There are two fundamentally different answers to this question. The first has to do with an old problem in astrophysics and cosmology, and is purely a numbers game. Scientists have known for decades that all the visible matter in galaxies does not account for nearly enough mass to explain the rotation rates observed - the gravitational force would not be great enough to prevent the galaxies from flying apart. This is known as the dark matter problem - there has to be a significant amount of mass in some form that we cannot see. The mass of everything we see, from stars to Starbuck's, gorillas to galaxies is mostly accounted for by the mass of protons and neutrons. Here's where the numbers come in. For every proton or neutron in the universe there are about a billion neutrinos! So if neutrinos have even tiny masses, they may make up a significant fraction of the mass of our universe, and at least help to solve the dark matter problem.
The second reason for excitement is more subtle. Our present understanding of the workings of our universe can be summed up in what is called the 'Standard Model', in which all matter is constructed using six quarks and six leptons, and the interactions among the quarks and leptons are mediated by four particles called gauge bosons. In the simplest standard model, neutrinos have zero mass. Thus this evidence for neutrino oscillations is the first experimental evidence that the standard model in its present form is not quite right, and scientists love it when something is not quite right - it gives them new puzzles to work on!
The question of neutrino mass is not a new one, but because the mass, if not zero, is at least very small, it is not directly measurable. You can't just grab a neutrino and throw it on a balance! Indeed direct measurement techniques have set an upper limit on the electron neutrino mass of ten electron volts (eV). By comparison the electron has a mass of about half a million eV. Upper limits on the other two neutrino flavors are significantly higher.
This is where oscillations come in. If the various neutrino flavors have different masses the possibility for quantum mechanical mixing arises. Whenever we detect a neutrino, we see it as a particular flavor, but each flavor may be a mixture of two or more mass states, in which case the probability of seeing a particular flavor will oscillate as the neutrino travels away from its source. An ideal neutrino oscillation experiment would then create neutrinos of one particular flavor and attempt to detect one of the other flavors some distance from the source. The LSND experiment at Los Alamos is such an experiment, and has seen some evidence for muon neutrinos oscillating into electron neutrinos, but has seen fewer than one hundred events. The BooNE experiment presently being planned at Fermilab will be able to confirm (or rule out) this result with an order of magnitude more data.
The Super Kamiokande experiment did not use an accelerator-produced neutrino beam, but rather relies on a well-known interaction in the upper atmosphere. As high energy cosmic rays smash into air molecules, unstable particles called pions are produced. These then decay into a muon and a muon neutrino. Most of the muons decay themselves, producing another muon neutrino, an electron and an electron neutrino. Muon neutrinos should thus outnumber electron neutrinos two to one. Super K measures this ratio and finds it to be much lower than expected. Furthermore, they see a change in this ratio with zenith angle - the muon neutrino deficit depends on the distance the neutrino has travelled, just as one would expect with oscillations.
The third bit of evidence for oscillations comes from the four operating solar neutrino detectors. We think we know pretty well what nuclear reactions make our sun shine, and therefore how many neutrinos are being produced, but four experiments using three different detection methods all see way too few neutrinos from the sun! Oscillations would be one possible explanation, as these detectors are only sensitive to the electron neutrino flavor.
There are of course complications! The three different types of experiment - solar, atmospheric, and accelerator - all need different values of oscillation parameters to explain their results. That's OK though, since there are three neutrino flavors. I expect, with several new experiments coming on line within the next few years, a pretty good solution to the neutrino puzzle is not far off. Stay tuned!
