decay rho-->eta + pion is forbidden.why?

[nur.alifah]

is it only violation of charge conjugate or its not conserve the energy & momentum.

[Vanadium 50]

It violates G-parity.

C is not a good symmetry for charged particles.

[Norman]

It violates G-parity.

C is not a good symmetry for charged particles.

To expand a bit on this (and please feel free to correct me if I am wrong), G-parity is conserved in strong interactions but not in weak or electromagnetic. So, \rho \rightarrow \pi\pi dominates the decay of a rho since it is a strong decay, but the decay \rho \rightarrow \eta \pi should still be allowed for a weak process, albeit suppressed in comparison to the 2 pion channel due to phase space considerations.

[Vanadium 50]

Yes, but weak decays are suppressed by the couplings. You'll be down 10, 12 maybe 15 orders of magnitude. The small isospin violation in the strong interaction is probably more important.

[nur.alifah]

the strangeness is conserve,
so,is it only the isospin violate make this decay forbidden?

[clem]

It is only the G parity. The decay occurs via the EM interaction, with an intermediate virtual photon.