Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Neutrino flavor oscillation

Learn more physics!

Most recent answer: 10/22/2007
Q:
I read in an article that muonic neutrinos could transform themselves into taonic or electronic ones and that it implies neutrinos have masses. (It was in reference with an experiment made at SuperKamioKande in Japan.) Doesn’t that violate basic principles of the standard model (e.g. conservation of leptonic numbers)? If so, why do physicists accept the news quietly while it seems to refute one of their major theories?
- David
Montréal
A:
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)

Follow-up on this answer.