for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos."

In our paper we present a global and unified analysis of all the results of the last generation of neutrino experiments. The phenomenological picture emerging from our analysis is rather well defined. On the one hand, the results of atmospheric experiments indicate a strong deficit of muon neutrinos with respect to the theoretical expectations, mainly visible in neutrinos coming from below the horizon, i.e., those which have crossed a good fraction of the Earth and have traveled many thousands of kilometers. The same data also show no visible deviation of nu_e events, thus suggesting that at the energy scale and travel distance relevant for contained atmospheric data—from 100 MeV to about 100 GeV, and from 10 km to 10,000 km—electron neutrinos do not play a relevant role in the "New Physics processes" responsible for the nu_mu disappearance. On the other hand, the data collected by solar experiments show a very strong deficit of electron neutrinos, at an energy scale of about 5 to 15 MeV. Recently, the SNO experiment has shown that this disappearance is accompanied by the appearance of an equal amount of the other two known neutrino flavors (nu_mu and nu_tau), so that the total number of neutrinos is conserved. We are therefore in front of a process of CONVERSION among different neutrino flavors. The most successful models which can account for ALL the experimental evidence explain these neutrino conversions by postulating that at least two of the three known neutrino states have a tiny but non-zero mass. This hypothesis, together with the assumption that the flavor eigenstates are not the same as the mass eigenstates, lead to a PERIODIC conversion among different flavors: we speak therefore of NEUTRINO OSCILLATIONS. In our work we focus on the simplest of such models, the CP-conserving three-neutrino oscillation model, discussing in detail how it succeeds in explaining each of the experimental results and carefully reconstructing the allowed ranges of its five parameters (two mass-squared differences and three mixing angles). In particular, we have put a special emphasis on the determination of one of the mixing angles, called theta_reactor, which at the moment is the only parameter of the neutrino mass matrix to be still unmeasured. Presently, the strongest constraint on this parameter is an upper bound which proceeds from the combination of atmospheric and reactor data. However, in Summer 2003, a reanalysis of the Super-Kamiokande atmospheric data has resulted in a weakening of this bound. In our work we have shown that a complementary bound on theta_reactor can also be obtained from the combination of solar and KamLAND data, thus increasing the robustness of the present determination of this parameter.

Michele Maltoni:

During my Ph.D. activity at Ferrara University, I became aware of the very quick growth undergone in the last few years by non-accelerator physics. In particular, I was fascinated by the outstanding results found in the field of neutrino phenomenology. For this reason, after the completion of my doctoral thesis I decided to enter the field of neutrino physics, and in February 2000 I joined the AHEP group in Valencia, which has had a long and well-consolidated experience on this subject. Since then, my research activity has mainly focused on the phenomenological aspects of the neutrino oscillation problem.