Messaggeri cosmici

Among the commonest particles in the Universe are neutrinos. Like photons, they outnumber the protons and neutrons of bulk matter by around one thousand millions to one. Each cubic centimetre of space contains a hundred or so neutrinos left over from the Big Bang at the beginning of the Universe. Every second, 60 thousand million neutrinos from the Sun pass through each square centimetre of the Earth surface and through you!

The Tarantula nebula and Supernova 1987a

When supernova 1987a blazed forth, the Earth was soaked with a tidal wave of neutrinos. Most passed through the Earth unheeded, but a few were caught in underground particle detectors.

Paradoxically, neutrinos are probably the least understood of particles. They have no electric charge, and interact with other matter only through the weak force. There are three types of neutrino, which are like electrically neutral versions of the electron, and its two heavier relatives, the muon and the tau. These neutrinos are all much lighter than their charged counterparts, but we have no idea how much lighter or whether indeed they have any mass at all. Neutrinos are so common in the Universe that even if they have only a small mass their total effect could dominate the Universe!

The masses of the neutrinos are certainly smaller than we can measure by standard methods, so experiments have to be more ingenious. One technique is to look for neutrinos changing or "oscillating" from one type to another. This would be possible if, but only if, one or more of the neutrino types has some mass. This exciting possibility could explain why fewer neutrinos reach Earth from the Sun than expected, as the present detectors pick up only one type of neutrino, the electron-neutrino. Experiments at CERN with beams of neutrinos produced from particle collisions are also testing whether neutrinos oscillate, at higher energies than the Sun produces.