The existence of the pion was predicted in 1935 by Hideki Yukawa, who theorized that it was responsible for the force of the strong interactions holding the atomic nucleus together. It was first detected in cosmic rays by C. F. Powell in 1947. The pion is actually a multiplet of three particles. The neutral pion, pi0, has a mass about 264 times that of the electron. The neutral pion is its own antiparticle , while the negative pion is the antiparticle of the positive pion.
Free pions are unstable.