A Neutrino Update
By: Professor Rabindra Mohapatra

Neutrinos are tiny particles that are emitted when nuclei undergo a process called beta decay and were postulated by Pauli to explain the apparent lack of energy conservation in these processes. They were discovered by Reines and Cowan in the mid-1950's and their interactions with matter were extensively studied in the ensuing decades. It was soon discovered that unlike other forces in Nature, neutrino interactions are always left handed. There was no evidence for the right handed neutrino. Thus, while all other particles were “full” particles, neutrino remained “half” a particle. Physicists became resigned to live with this unpleasant asymmetry. In the 1960's and 1970's, two more species of neutrinos with very similar properties the beta decay neutrino were discovered. Subsequent studies provided key confirmation of the standard model of particle physics which is now the accepted theory of electroweak and strong interactions. Thus neutrinos have been playing a key role in broadening the horizon of our knowledge of fundamental forces and matter.

They are also extremely important in our understanding of the Cosmos. Born at the moment of the big bang, they have survived as relics until today and fill the Universe almost as abundantly as cosmic black body radiation. They were responsible for the formation of light nuclei in the early stage of the Universe, which eventually led to the formation of heavier nuclei that we use in every day life. The nuclear fusion at the core of the Sun responsible for sunlight is also possible due to the existence of the neutrino.

Despite a great deal of knowledge about how the neutrinos interact, the very basic properties that characterize every matter in the universe (i.e. how much it weighs) remained a deep mystery for the neutrinos until about seven years ago. In June of 1998, the results of the Super Kamiokande experiment in Japan looking for neutrinos from the Sun and cosmic rays were announced and our ideas about neutrinos changed forever. They announced evidence for neutrino oscillations which can occur only if neutrinos have a mass. Furthermore, observing neutrinos from different directions in the sky, they could give an idea about how much the mass is likely to be. This also confirmed an earlier indication of such oscillations by Ray Davis who was searching for neutrinos from the Sun. During the past seven years a large number of other experiments have provided conclusive evidence that neutrinos have a very tiny weight.

The weight of about ten billion neutrinos equals that of a proton and weight of about ten million neutrinos equals that of an electron. It was also found that they can transmute from one species to another giving rise to the phenomenon of neutrino oscillation and providing an understanding of two puzzling observations. For example, most of the neutrinos produced in the solar core and in the cosmic rays seemed to be missing on the way to the Earth.

These observations have raised several profound puzzles for theorists. The key issue is the tiny mass of the neutrinos compared to the masses of familiar particles such as protons and electrons. This puzzle was solved by the work of Rabi Mohapatra and Goran Senjanovic in 1979 (and independently in four other papers) who suggested the so-called seesaw mechanism. According to this mechanism, the neutrinos, which are known to be left handed, are light because they are accompanied by neutrinos, which are right handed with superlarge mass. Mohapatra and Senjanovic, at that time, were studying a class of models called left-right models which postulated that at very short distances beta decay forces that neutrinos participate in, are parity conserving at a fundamental level.

The fact that all observed weak processes appear parity asymmetric is explained in this theory by the assumption that the right handed neutrinos processes are suppressed due to their superlarge masses, which is precisely what is required to work out the seesaw mechanism. So they concluded that weak interactions are parity asymmetric because neutrinos have such tiny masses. This mechanism has become a standard paradigm in neutrino mass physics.

A prediction of the seesaw mechanism is that the mass of the right handed neutrino is around ten thousand trillion times the mass of the proton. This is of course an extremely high mass and is therefore unlikely to be ever seen directly. However, the effect of such masses had already been considered in a different context in the 1970's when it was realized that as we go up in energy, the strengths of the known forces become similar and at precisely around the seesaw scale, they become unified into one force giving rise to the so called grand unified theories. This raised the possibility that the two ideas, grand unification and small neutrino masses must go together. It turns out that there is a symmetry group called SO(10) group which provides just the right setting for understanding the origin of forces as well as the origin of neutrino masses. Mohapatra has studied the physics implications of SO(10) grand unified theories prior to the discovery of neutrino masses. With the discovery of neutrino masses, has applied this group to understand the neutrino properties. Along with his recent collaborators (research associate Salah Nasri and former students Hock Seng Goh and Siew-Phang Ng, two collaborators in Canada , Bhaskar Dutta and Y. Mimura) he has returned to these theories and proposed them as the theory of all forces and matter beyond the standard model and studying their implications for proton decay and origin of matter in the Universe.