Khachatryan, V., Sirunyan, A.M., Tumasyan, A., Gülmez, E., Kaya, M., Kaya, O., Yetkin, E.A., Yetkin, T., "Observation of Gamma(1S) Pair Production in Proton-Proton Collisions at root s=8 TeV", Journal of High Energy Physics, 5, 2017.