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Particle resonances. The red diagonal band in this 'Dalitz plot' is due to a resonance called the rho-minus, an excited state of the pion. Each point in the plot corresponds to a proton- antiproton annihilation event detected by the Crystal Barrel experiment at CERN, the particle physics laboratory near Geneva. Crystal Barrel studied proton-antiproton annihilations at the LEAR (Low-Energy Antiproton Ring) accelerator. The fact that many subatomic particles, such as pions and protons, resonate when excited by energy indicates that they are not elementary particles but are made of smaller constituents - quarks.
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