 Diphoton invariant mass

Adding two null photon four-momenta gives the displayed invariant mass, with opening angle $\theta$ measured in the same frame as the energies. A nonzero mass of the pair does not give either photon a rest mass. At fixed energies it vanishes for parallel photons and is greatest for opposite directions. A reconstructed mass peak must be assessed with background, resolution and identification controls before assigning a parent particle.