Griffiths Introduction to Elementary Particles · Problem 4.37

Bound States — David J. Griffiths

Physics — QFT · eta-meson-decays-symmetry

Problem

The dominant decays of the

\eta

meson are

\eta \to 2\gamma\ (39\%), \quad \eta \to 3\pi\ (56\%), \quad \eta \to \pi\pi\gamma\ (5\%)

and it's classified as a ``stable'' particle, so evidently none of these is a purely strong interaction. Offhand, this seems odd, since at 549 MeV/

c^2

the

\eta

has plenty of mass to decay strongly into

2\pi

3\pi

. (a) Explain why the

2\pi

mode is forbidden, for both strong and electromagnetic interactions. (b) Explain why the

3\pi

mode is forbidden as a strong interaction, but allowed as an electromagnetic decay.