Thursday, June 24, 2010 :::
Do neutrinos have a different mass from antineutrinos?
PhysOrg, insofar as I've been following them, seems to tend toward the sensationalistic, and neutrino masses are a complicated enough subject that they may be spinning this a bit hard. For an electron, the mass term effectively couples a left-handed electron to a right-handed electron; if the electron were massless (so that it traveled at the speed of light) the two handednesses would be well-defined and separable, but because the electron is massive they kind of intermingle. The left-handed electron, though, can't turn into a right-handed positron in the same way, due to conservation of electric charge. Because neutrinos are uncharged (with respect to the electromagnetic force, but also the weak and strong forces), that's not necessarily true of neutrinos, and some people consider "the standard model" — which I take to mean any of a number of similar models, mostly differing in the Higgs and neutrino sectors — to be the model in which there is no right-handed neutrino per se, and the left-handed neutrino is simply coupled, by the mass term we've known for 12 years must exist, to its own antiparticle. If you have left- and right-handed neutrinos in addition to their antiparticles, and you have an assortment of mass couplings between them, you can find that your actual mass eigenstates are a bit strange, and you have two mass eigenvalues. (One interesting suggestion, in fact, is that the usual self-couplings are of the order of the electroweak scale and the unusual coupling with the antiparticle is on the order of the Plank scale, which could very naturally give a light mass eigenvalue on the order mGUT/mEW2, which is about what we observe.) Flavor-mixing just makes things that much more complicated.
That got longer than I meant. Anyway, it's possible they're playing up something that can fit very well into the standard model. If they're not simply spinning something of this nature, though, this is a big deal — and should, accordingly, be approached with a very strong prior against it.
::: posted by dWj at 10:34 PM
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