One in ten billion: after a 40-year chase, physicists catch a particle doing the rarest thing it can do

Some hunts take a career. This one took more than forty years. Physicists have been chasing a particular vanishing act since the early 1980s: a particle called a B0 meson — made of a 'beauty' quark and a 'down' antiquark — decaying into just two muons. It happens, theory says, only about once in every ten billion decays, which is exactly why it matters: the rarer a decay, the more sensitive it is to the quiet influence of particles nobody has discovered yet. Today, at a seminar at CERN in Switzerland, the LHCb collaboration announced the first evidence for the decay from a single experiment. Their detector sifts through hundreds of millions of proton collisions every second — four terabytes of data a second — and from that flood the signal emerged at 3.2 standard deviations, meaning only about a one-in-1,500 chance it is a background fluke. The rate matches the Standard Model, physics' reigning rulebook, yet again. 'Physicists have been chasing this decay for more than forty years,' said co-lead Marco Santimaria of Italy's INFN. A planned upgrade will let LHCb collect ten times more data — and chase the particles this decay may be whispering about.