by S. Prasad Ganti

Last year I wrote about neutrinos, the mystery particles, based on the book I read entitled “The Neutrino Hunters” by Ray Jayawardhana. This year, the Nobel Prize in Physics went to Takaki Kajita and Arthur McDonald who led two different neutrino experiments in two different parts of the world. In fact, the book by Jayawardhana is referenced on the http://www.nobelprize.org site, which has more details on the prize and the prize winners. The site is very informative compared to what gets reported in the general media.

Around the turn of the millennium, Takaaki Kajita discovered that neutrinos from the atmosphere switch between two identities on their way to the Super-Kamiokande detector in Japan. Meanwhile, Arthur McDonald in Canada demonstrated that neutrinos from the Sun were not disappearing on their way to Earth. Instead they were captured with a different identity when arriving at the Sudbury Neutrino Observatory.

Neutrinos are mysterious particles. Italian for “little one”, they weigh very little and do not have any electrical charge. Earlier in the nineteenth century, Wolfgang Pauli found some energy missing when a beta decay occurs. A beta decay is a form of radioactivity in which unstable elements change their form and structure. For example, C14 (carbon) has eight neutrons and six protons. During a beta decay, one of the neutrons splits into an electron and a proton. The new nucleus with seven protons and seven electrons is N14 (nitrogen). Pauli explained the missing energy as that of a neutrino.

When passing through a huge tank of water in a detector, a large number of neutrinos may produce a few muons that produce a blue light called Cherenkov cones. Cosmic rays also generate muons in Earth’s atmosphere. Going deep inside Earth’s surface or underwater is filters out the cosmic ray muons. Cosmic rays decrease as one goes deeper inside the Earth, but upward moving neutrinos from other side of the Earth also pass through the detector.

In 1956, Frederick Reines and Clyde Cowan detected neutrinos near a nuclear reactor for the first time. The 1995 Nobel Prize in Physics went to Reines. In 1968, Ray Davis detected solar neutrinos deep inside Homestake mine in South Dakota. Kamiokande detector in Japan, also known as Super K, detected the first neutrinos from outside of the solar system from the 1987A supernova in the Large Magellanic Cloud, a satellite galaxy of the Milky Way located 160,000 light years away. Ray Davis and Masatoshi Koshiba,the Director of Super K, won the Nobel Prize in Physics in 2002.

The third Nobel Prize for neutrinos in 2015 was given for discovering a new phenomenon – neutrino oscillations, which means that neutrinos change their form between three different types. Unless all the three type are detected, it is not possible to account for all the neutrinos generated in the Sun and come to the Earth. That is what Takaki Kajita and Arthur McDonald did to win the prize in 2015. They solved the mystery of the missing neutrinos.