UK Neutrino Network

In 1930, the Austrian physicist Wolfgang Pauli proposed the existence of particles called neutrinos as a ``desperate remedy'' to account for the missing energy in a type of radioactivity called beta decay. He deduced that some of the energy must have been taken away by a new particle emitted in the decay process, the neutrino. Since then, after decades of painstaking experimental and theoretical work, neutrinos have become enshrined as an essential part of the accepted quantum description of fundamental particles and forces, the Standard Model of Particle Physics. This is a highly successful theory in which elementary building blocks of matter are divided into three generations of two kinds of particle - quarks and leptons. It also includes three of the fundamental forces of Nature, but does not include gravity. The leptons consist of the charged electron, muon and tau, together with three electrically neutral particles - the electron neutrino, muon neutrino and tau neutrino. Unlike the case for quarks and charged leptons, however, the Standard Model predicts that neutrinos have no mass! This might seem curious for a matter particle, but the Standard Model predicts that neutrinos always have a left handed spin - rather like rifle bullets which spin counter clockwise to the direction of travel. If right-handed neutrinos were to be added to the Standard Model, then neutrinos could have the same sort of masses as the quarks and charged leptons, and the theory would also predict the existence of antineutrinos. However, even without right-handed neutrinos, neutrinos with mass are possible, providing that the neutrino is its own antiparticle. Such a mass is then called a Majorana mass, named after the Sicilian physicist, Ettore Majorana. But the current Standard Model forbids such Majorana masses. These subtle theoretical arguments about the nature of neutrinos have now come to the fore, as the results from experiments detecting neutrinos from the Sun, as well as atmospheric neutrinos produced by cosmic rays, suggest that they do have mass after all. These results have subsequently been confirmed by terrestrial sources of neutrinos from nuclear reactors and beams. Experimental information on neutrino masses and mixings implies new physics beyond the Standard Model, and there has been much activity on the theoretical implications of these results. An attractive mechanism for explaining small neutrino masses is the so-called see-saw mechanism proposed in 1977 by Peter Minkowski and 1979 by Murray Gell-Mann, Pierre Ramond and Richard Slansky working in the U.S., and independently by Tsutomu Yanagida of Tokyo University. The idea is to introduce right-handed neutrinos into the Standard Model which are Majorana-type particles with very heavy masses, possibly associated with large mass scale at which the three forces of the Standard Model unify. The Heisenberg Uncertainty Principle, which allows energy conservation to be violated on small time intervals, then allows a left-handed neutrino to convert spontaneously into a heavy right handed neutrino for a brief moment before reverting back to being a left-handed neutrino. This results in the very small observed Majorana mass for the left-handed neutrino, its smallness being associated with the heaviness of the right-handed neutrino, rather like a flea and an elephant perched on either end of a see-saw. An alternative explanation of small neutrino masses comes from the concept of extra dimensions beyond the three that we know of, motivated by theoretical attempts to extend the Standard Model to include gravity. The extra dimensions are 'rolled up' on a very small scale so that they are not normally observable. The purpose of this proposal is to enable UK theorists and experimenters to hold meetings in which the latest theories and experimental results relating to neutrino physics can be discussed, and hopefully new ideas and collaborations can be formed or fostered.