In a vast, dark hall, they're searching for a single person who makes almost no sound. That's how physicists hunt for sterile neutrinos — ghostly particles that pass through matter, leaving barely a trace.
The kaon is a long-lived particle by microcosm standards: it lives for billionths of a second before breaking apart into lighter particles. In the KOTO experiment in Japan, physicists catch the rarest variants of such decays. One of them is when a kaon transforms into two neutral pions plus another particle X, which immediately decays into two photons (particles of light).