[UPSC IFoS 2016] Particle Physics (Q265, 20M)

What are leptons? How are they different from other baryons? Define their quantum numbers and identify the forbidden ones in the following scattering or decay processes, justifying your conclusions :

(i) \pi^0 \rightarrow \gamma + \gamma

(ii) \pi^0 \rightarrow \gamma + \gamma \rightarrow (e^+ + e^-) + (\mu^- + \mu^+)

(iii) n \rightarrow p + e^- + \bar{\nu}_\mu

(iv) \pi^+ + p \rightarrow p + p + u_\tau + \bar{\nu}_e

(v) $ u_e + p \rightarrow u_\mu + p + \pi^0$