The puzzle of the A2 meson

Particles in high‐energy physics have become so numerous that a new “resonance,” as a short‐lived unstable particle is called, must do something unusual to attract attention. The A2 meson, an otherwise ordinary resonance, stands out from the crowd by exhibiting a peculiar structure in its mass spectrum. Instead of the single peak of a normal resonance, the A2 has two closely spaced peaks with the separation between the peaks roughly equal to their width. This double peak implies that the A2 is really two particles with nearly the same mass, or perhaps a single object of an entirely new type, a double resonance or “dipole.”