25 Days of Particles: Day 15

Muon/Tau Neutrinos

Classification: lepton, fermion
Fundamental: yes
Family: second and third
Mass: < 0.000002 MeV
Interactions: Weak, Gravity (barely)
Spin: 1/2
Lifetime: stable

Today we are almost going to revisit a particle we met earlier. If you'll recall, the electron had a little buddy, the electron neutrino. This little neutral particle was appeared with electrons in weak decays and was one of the four members of the first family. The second and third families have their neutrino members as well, the muon and tau neutrinos. These neutrinos only appear in weak reactions with muons and tau leptons, respectively.

Now, neutrinos are not the easiest particles to detect and count, but it became apparent in the 1960s that some electron neutrinos were missing. One of the major sources of neutrinos in our solar system is from fusion reactions in our sun, which primarily produce electron neutrinos. The volume of electron neutrinos the sun should produce can be calculated, and experiments sensitive to electron neutrinos set about to measure that volume. They came to the conclusion that only between 1/3 and 1/2 of the expected number of neutrinos were making it to Earth.

This leaves two possible scenarios. Either we over-estimated how many neutrinos the sun makes, or something we don't know about is happening to the neutrinos in transit. What exactly was going on remained a mystery for over thirty years.

The answer was that something was happening to the neutrinos in transit. Sometimes, the particles we see in real life (the mass states) don't exactly correspond to how particles interact with each other (the electroweak or strong states). We've seen this a little already, as some of the mesons exist as linear combinations of different possible quark combinations. This phenomena is called mixing, and it allows one type of particle to turn into another type. When one of the particles is much more massive than the other, the change will only go in one direction. The high mass particle will turn into the low mass one, and we call this a decay. When the particles have about the same mass, they can change in either direction, and we call this oscillation.

So any given neutrino is a mix of electron-, muon-, and tau-ness, and can change from being dominant in one to being dominant in a different one. The electron neutrinos were changing into muon and tau neutrinos (and back again) as they traveled from the sun to the earth. The excess muon and tau neutrinos were measured as expected, finally solving the mystery in the late 1990s.

It should be noted that this mixing can only happen for particles that have some mass. Thus even though neutrinos are so light we haven't been able to measure their masses, we know they must have some. Neutrinos are a little weird that way.

And one final grace note about the weirdness of neutrinos: they are the only particle that can't exist in both helicity states. Neutrinos must have their spin vectors anti-parallel to their momentum vectors, and anti-neutrinos must have the two pointing in the same direction. There is no good explanation for this bit of weirdness, but it does affect particle decays involving neutrinos. Such decays must preserve spin, and the neutrino will take spin-1/2 away with it, and the neutrino will travel away from the decay in a direction that keeps its spin and momentum lined up as it needs to. This means that weak decays involving neutrinos will have some direction to them, instead of particles flying in every possible direction.

In summary, neutrinos don't do much, but the little bit they do ends up being weird and making us rewrite all our theories. This is why they are still the subject of serious study and puzzlement almost eighty years after their discovery. I hope they take that as a compliment.