598 10 Particle Physics – II
For the neutral pion there are two vertices of strength√
αembut the propa-
gator term is absent because photon has zero rest mass. Thus the decay rate
will be proportional toα^2 em.Butαw∼=αemaccording to the electro-weak
unified theory. Therefore, the ratio of the decay rates
ω(π±→μν)
ω(π^0 → 2 γ)≈1
MW^4
which is quite small and so charged pion lifetime (∼ 10 −^8 s) is much larger
than that of neutral pion (∼ 10 −^17 s).10.107 (a)
Fig. 10.25(a)Dominant Feynman diagrams for the Bremsstrahlung processe−+(Z,A)→e−+
γ+(Z,A)
(b) The pair production processγ+(Z,A)→e−+e++(Z,A)Fig. 10.25(b)