Massless Particles (photons)?

The rest mass of the pion should not matter. In the pion's frame of reference, the momentum and energy should be equally divided between the photons -- and the net momentum will be zero. In the laboratory frame, it won't be. If you do a relativistic transformation of coordinates, you should be able to get the indicated result.. But it has been too many years since I did this stuff to be any more specific.