The discovery of the neutrino and its partners was a major breakthrough, solving an apparent problem with radioactive decay and expanding our knowledge of particle physics. But more than this, it also allowed us to create new ways of studying other aspects of the universe.
As detector technology improved, it became possible to investigate neutrinos from astronomical sources such as the Sun.
Our home star is a natural fusion reactor that produces electron neutrinos via the proton–proton chain. Caltech’s John N Bahcall helped to develop what became known as the standard solar model, and calculated the number of neutrinos we should expect to be able to detect from Earth.
Ray Davis Jr of Brookhaven National Laboratory led an experiment to count them, beginning in 1967 and using 100,000 gallons of dry-cleaning fluid in the Homestake gold mine in South Dakota for his detector. (The depth of the mine helped to prevent interference from cosmic rays.)
The experiment ran from the late 1960s through the mid-1970s, and succeeded in detecting neutrinos coming from the Sun. The trouble was, there were only a third as many as predicted.
A check of Bahcall’s calculations revealed no error, and the same result was found by the Sudbury Neutrino Observatory in Canada and at Kamiokande in Japan.
Davis and Japanese physicist Masatoshi Koshiba won a share of the 2002 Nobel Prize for their work on detecting solar neutrinos, despite the unexpected result. And that result was perplexing indeed. If there was no experimental error, it suggested that of our knowledge of the Sun, of fusion and of particle physics, one of them had to be wrong.
It would prove to be the latter.
- 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.