In May an international group of physicists studying the elusive particles known as neutrinos announced that they had spotted one spontaneously transforming from one type to another. Such an ability indicates that neutrinos, long thought to be weightless, have mass, a finding with profound theoretical and cosmological implications.
Neutrinos come in three varieties: muon, tau, and electron. Previous experiments had suggested that one variety can turn into another, and scientists finally caught one in the act. They fired a billion billion muons from CERN in Switzerland toward the OPERA detector in Italy, hoping to discover one of them transforming into a tau neutrino, and in late 2009 they did. The standard theory of particle physics does not allow that to happen. (A separate 2010 experiment at Fermilab found evidence of a fourth type of neutrino—another major puzzle.) OPERA physicist Antonio Ereditato notes that neutrinos are so common that although their mass is tiny, collectively they must account for a noticeable fraction of the bulk of the universe.