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A coin landing on its edge is about as rare as a heavy B meson decaying into a light pion and an electron-positron pair. Over a decade of collider operation, only a handful of such events have accumulated, but physicists have finally managed to spot them.
The catch is that other, more common processes can leave similar traces. To filter out false signals, the LHCb detector worked like an ultra-sensitive sieve, discarding anything that didn't match in mass and energy. The remaining 121 events became the first evidence of the long-awaited decay.
This process is highly sensitive to any unaccounted-for particles — even those too heavy to be produced directly in the collider. So far, the measured rate matches predictions, but now that the decay has been caught, the hunt for tiny deviations can begin.
B mesons live for only about 1.5 picoseconds — in that time, light travels less than half a millimeter. Catching their decay is like photographing a bullet in flight.