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Before the discovery of neutrino mass, how did people aware electron and muon neutrinos are different?

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First of all, the flavor basis and mass basis for neutrinos are not the same (in other words a muon neutrino doesn't have a well-defined mass, it is a linear combination of the mass eigenstates). This is what allows neutrino oscillations to occur. By the way, neutrino masses cannot be directly extracted, we rely on indirect measurements. Secondly, we know that there are different types of neutrino from their different interactions. – Will Jul 6 '13 at 1:03
what different interactions? thx – user26143 Jul 6 '13 at 1:07
For example, $W$ decay. We can have: $W^+\rightarrow e^+ + v_e$, $W^+\rightarrow \mu^+ + v_\mu$, or $W^+\rightarrow \tau^+ + v_\tau$. You would never get something like $W^+\rightarrow e^+ + v_\tau$ etc, due to lepton conservation. Now you will ask, how do we know that these are different? It comes down to experimental measurements. The mass oscillation is one indicator - this only makes sense if there are other types to oscillate into, and our model of three neutrinos matches the data well. – Will Jul 6 '13 at 1:19
This question is very similar: – Will Jul 6 '13 at 1:21
I heard mass oscillation was confirmed at 2001. Before that, is there any other experimental result indicates $\nu_e$ and $\nu_{\mu}$ are different? – user26143 Jul 6 '13 at 1:57
up vote 9 down vote accepted

Neutrinos are leptons, they have leptons number just like the charged leptons (electron, muon, tau).

Weak interaction conserve not only the global lepton number, but the lepton flavor numbers as well. And that is how we identify their flavors: electron neutrinos participate in reactions that involve electrons and muon neutrinos participate in reaction that involve muons.

We know that they are not the same because we have intense sources of muon neutrinos and muon anti-neutrinos (from cosmic rays and accellerator created muons and anti-muons) and we have intense sources of electron anti-neutrinos (reactors). And when we put detectors in front of these sources the two flavor behave differently; the most diagnostic interaction (and a common one) is quasi-elastic scattering in which a charged lepton out---a muon or an electron depending on the flavor of the beam. We don't have any intense sources for tau neutrinos, but they have been identified in the oscillated input to both OPERA (which was designed for that measurement) and IceCube.

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"how we identify their flavors: $\nu_e$ participate in reactions that involve $e^-$ and $\nu_{\mu}$ part. in. react.s that involve $\mu^-$." -- That's however not consistent with how we identify flavors of quarks: we don't distinguish flavors of "charge -1/3 quarks" (a.k.a. "d-type") by whether they participate in reactions that involve "u", or "c", or "t" quarks; but instead we distinguish quark flavors by their mass (or, perhaps rather, by the mass of the hadrons they form) and their charge. If there's a different convention in the lepton sector, then only for historical reasons. – user12262 Jul 6 '13 at 7:11
thank you very much for your explanations! – user26143 Jul 6 '13 at 11:26

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