interest are selected using a two-tiered trigger system [@Khachatryan:2016bia]. The first level (L1), composed of custom hardware processors, uses information from the calorimeters and muon detectors to select events at a rate of around 100 within a time interval of less than 4. The second level, known as the high-level trigger (HLT), consists of a farm of processors running a version of the full event reconstruction software optimized for fast processing, and reduces the event rate to around 1 before data storage.

A more detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in Ref. [@Chatrchyan:2008zzk].

Data samples and simulated events {#sec:samples}
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The events used in the analysis were selected by a diphoton trigger with asymmetric thresholds of 30 (18)on the leading (sub-leading) photon, a minimum invariant diphoton mass [$m_{\gamma\gamma}$]{}of 90, and loose requirements on the calorimetric isolation and electromagnetic shower shape of the photon candidates. The trigger selection is $>$99% efficient at retaining events passing the selection requirements described in Section \[sec:selection\].

A detailed simulation of the CMS detector response is based on a model implemented using the  [@Agostinelli:2002hh] package. Simulated events include the 