The solar neutrino problem relates to the discrepancy between the proportions of the different flavours of neutrinos emitted by the sun in the theoretical model as opposed to experimental measurements. Whilst the sun primarily emitts electron neutrinos, neutrino observatories such as SNO+ detected neutrinos in roughly equal proportions of the three flavours; furthermore the quantity of electron neutrinos detected was less than the theoretically predicted value. Both of these can be explained by neutrino oscillation - in which the neutrinos alter their mass to change their flavour (ie. an electron neutrinos gain mass to change to a muon neutrino). This would also explain the relative lack of electron neutrinos, thus solving the solar neutrino problem!

An antielectron neutrino is an antileptonic elementary particle - the antiparticle of an electron neutrino.

An antimuon neutrino is an antileptonic elementary particle which is the antiparticle of a muon neutrino.

The problem was that the Sun should output a lot more electron neutrinos then were measured. This meant that the model describing the interior of the Sun would be wrong, but it was working very well in predicting other things.

It was finally solved when something called neutrino oscillation was discovered. It turned out that (this might be a bit technical) the interaction state of a neutrino was not equal to its mass or propagation state. In short, this meant that electron neutrino's could become muon or tau neutrino's after a while (and change back again after that).

After this people began looking for muon and tau neutrinos coming from the Sun and together with the electron neutrino number they added up to the amount the Solar model predicted. The problem was thus solved; the Sun DOES output more electron neutrino's but some of these change into muon or tau neutrinos before they reach the Earth, and since we were initially only looking for electron neutrinos we missed some.

Muon neutrino was created in 1962.