Missing Neutrinos

19 July, 2008

Neutrino DetectorThe deepest mine in Canada is located in a ghost town in Ontario. Two kilometres below ground, within this nickel mine, is a solar observatory that found the answer to a puzzle about some of the fundamental particles of nature. It may seem an odd place to look at the Sun, but deep underground is the only place quiet enough and hidden enough to find the elusive radiation that signals events that took place in the heart of the Sun.

Studying the surface of the Sun is done from many observatories around the world and even from telescopes in space. But the surface is a tiny part of this blazing ball of gas that we spin round once a year. How can we see past the surface and inside the Sun? There is a way. The light that reaches us from the Sun takes only eight minutes or so to get to the Earth once it leaves the surface. But those photons may have been produced millions of years ago in the centre of the Sun. There the photons will bounce around, be absorbed, emitted, reabsorbed and generally messed about with until the reach the surface. By the time they do so there’s no information to be gleaned about their origin (though much to be learned about the top layers of the Sun). Photons are of no help, but there are other particles produced in the nuclear furnace deep within the Sun that do not interact much with the other matter around. These are neutrinos, and instead of taking millions of years to get out of the Sun they do it in minutes.

Neutrinos have no charge and so aren’t affected by electromagnetic fields. They fly straight and true and don’t interact much with matter either. Millions of neutrinos pass through your body every second, and indeed through the entire Earth. Rarely, very rarely, a neutrino will hit the nucleus of an atom and a reaction may occur. The neutrino detectors look for these rare events. Deep within a mine, a huge tank of heavy water is surrounded by 10,000 photomultipliers, watching for incredibly faint flashes of light. When a particular type of neutrino hits the neutron in the centre of the deuterium in the heavy water, the neutron is converted to a proton and the neutrino becomes an electron. This process emits light and can be detected.

However this process only detects Electron Neutrinos, and there are two other flavours of neutrino, Muon Neutrinos and Tauon Neutrinos. Until recently there was a problem. All the detectors around the world were picking up the light caused by Electron Neutrinos interacting with them. But there were far fewer Electron Neutrinos being detected than solar models predicted. These models fit very well with the results from observations of the photosphere and of helioseismology and so scientists were reluctant to throw them away. Could anything explain where the missing neutrinos were?

In the Candian mine, below the ghost town, a new observatory was completed in 1999. It was able to detect all flavours of neutrinos and soon discovered that the total number of neutrinos it observed was exactly in line with the number predicted by the solar models. There were less Electron Neutrinos but more of the other two flavours. Particles can change from one form to another, but for neutrinos to do so must mean that they have a rest mass of greater than zero. Unlike photons, these things have a bit of weight to them.

And so every second the Sun pours forth its trillions of neutrinos, they shoot through the deep hot layers of the star and into space. Somewhere along the line many of them change flavour and then some of them pass straight through the Earth, while a tiny few others get slammed into some water where someone in a mine is watching for it. They interact with almost nothing in the universe, and yet we manage to spy on them. Underneath a ghost town in Canada, we watch the ghost particles of the Sun.