Lecture 28 - The case of the missing neutrinos

On many occasion Astrophysics has played a pivotal role in changing the direction of fundamental physics. The problem of the `missing' solar neutrinos has been one such. The solution has taken more than four decades to arrive at and has the startling implication that the netrinos have mass (even if tiny). The most significant consequence of this is the need for a major revision of the current `Standard Model' of Particle Physics!

This lecture is about a most remarkable `detective story'. The relentless hunt for the missing neutrinos from the Sun. In August 1920, Sir Arthur Eddington made the remarkable suggestion that the energy radiated by the sun and the stars is produced when Hydrogen is transmuted to Helium - the deficit mass is radiated away as energy. The details of this fusion of hydrogen to helium was worked out by Hans Bethe in 1938. If it is true that the sun is converting hydrogen into helium, then the sun must be emitting 10^38 neutrinos per second; and a good fraction of this must pass through the earth. Raymond Davis, a very brave chemist, decided to build a detector to `detect' the solar neutrinos. By 1968, he had succeeded. But there was a twist. The 'number' of neutrinos he detected was just one third of what was expected. Thus began the hunt for the missing neutrinos. And it took more than 30 years to solve this mystery - and it was solved! In this lecture, I describe how the story evolved.