Light-by-light scattering contributions to positronium decay rates

Abstract
We evaluate the contributions of final-state light-by-light scattering to the parapositronium and orthopositronium decay rates. Our results are 1.29392(4) for parapositronium and 0.350(4) for orthopositronium in units of (α/π)2 times the respective lowest-order rates.