Neutrinos are the most elusive of the Standard Model particles and are key to important aspects of fundamental interactions. There are three species of neutrinos that can morph into each other (neutrino oscillations) while they travel over long distances.
The experimental investigation of neutrino oscillations (intimately related to the neutrino masses) started long time ago, in the 1970s, with the pioneer Homestake experiment, led by Raymond Davis. In this experiment the observed rate of solar electron-neutrino was found to be significantly smaller than the rate predicted by the Standard Solar Model. This discrepancy was known as the solar neutrino puzzle. Since then, neutrino physics has evolved immensely. Without any doubt, the most important recent breakthrough in this field, has been the discovery of neutrino oscillations by the Super-Kamiokande and the Sudbury Neutrino Observatory (SNO) experiments.
In 1998 the water-Cherenkov Super-Kamiokande experiment, led by Prof. Kajita, measured a significant up-down asymmetry of the high-energy atmospheric muon neutrino events. It was discovered in this experiment that the number of up-going high-energy muon neutrinos, passing through the Earth, is about twice smaller than the number of the down-going muon neutrinos coming directly from the atmosphere. This clearly suggested that an oscillation of neutrinos happens. The key theoretical observation is that oscillations are direct consequence of the existence of non-zero neutrino masses and mixing.
Disappearance of solar electron-neutrinos was discovered in 2001 by the SNO experiment, led by Prof. McDonald. At SNO solar neutrinos were detected through the observation of charged-current and neutral-current interactions. SNO was therefore the first experiment to prove oscillations of solar neutrinos thus resolving the long-standing solar neutrino problem.
These discoveries demonstrated that neutrinos have masses albeit we do not yet know their absolute mass and nature. It can be viewed as a clear indication of physics beyond the Standard Model.
We are very proud to have at CP3-Origins young associates such as Aurora Meroni and Emiliano Molinaro that are leading experts in Neutrino Physics, and are currently working on this fascinating topic that is intimately related to the origin of mass problem.