The Nobel Prize in Physics 2015 has recognised Takaaki Kajita in Japan and Arthur B. McDonald in Canada, for their key contributions to the experiments which demonstrated that neutrinos change identities. This metamorphosis requires that neutrinos have mass. The discovery has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe.
Around the turn of the millennium, Takaaki Kajita presented the discovery that neutrinos from the atmosphere switch between two identities 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 from the Sun were not disappearing on their way to Earth. Instead they were captured with a different identity when arriving to the Sudbury Neutrino Observatory. So, a neutrino puzzle that physicists had wrestled with for decades had been resolved. Compared to theoretical calculations of the number of neutrinos, up to two thirds of the neutrinos were missing in measurements performed on Earth. Now, the two experiments discovered that the neutrinos had changed identities.
The discovery led to the far-reaching conclusion that neutrinos, which for a long time were considered massless, must have some mass, however small.
After photons (the particles of light), neutrinos are the most numerous in the entire cosmos. The Earth is constantly bombarded by them. The new observations have clearly showed that the Standard Model used by empirical science could not be the complete theory of the fundamental constituents of the universe, as it required neutrinos to be massless.
Many neutrinos are created on the Sun itself, while others are created in reactions between cosmic radiation and the Earth’s atmosphere. Thousands of billions of neutrinos are streaming through our bodies each second. Hardly anything can stop them passing; neutrinos are nature’s most elusive elementary particles. And so we learn a little more about the everyday reality of our Solar Ancestors!
Main information source: www.nobelprize.org/nobel_prizes/physics/laureates/2015/press.html