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In physics, pion is a shortened form of the name Pi meson; this subatomic particle comes in three forms: the pi-zero (π0), pi-plus (π+) and pi-minus (π-). Pi mesons are the lightest mesons.

Pi mesons have zero spin and are composed of first generation quarks. An up and an anti-down quark compose a pi-plus (π+), while a down and an anti-up quark compose the pi-minus (π-), its antiparticle. Combinations of up and anti-up, or down and anti-down, would both be neutral, but because they have the same quantum numbers are only found in superpositions. The lowest energy superposition is the pi-zero, and it is its own antiparticle.

The pi-plus (π+) and pi-minus (π-) may combine to form an exotic atom called pionium, if they are created close to each other with a small relative momentum.

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