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).

The Standard Model, the best current map of elementary particles, does not predict such a particle X. But its existence is allowed by theories that try to explain why our Universe has so much matter and almost no antimatter. Particle X could be axion-like — light, almost as elusive as a ghost passing through walls of ordinary matter.

Physicists analyzed data collected in 2021. They looked for traces of X in the mass range from 160 to 220 MeV (roughly 200–300 times heavier than an electron). In the region where a signal should appear, three events were found. Two of them landed close together, as if pointing to a mass around 177 MeV. But three events are too few to claim a discovery: such a spike could be a random fluctuation, a game of statistics.

So the result was presented as upper limits: the probability of a kaon decay with the production of X does not exceed (1–20)×10⁻⁷ depending on the particle's mass. This means that for a billion kaon decays, there are no more than one or two hundred such events. In addition, for the first time, they searched for the kaon decay into two pions and two photons without an intermediate particle, and found no events, setting a limit of 1.69×10⁻⁶.

The next step is to collect more data. If a particle with a mass around 177 MeV truly exists, it will show up in new batches of decays. For now, this is like a footprint in the sand: perhaps it was left by an unseen beast, or perhaps by the wind.