can only be done indirectly by exploiting the properties of each reconstructed vertex. Three discriminating variables are calculated for each reconstructed vertex: the sum of the squared transverse momenta of the charged-particle tracks associated with the vertex, and two variables that quantify the vector and scalar balance of between the diphoton system and the charged-particle tracks associated with the vertex. In addition, if either photon has an associated charged-particle track that has been identified as originating from a photon conversion to an electron-positron pair, the conversion information is used. The variables are used as the inputs to a multivariate classifier based on a boosted decision tree (BDT) to choose the reconstructed vertex to be associated with the diphoton system. The average vertex finding efficiency of this algorithm is about 81% [@CMS-PAS-HIG-16-040]. The vertex is considered to be correctly identified if it is within 1 of the true vertex in the longitudinal direction. The contribution to the diphoton mass resolution from vertex displacements smaller than 1 is found to be negligible compared to the contribution from the photon energy resolution of the calorimeters.

A photon identification algorithm separates prompt photons from photon candidates resulting from the misidentification of jet fragments [@Khachatryan:2014ira]. 