Great question!
Yes, the SuperKamiokande experiment has provided compelling
evidence that neutrinos change from one kind (e, mu, or tau) into
another and back. This was clearly a sensation and we were quite
excited to learn that this happens. It's a lot of fun to see when
science advances in this way, because that's why we do it in the first
place -- to learn new things. Since then, other experiments have
confirmed that neutrinos oscillate -- the the Sudbury Neutrino
Observatory (SNO) experiment in Canada has helped to elucidate the
neutrino mass and mixing parameters.
People didn't raise much fuss about it because it was, in a way,
expected (or at least we didn't have any good reason to believe it
wouldn't be true, that neutrinos would have mass and change from one
kind to another). One of the reasons we weren't entirely suprised is
because the same sort of thing happens for quarks. Each kind of
down-type quark which is observed in nature is really a mixture of the
different flavors of quarks, and the fractions of each kind have been
measured and are being measured ever more carefully in high-energy
physics laboratories today.
Quarks and leptons each come in three generations, and the
Standard Model is not consistent with a different number of generations
for quarks and for leptons. It's not a surprise that some feature of
the quarks is also a feature of the leptons.
We know of a physical principle which requires photons to be
massless, but do not know of any which requires neutrinos to be
massless (if we did, we'd be proved wrong and would have to discard
that!). If photons were found to be massive, we'd have to modify
theories much more. Neutrino masses and mixings are just more
parameters of the Standard Model. The fact that they are not zero does
not, within the Standard Model, produce a prediction which is in
conflict with any other experiment.
The conservation of the numbers of each lepton type is now known
to be only an "approximate" conservation law. It works great for
interactions which happen very fast in particle experiments, but even
then this rule may be broken a very tiny fraction of the time, as
neutrino oscillations are very slow compared to the time a high-energy
collision needs to take place. The conservation of the total number of
leptons (adding up the numbers for e, mu, and tau) seems to be still
intact.
Tom J.
(published on 10/22/2007)
