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In particle physics, a pion (short for pi meson, denoted with π) is any of three subatomic particles: π0
, π+
, and π
. Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force.


Basic properties

Pions are bosons with zero spin, and they are composed of first-generation quarks. In the quark model, an "up quark" and an anti-"down quark" make up a π+
, whereas a "down quark" and an anti-"up quark" make up the π
, and these are the antiparticles of one another. The uncharged pions are combinations of an "up quark" with an anti-"up quark" or a "down quark" with an anti-"down quark", have identical quantum numbers, and hence they are only found in superpositions. The lowest-energy superposition of these is the π0
, which is its own antiparticle. Together, the pions form a triplet of isospin. Each pion has isospin (I = 1) and third-component isospin equal to its charge (Iz = +1, 0 or −1).

Charged pion decays

The π±
mesons have a mass of 139.6 MeV/c2 and a mean lifetime of 2.6×10−8
. They decay due to the weak interaction. The primary decay mode of a pion, with probability 0.999877, is a purely leptonic decay into a muon and a muon neutrino:

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