The 1930s saw a proliferation of new particles, and the neutrino would eventually prove to be one of three generations, or flavours, with similar properties but different masses.
Shortly after having discovered the positron, Caltech’s Carl D Anderson was studying cosmic radiation when his experimental data revealed another previously unknown particle, which later came to be named the muon.
The track this new particle created in Anderson’s cloud chamber when passing through a magnetic field curved more than an electron’s but less than a proton’s, distinguishing itself from either and suggesting a mass somewhere between the two.
Briefly mistaken for another, completely different new particle predicted earlier by Hideki Yukawa, it in fact turned out to be another lepton – essentially, the electron’s bigger brother. And, like the electron, it should have a corresponding partner of neutral charge and much lower (or zero) mass.
Sure enough, the muon neutrino was detected by Leon M Lederman, Melvin Schwartz and Jack Steinberger at Brookhaven National Laboratory in the US in 1962, with the trio later being awarded the 1988 Nobel Prize.
There was more to come. In experiments at the Stanford Linear Accelerator facility during the period of 1974–77, Martin Lewis Perl and colleagues spotted several events that appeared anomalous in much the same way as those that first motivated the search for the neutrino, witnessing decays that couldn’t be explained by any known interaction unless another unseen particle was involved.
This would turn out to be another lepton even more massive than the muon – the tau. The discovery of this one’s diminutive partner, the tau neutrino, would be made in 2000 by Fermilab’s DONUT collaboration, which was dedicated to the search for it. Perl shared the 1995 Nobel Prize with neutrino pioneer Frederick Raines.
Between them, these six particles make up all the leptons in standard model of particle physics – one of the most complete and best-tested models we have. But observations of neutrinos from the Sun would throw up another puzzle for physicists to solve.
- 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.