I have an interesting question that I'm not sure how to go about solving. This question has a little general relativity and (maybe) a little QM, but I wasn't sure where to post it. Question: Imagine that a [itex]\pi[/itex]0 meson traveling along the z-axis (velocity v=0.99c, rest mass M) decays into two photons. The angular distribution of the photons is isotropic in the rest frame of the pion. If in the lab frame the [itex]\pi[/itex]0 meson travels with velocity v in the z direction, what is the probability P(θ)dΩ that a photon is emitted into the solid angle dΩ? We also know [itex]\int[/itex]P(θ)dΩ=1. My ideas: I know the Lorentz transformations, so switching between frames is no biggie. I know Ω[itex]\equiv[/itex]A/r2, and I know the differential solid angle. What's confusing to me is P(θ). Do I need to get the particle's wave function, as in P(θ)=ψ2(θ)? How would one do this? Any hints would be helpful.