A particle of mass m decays in flight to give two photons of energies E₁ and E₂. Show that the opening angle θ between the photon directions is given by

cos θ = 1− m²c⁴/2E₁E₂

(Several short-lived particles – for example π⁰(140), η⁰(547) – can decay in this way, and since they have different masses the above relation gives a simple means of identifying them.)