- Takaaki Kajita Kajita (Super Kamiokande)
- Arthur McDonald (Sudbury Neutrino Observatory - SNO)
I think this is a well deserved recognition to a field of physics which has seen an incredible rise in the last twenty years. Neutrino physics has boomed when it was discovered that neutrinos oscillate between different flavours, thus explaining a 30-year-long puzzle concerning the deficit of neutrinos coming from the sun.
Neutrinos are the lightest elementary particles we know - they are so light that they have been considered massless ever since their original hypothesis (Pauli, 1930) and then since their first experimental detection (by Reines and Cowan, in 1956).
Of course, measuring the mass of a very light particle that almost never interacts with visible matter is an incredible challenge, so physicists considered neutrinos massless and this working approximation has paid good service for over sixty years. In retrospect, the non-zero mass of neutrinos should not have come as a big suprise - why zero, after all ? Nonetheless the discovery that receives this Nobel Prize is a groundbreaking one.
The challenge we face 17 years after the Superkamiokande discovery is not very different, although in the meantime we have covered a lot of ground in neutrino physics. The puzzle is still the one of understanding whether the neutrino masses follow a "normal" hierarchy, i.e. one where the electron-type neutrino is the lightest, and the tau-type neutrino is the heaviest, or if instead there is an "inverted" mass hierarchy. This fact bears huge consequences for the phenomenology of the whole field. It is a question to which we hope we will give an answer in the course of the next decade. And as for determining the exact value of the masses of neurinos, well, that is even farther in the future.
I think today is a big day for a lot of my colleagues, who have devoted their life to the study of neutrinos and the building of experiments where their properties could be measured. So I hope I will get to drink some bubbly stuff very soon - but in the meantime, my congratulations to all !