If neutrinos had personalities, they'd be known best for their aloofness. These tiny particles, which can travel almost as fast as the speed of light, don't have much regard for matter; they sail right on through walls and buildings and planets. If you sent a group of neutrinos through a light-year of lead shielding—six trillion miles thick—half of them would still cruise on through.

Physicists attempt to explain all fundamental particles, including neutrinos, with a set of rules known as the Standard Model. Mostly it works great, but it has a few flaws. One mystery that the theory fails to explain is the snobbish behavior of the tiny neutrino, which comes in three different flavors: muon, electron and—most elusive of all—the tau neutrino.

Now, particle physicists have experimental proof that muon neutrinos can change into tau neutrinos, a finding that confirms the existence of a big blind spot in the Standard Model. On Tuesday, Italy's Gran Sasso Laboratory announced that the Oscillation Project with Emulsion-tRacking Apparatus, or OPERA experiment, had detected a muon-turned-tau neutrino—just the fifth time the transition had ever been observed.