Neutrinos are fundamental particles which are created in radioactive decays, in fusion processes in stars, such as our Sun, and in cosmic ray interactions with the Earth’s atmosphere. Even though neutrinos are very abundant in nature and billions pass through our bodies each second, their very weak interaction make observation challenging.
Similar to other fundamental particle of the Standard Model, neutrinos come in three flavours: electron-, muon- and tau-neutrino.
The 2015 Nobel prize for Physics was jointly awarded to Prof. Arthur McDonald of the Sudbury Neutrino Observatory (SNO) in Canada and Professor Takaaki Kajita of the Super-Kamiokande (Super-K) collaboration in Japan. The experimental teams led by the laureates showed that neutrinos can switch (or oscillate) between the different flavours. This important observation implies that neutrinos have a small but non-zero mass.
The SNO experiment focused on studying neutrinos produced in our Sun. Previous measurements, for which Ray Davies was awarded the 2002 Nobel prize, indicated that there was a deficit in the electron-neutrinos produced compared to expectations from solar models. The measurements from SNO solve this discrepancy by showing that the electron neutrinos produced were oscillating into muon- or tau-neutrinos. The Super-K experiment is a large underground detector in Japan, consisting of a cylindrical stainless steel tank that is 41 m tall and 39 m in diameter holding 50,000 tons of ultra-pure water. It’s large size makes it ideal to detect and study neutrinos produced in the atmosphere and the experiment demonstrated the oscillation phenomenon applies also to muon-neutrinos, changing flavour into tau-neutrinos.
'The award of the Nobel prize for neutrino physics is fantastic news and well deserved. The fact that neutrinos oscillate and have mass was totally unexpected’.
Dr Greig Cowan, University of Edinburgh
Scientists at the University of Edinburgh, including Greig, have recently joined the planned successor to the Super-K experiment, called Hyper-K. Hyper-K is also an underground detector located in Japan, filled with 1 million metric tons of ultrapure water, a volume approximately 20 times larger than that of Super-K. Start of science operation is foreseen from around 2025.
The behaviour of neutrinos could well surprise us in the future, in particular Hyper-K aims to study with unprecedented accuracy the differences in the switching properties for neutrinos and their anti-matter counterpart. During the Big Bang equal amounts of matter and anti-matter should have been produced. However, our Universe we see today is dominated by matter.
As Dr Cowan explained 'There must be new yet unknown physics processes that distinguish matter and anti-matter. One possibility is that neutrino oscillations are not the same for neutrinos and anti-neutrinos, which could help resolve the puzzle of the matter asymmetry of the Universe. If this is the case we will pin this down with Hyper-K in the coming years. The next years are going to be very exciting times for neutrino physics.'