Dawa Bay

I have been remiss. The Dawa Bay experiments have found that the "mixing angle" between electron neutrinos and other varieties was surprisingly large. (I interviewed to work on that experiment, but they weren’t hiring right then.)

Before you click Back, or even click on the link, I should explain a few things.

The first is that the “nature” of a particle not does not predict its mass. In fact, if you try to find the mass of a quark, you discover you aren’t so sure what kind it is, and if you are sure what kind it is you can’t be sure of the mass. The same is true for neutrinos, though at very much smaller masses.

So you have to think of an electron neutrino as being made up of a little of mass type I, a little of mass type II, and a little of mass type III—in a quantum-mechanical way, of course. You don’t chop a little of this and a little of that into a lumpy neutrino salad. You have a blend that will appear to be either type I or II or III, with some probability for each. Likewise a type I will, if you poke at it, be an electron, muon, or tau neutrino with some probability for each.

A neutrino (or anti-neutrino) flying away from the reaction that generated it has to, in some sense, find which mass type it is going to be as it flies. And as it travels through matter, it is in some weak sense poked a little. So the picture is of an electron anti-neutrino that finds itself as a mass type I anti-neutrino as it flies through air and rock, but when it interacts with the detector far away it turns out to be a muon anti-neutrino. It "oscillated" from being one type to another (and it can do that even without matter around.)

Those mixing proportions involve the "mixing angles." The handiest way to describe this sort of relationship between types of neutrinos is with vectors and matrices, which are just ways of bundling lots of detailed equations into something simple to handle. (Trust me, it makes things much easier to keep track of. Remember solving linear equations back in high school algebra? A pain. But a lot easier when you use matrices.) And it turns out to be handy to describe the various components of the matrix in terms of some angles.

We don’t know if there are only 3 types of neutrinos (and their anti-particles, of course): a fellow named Majorana developed a theory with a different kind 70 years ago, and nobody has been able to prove him wrong.

His neutrinos were their own anti-particles, just like photons are anti-particles for photons. He disappeared mysteriously—perhaps he was his own anti-Majorana.

If you know the components of the mixing matrix, you can tell if there’s something missing; some new type of neutrino we haven’t seen yet. That’s why people are especially interested in these details. At any rate, two of these angles are quite small, and this is rather large: about 6 degrees. At Dawa Bay that means that some electron anti-neutrinos coming from the reactors were detected as electron anti-neutrinos at the close detector but a measurable proportion were missed at the far detector. They had "oscillated" into muon anti-neutrinos, but they didn’t have the kinetic energy to generate a muon in the detector, so the detector couldn’t see them. I suppose if we had detectors where muons instead of electrons orbited the nuclei, that might have spotted some of those missing neutrinos. Of course muons decay quickly, so we can’t build any such detectors. Phooey.

Ok, now try the link. (Dawa Bay is in China. There are several reactors there, and an international physics neutrino experiment. The area is high security, of course, and since the French built some of the facilities I understand that the restaurants are very good. They'd better be--you don't get to get out much.)

Update: changed 4'th paragraph to be more precise. If you "poke at" a neutrino, you are probing it with matter and will find out its "flavor" or electron/muon/tau neutrino type. A moving neutrino, though, has to be within shouting distance of being of mass type I, II, or III.