Landau-Yang theorem
= Landau-Yang theorem
{c}
{wiki=Landau–Yang_theorem}
A massive <spin>-one state cannot decay into two identical on-shell <photons>. In their <centre-of-momentum frame>, opposite <photon helicities> have helicity difference two and require $J\geq2$. Equal helicities have difference zero; exchanging the two identical <bosons> multiplies their total-<angular momentum> wave by $(-1)^J$, so <Bose-Einstein statistics> requires even $J$. Neither case permits $J=1$. The on-shell and identical-particle assumptions are essential; the theorem does not exclude a decay with one virtual <photon>.