Nobel Prize Physics 2015

The Noble Prize in Physics 2015 has been awarded jointly to TakaakiKajita and Arthur B. McDonald. TakaakiKajita is director of the Institute for Cosmic Ray Research and professor at the University of Tokyo. He discovered that neutrinos from the atmosphere switch between two identities. Arthur B. McDonald is a professor emeritus at Queen’s University in Kingston, Canada. The experiments of Arthur B. McDonald discovered neutrino oscillation which further led to the concept that neutrinos have mass.

What are Neutrinos?

Neutrinos are one of the elementary particles, which make up the universe. They are similar to electrons except that they do not carry electric charge. As they are electrically neutral, they are not affected by the electromagnetic forces like electrons or protons. Neutrinos are only leptons with no electric nature. Other leptons, which carry electric charge are electrons, muons and taus. Presently three types of neutrinos are known. They are electron neutrino (close to electron), muon neutrino (close to muon) and tau neutrino (close to tau).

Neutrinos are though not affected by the strong forces such as electromagnetic force, yet they are affected by the “weak” sub-atomic force and gravity. Therefore they are able to pass through great distances in matter at almost speed of light and without being affected by the matter. They are abundantly found in nature and can be created in several ways, including nuclear reactions and general atmospheric phenomena. The majority of the neutrinos present in the vicinity of the Earth are from nuclear reactions in the sun. Other neutrinos are created in nuclear power stations, nuclear bombs. Neutrinos also arise from  cosmic phenomena such as death of stars.

What is Neutrino Oscillation and how it is important?

The existence of neutrinos was first proposed in 1930. The first experimental detection of neutrinos was achieved in 1956. During the later experiments on solar neutrinos it was found that there is a mismatch between yielded results and theoretical calculations. The solar neutrinos detected were about a third to half of what was expected. In subsequent experiments also the same results were found and scientists found that neutrinos were disappearing mysteriously.

This led to the question of whether the mechanism by which the Sun shine is different from that predicted in theory or is there any other process that preventing all the neutrinos from being detected? The experiments of TakaakiKajita and Arthur B. McDonald demonstrated that the neutrinos from the sun were not disappearing on their way to Earth. Rather, in their journey from Sun’s core to the detector underground, some of the solar neutrinos (which are all electron-neutrinos) were transforming themselves to other forms of neutrinos, and thus escaping detection. This property of morphing of electron neutrinos to muon-neutrinos and tau-neutrinos is called “neutrino oscillation”. The neutrino oscillation was the reason why in the theoretical calculations, up to two-third of the neutrinos were missing.  It was found that all three neutrinos are able to morph into each other.

However, this raised another question. The neutrino oscillation requires that the neutrinos must have some finite mass. This was against the hitherto assumption that neutrinos are massless. The neutrino oscillation gave reversed the existing theory and proved that they indeed have mass.

What are implications of Neutrinos having Mass?

Currently, physicists know that the neutrinos exist in three types (flavours)  and that they oscillate, which means that the morph into three types as they move in space and time. The observed oscillations imply that neutrinos have mass. In academic terms, this is far reaching discovery because so far scientists were shy to include neutrinos in the standard model of particle physics as some building blocks having mass (so far they were put in as massless). However, lots of questions still need to be resolved that might change our understanding of basic physics. For example – What is the origin of neutrino mass? How are the neutrinos masses ordered (referred to as mass hierarchy)? What are the masses? Do neutrinos and antineutrinos oscillate differently? Are there additional neutrino types or interactions? Are neutrinos their own antiparticles? The present experiments and observatories around the world are trying to answer various other questions related to neutrinos and their properties and usefulness in development of science.

It’s possible that India’s Neutrino Observatory is helpful in resolving some of these question.