by S. Prasad Ganti

The first half of twentieth century was a very interesting period for particle physics. All the matter in this Universe can be broken down into atoms and atoms were found to contain subatomic particles called electrons, protons and neutrons. Among the zoo of particles discovered since then, neutrinos are mysterious particles. Italian for “little one”, they weigh very little and do not have any electrical charge.

There are millions and millions of neutrinos coming to Earth from space every second, particularly from our own Sun. Some get produced in the vicinity of nuclear reactors. They do not interact with matter. Millions may pass through our body without any effect. This makes them very difficult to detect or trap. If they are so invisible yet ubiquitous, how did we come to know about them? It is an interesting scientific detective story.

Wolfgang Pauli found out that some energy is 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 to be that of a neutrino. Thus was born a theory whose quest triggered a multitude of scientific experiments over several decades.

Detectors have grown pretty sophisticated like the optical telescopes used to gather and study light. Neutrinos may generate muons when they hit water. A large number of neutrinos when passing through a huge tank of water may produce a few hits that produce a blue light called Cherenkov cones. This is the basis for a neutrino detector. The problem is that cosmic rays also generate muons in Earth’s atmosphere. Going deep inside Earth’s surface or under water is a way to filter 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. Detectors were built, each more sophisticated than the previous one. There are neutrino detectors at the bottom of Lake Baikal and at the South Pole called the Ice Cube.  Eventually the mysterious neutrinos were detected.

In 1956, Frederick Reines and Clyde Cowan detected the 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 in 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.

An interesting book on this topic is “The Neutrino Hunters” by Ray Jayawardhana.