The Nobel Prize in Physics 2015 was awarded jointly to Takaaki Kajita in Japan and Arthur B. McDonald in Canada, for their key contributions to the experiments which demonstrated that neutrinos change nature over time in an oscillatory fashion. This metamorphosis of quantum nature, requires that neutrinos have mass, contradicting the established theory of particle physics, the so-called standard model. Therefore, the discovery raised new challenges for theoreticians, while having consequences on the mass of the Universe, as an enormous population of neutrinos exists, produced by e.g. nuclear reactions in stars.
Neutrinos are tiny particles having a very weak interaction with matter. Thus, they require gigantic and highly sensitive detectors to be tracked. Several experiments in the last decades of the XXth century, progressively showed that out of the total number of electron neutrinos produced by solar nuclear reactions, only a fraction of them were reaching the earth.
Around the turn of the millennium, Takaaki Kajita presented the discovery that neutrinos created in the atmosphere by cosmic rays switch between muon neutrinos to electron neutrinos on their way to the Super-Kamiokande detector in Japan. Meanwhile, the research group in Canada led by Arthur B. McDonald could demonstrate that the neutrinos produced from the Sun were not disappearing on their way to Earth. Instead they were detected as tau neutrinos when arriving to the Sudbury Neutrino Observatory.