598
10 Particle Physics – II
For the neutral pion there are two vertices of strength
√ α em but the propagator term is absent because photon has zero rest mass. Thus the decay rate
will be proportional to α
2
em . But α w ∼ = α em according to the electro-weak
unified theory. Therefore, the ratio of the decay rates
ω(π
± →μν)
ω(π 0 →2γ )
≈
1
M
4
W
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 process e
− + (Z , A) → e
− +
γ + (Z , A)
(b) The pair production process γ + (Z , A) → e
−
+ e
+
+ (Z , A)
Fig. 10.25(b)
10 Particle Physics – II
For the neutral pion there are two vertices of strength
√ α em but the propagator term is absent because photon has zero rest mass. Thus the decay rate
will be proportional to α
2
em . But α w ∼ = α em according to the electro-weak
unified theory. Therefore, the ratio of the decay rates
ω(π
± →μν)
ω(π 0 →2γ )
≈
1
M
4
W
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 process e
− + (Z , A) → e
− +
γ + (Z , A)
(b) The pair production process γ + (Z , A) → e
−
+ e
+
+ (Z , A)
Fig. 10.25(b)