Protonium: Discovery and prediction

The Beijing Spectrometer (BESIII) Collaboration reconstructed the invariant mass of three pairs of positive and negative pions by studying the decay process of charmonium to a photon and three pairs of positive and negative pions. They discovered the resonant structures X(1840) and X(1880), which are interpreted as the predicted proton-antiproton bound states, also known as protonium. This article briefly introduces the experimental discovery processes of these resonant structures and discusses the theoretical explorations inspired by them. The predictions proposed by these theoretical explorations offer a new perspective for studying the nature of these particles and new decay modes. Therefore the collaborative exploration of experiments and theory plays a positive role in deepening understanding of the fundamental laws of nature.