Griffiths Introduction to Elementary Particles · Problem 10.11

Weak Interactions — David J. Griffiths

Physics — QFT · pion-decay-massless-electron-forbidden

Problem

Show that the decay

\pi^- \to e^- + \bar{\nu}_e

would be forbidden if the electron were massless. [Hint: The factor

(1 - \gamma^5)

in the weak vertex factor couples only to left-handed particles.]