The 2015 Nobel Prize in Physics was awarded to Takaaki Kajita and Arthur McDonald for the discovery of neutrino oscillations. The existence of neutrino oscillations means that neutrinos have non-zero masses. This is a piece that does not quite fit to the original Standard Model, which has been otherwise very successful. (At least that is the extent of my knowledge.) It was one of the biggest discoveries in elementary particle physics in the last few decades before the discovery of Higgs boson and it is not a huge surprise that they finally decided to award the Physics Prize for the discovery of neutrino oscillations this year. Takaaki Kajita worked for the Super Kamiokande project in Japan. Arthur McDonald worked for the SNO project in Canada.
It was also a somewhat bittersweet announcement. Yoji Totsuka, who lead the Super Kamiokande project, is no longer alive. He passed away in 2008 at the age of 66. He most likely would have shared the Nobel Prize had he been alive.
In my previous post, I guessed that the discovery of neutrino oscillations could win the Nobel Prize in Physics. I thought this was an unquestionably important discovery. The discovery of exoplanets, predicted to win the Physics Prize by some, may be intriguing and is no doubt a great technical feat, but we already knew that planets exist, at least in our solar system. The discovery of neutrino oscillations was more fundamental and surprising. I also think that the discovery of neutrino oscillations was probably on a more solid foundation than some other contenders. The tests of Bell's inequality are important, but there are arguments of possible loopholes. The existence of dark matter was only inferred from indirect methods and we still don't know what dark matter really is. The only question was how to deal with the death of Totsuka. It seems that the answer was to choose Kajita as one of the recipients.
I'm not sure if only awarding the leaders of these big projects a prize is a good thing. Nevertheless, I'm sure that this is an exciting news for the scientists who worked on the Super Kamiokande project and the SNO project. I would like to congratulate them for the job well done.