On the challenge of estimating diphoton backgrounds at large invariant mass J. F. Kamenik, G. Perez, M. Schlaffer and A. Weiler Eur. Phys. J. C, 77 2 (2017) 126 Published online: 24 February 2017 DOI: 10.1140/epjc/s10052-017-4687-y