Pion - Basic Properties - Charged Pion Decays

Charged Pion Decays

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

π+	→	μ+	+	ν μ
π−	→	μ−	+	ν μ

The second most common decay mode of a pion, with probability 0.000123, is also a leptonic decay into an electron and the corresponding electron antineutrino. This mode was discovered at CERN in 1958:

π+	→	e+	+	ν e
π−	→	e−	+	ν e

The suppression of the electronic mode, with respect to the muonic one, is given approximately (to within radiative corrections) by the ratio of the half-widths of the pion–electron and the pion–muon decay reactions:

and is a spin effect known as the helicity suppression. Measurements of the above ratio have been considered for decades to be tests of the V − A structure (vector minus axial vector or left-handed lagrangian) of the charged weak current and of lepton universality. Experimentally this ratio is 1.230(4)×10−4.

Besides the purely leptonic decays of pions, also observed have been some structure-dependent radiative leptonic decays (that is, decay to the usual leptons plus a gamma ray).

Also observed, for charged pions only, is the very rare "pion beta decay" (with probability of about 10−8) into a neutral pion plus an electron and electron antineutrino (or for positive pions, a neutral pion, positron, and electron neutrino).

π−	→	π0	+	e−	+	ν e
π+	→	π0	+	e+	+	ν e