In case anyone missed it (it really hasn't hit the non-scientific news in a big way, which isn't that surprising), the OPERA experiment at Gran Sasso National Laboratory, Italy has detected — with 98% confidence, they say — a tau neutrino in a beam of muon neutrinos fired from CERN, roughly 732km away.
Well, so what? One tau neutrino among billions of muon neutrinos. No big deal, right? Everyone makes mistakes?
Well, here's why it's a big deal: The originating beam from CERN is known to contain solely muon neutrinos, because the process that creates the beam can emit only muon neutrinos. This means that in only 732km, a trip lasting 2.4 milliseconds, one of those muon neutrinos changed into a tau neutrino. And that's important because it means that neutrinos, long believed to be massless, must in fact possess rest mass, be it ever so tiny a mass.
And that may not sound like a lot, but believe me, it's a lot more important than it sounds — not least because it's another datum that means the Standard Model of particle physics is, at best, incomplete, because the Standard Model as it currently exists does not allow the possibility of neutrinos having rest mass. This is at least the third recent major result to throw serious doubt upon the Standard Model. (Others include the recent result showing significant matter/antimatter asymmetry from the DZero experiment at Fermilab's Tevatron facility, and the Brookhaven muon (g-2)experiment to measure the anomalous magnetic moment of the muon, which yielded a result 2.6 standard deviations outside the range possible under the Standard Model. This latter result also possibly presents experimental evidence in support of the theory of supersymmetry.)
It's an interesting footnote that from the day they began drawing up the blueprints for GSNL, the precise location and alignment of the laboratory were chosen specifically to facilitate constructing this experiment.