= Diphoton invariant mass
{title2=$M^2c^4=2E_1E_2(1-\cos\theta)$}
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.
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