Neutrinos: Oscillations and open questions

 

The IceCube neutrino observatory located in the south pole detected neutrinos coming from outer space.

Textbooks of particle physics, even in the 1990s, used to describe the neutrino as a particle which had no charge or mass. So if neutrinos have no charge or mass, how does one detect them? 

In fact, Austrian physicist Wolfgang Pauli, who postulated the existence of this particle, is said to have written in a letter: “I have done a terrible thing. I have postulated a particle that cannot be detected.” 

These textbooks had to be corrected soon, as, through independent experiments in Japan and Canada, it was shown (in 1998 and 2001) that the neutrinos do indeed possess a small mass. This discovery is what has led to the researchers, Takaaki Kajita and Arthur B McDonald, being awarded the Nobel Prize in Physics this year. 

Metamorphoses

Neutrinos come in three flavours — electron neutrino, muon neutrino and tau neutrino — the names indicating that they are associated with processes involving the electron or its close cousins the tau particle or the muon. 

The two groups, working in Super Kamiokande detector near Tokyo and the Sudbury Neutrino Observatory (SNO) in Ontario, Canada, made this discovery indirectly, by observing that on their route to the earth, the neutrinos undergo a change from one type to the other, through a process called neutrino oscillations. This process cannot take place if the neutrinos had no mass. 

The Super-Kamiokande detector became operational in 1996 in a zinc mine some 250 km from Tokyo, and is deep inside at a depth of 1,000 metres below the ground. It is built to detect Cosmic neutrinos — those that are produced through cosmic radiations that fall on the earth from all directions. The Sudbury Neutrino Observatory, on the other hand, is built to study Solar neutrinos — neutrinos created deep within the Sun. 

In 1998, the Super-Kamiokande first detected that there was a difference in the number of muon neutrinos falling on the detector from above and those incident from below after passing through the mass of the globe. One explanation for this puzzle was that the muon neutrinos were “oscillating” into a different type. They further suspected that the muon neutrinos were actually changing into Tau neutrinos. This was corroborated by the Sudbury Neutrino Observatory, which was built to study electron neutrinos coming from the Sun, and which in 2001 detected a difference in the number between what was calculated and what was observed.

Theoretically explaining this puzzle meant making a big dent in the so-far accepted Standard Model of particle physics, because it meant that the neutrino had to have a small mass. 

Open questions

Even today, while the difference between masses of the three types of neutrino are known, the absolute mass of the lightest is not, as Prof. McDonald said over the telephone to the Nobel committee and the press. 

Another question is about the hierarchy of masses of the three flavours. Would the electron neutrino be heavier than the Tau and muon neutrinos, or is it the other way around? 

Every particle known so far has a unique antiparticle. For instance, the antiparticle of the electron is the positron, and that of the proton is the anti-proton. Similarly, would neutrino have an antiparticle which is different from itself or is each neutrino its own antiparticle? 

The Nobel Prize has given a boost to neutrino hunters across the globe as they gear up to pursue these questions.