Only a third of the expected number of neutrinos from the Sun could be detected. And there are three different flavours of neutrino. That numerical match-up would turn out not to be a coincidence.
As far back as 1957, Italian physicist Bruno Pontecorvo had shown that neutrinos might oscillate between their different flavours, switching from one type to another in mid-flight – but only if they have mass.
In the standard model, however, neutrinos are massless. And yet that oscillation is precisely what happens.
Results from the Super-Kamiokande facility in Japan, published in 1998, were the first real hint. Muon neutrinos are among the shower of particles produced when cosmic rays hit the Earth’s upper atmosphere, but Super-Kamiokande detected more of them coming from directly above the detector compared to those that had come through the Earth. This was consistent with them changing into tau neutrinos during their longer journey through the Earth, and showed that the solar neutrino problem didn’t lie in our understanding of the Sun.
The Sudbury Neutrino Observatory confirmed the existence of neutrino oscillation in 2001, announcing the detection of different flavours of neutrino coming from the Sun. For its detection medium, Sudbury used heavy water (in which deuterium replaces ordinary hydrogen), allowing it to detect the heavier neutrinos as well as the electron neutrino, although it couldn’t distinguish between the muon neutrino and tau neutrino themselves.
Sudbury’s Professor Arthur B McDonald and Super-Kamiokande’s Takaaki Kajita shared the 2015 Nobel Prize in Physics for their discovery that neutrino oscillation is real and that neutrinos must therefore have mass.
This means, then, that the standard model of particle physics – one of the most successful, well-tested theories we have – is incomplete.
One possible resolution involves a hypothetical fourth type of neutrino, which interacts with other matter only via gravity and not through any of the other fundamental forces. CERN has approved the building of a new detector to look for these sterile neutrinos, which may also be a dark-matter candidate.
In once again being proposed as a solution other scientific problems, the neutrino has come full circle. But on the way, it’s helped particle physics advance further than the physicists of the 1930s could ever have imagined.
- The 27th International Conference on Neutrino Physics and Astrophysics is taking place at Imperial College London from 4–9 July. The keynote speakers are Nobel Prize winners Professor Takaaki Kajita of the University of Tokyo and Professor Arthur B McDonald of Queen’s University, Canada. Follow the action from the conference on Twitter.