Neutrinos are tricky. These chargeless,
weakly-interacting particles can
easily pass right through the Earth. Detecting them, much less
collecting
large enough samples to understand their properties, is a colossal
undertaking. They do, however, play important roles in nuclear
decays,
stars, supernovae, and cosmology. They may even be responsible
for
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.