Measuring the rarity of a particle decayIn the so-called MEG experiment at the PSI, researchers are searching for an extremely rare decay signature from a certain kind of elementary particles known as muons. More precisely, they are quantifying its improbability. According to their latest number, this decay occurs less than once in 2.4 trillion events. By means of this result, theoretical physicists can sort out which of their approaches to describing the universe will hold up against reality.