Neutrinos are tricky. These chargeless,
weakly-interacting particles can
easily pass right through the Earth. Detecting them, much less
large enough samples to understand their properties, is a colossal
undertaking. They do, however, play important roles in nuclear
stars, supernovae, and cosmology. They may even be responsible
generating the matter-antimatter asymmetry we see in the Universe.
Fifteen years ago, experimental data from astrophysical sources hinted that neutrinos had a rather amazing trick: They could spontaneously oscillate from one type to another. For this to happen, they would need to be massive, and the interaction that creates them must operate only on mixed quantum mechanical states.
Since then, a world-wide effort using man-made neutrino sources has clearly demonstrated that that these particles have finite but tiny masses and undergo neutrino oscillations over continental distances. This is physics beyond the standard model of particle physics. The discovery of neutrino mass and mixing has opened up a new regime in the subatomic world, which still remains largely unexplored.
I will summarize our current understanding and future plans, with an emphasis on the recently-completed MINOS long baseline neutrino oscillation experiment. MINOS investigated neutrinos in a beam created at Fermilab near Chicago and directed to a 5400 ton detector a half mile underground in northern Minnesota.