High-energy collision experiments have helped particle physicists understand many aspects of the behavior of quarks—the subatomic building blocks of protons and neutrons. For example, during the decay of quark conglomerations, researchers have observed the brief appearances of exotic subatomic particles, such as tetraquarks or loosely bound mesons. These exotic particles appear as resonance peaks in the mass spectra of the debris of electron–positron collision experiments. A decade ago, the BESIII Collaboration at the Beijing Electron Positron Collider observed one such peak [1]. The collaboration has now confirmed the existence of that peak [2]. But the new data and analysis reveal that the previously observed peak is actually two peaks, suggesting the existence of a new exotic particle.
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