S. Suzuki

A proton can't be seen as a simple ball. If you hit it with a photon, it can absorb energy and jump into an excited state, like a bell that starts ringing at a certain frequency after being struck. These states are called nucleon resonances, and they live very briefly before decaying into other particles.

Inside every atom lives a proton. For a long time, it was considered as eternal as matter itself. But theories that unify all forces of nature predict that, sooner or later, the proton will decay. The only question is how long we have to wait. In the Japanese detector Super-Kamiokande — a giant tank of ultra-pure water — physicists searched for signs of such a decay.

A strange visitor came to light in a fleeting flash. A beam of photons struck a deuterium nucleus (heavy hydrogen), and in the cascade of produced particles, detectors noticed something unusual. At a certain combination of masses and momenta, the signal sharply intensified, as if a new particle had appeared for a brief moment.