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A meson occurring either in a neutral form with a mass 264 times that of an electron and a mean lifetime of 8.4 × 10-17 seconds or in a positively or negatively charged form with a mass 273 times that of an electron and a mean lifetime of 2.6 × 10-8 seconds. The pion was once believed to be the particle that mediates the strong force, which holds nucleons together in the nucleus; it is now believed that the gluon is the mediator particle. Pions do interact with nucleons, however, and are able to transform neutrons into protons and vice versa. Also called pi-meson. See Table at subatomic particle.