Definition of pion
: a meson that is a combination of up and down quarks and antiquarks, that may be positive, negative, or neutral, and that has a mass about 270 times that of the electron
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The Xi decays into a particle called the lambda baryon and three lighter particles, the K- (or kaon) and two pions.
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The most famous of these two-quark particles are kaons and pions.
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