simple definition of the fiducial phase space so that it can be easily reproduced. Furthermore, the selection criteria in data, described in Section \[sec:selection\], are necessarily defined at the reconstruction level, while the fiducial phase space, for which theoretical predictions are computed, is defined without considering the effect of the detector response on the generator-level quantities. Because of the finite detector resolution, the two definitions do not exactly coincide, and for this reason events fulfilling the event selection criteria at the reconstruction level can originate from either inside or outside the fiducial phase space. To minimize the effect of events migrating, the selection criteria at the reconstruction level and the definition of this phase space are aligned as closely as possible.

The fiducial phase space for the analysis is defined by requiring that the generator-level ratio between the of the -leading (-subleading) photon and [$m_{\gamma\gamma}$]{}, $\pt^{\gamma_1}/{\ensuremath{m_{\gamma\gamma}}\xspace}$ ($\pt^{\gamma_2}/{\ensuremath{m_{\gamma\gamma}}\xspace}$), be greater than 1/3 (1/4), and that the absolute pseudorapidity of both photons be less than 2.5.

In addition, the sum of the generator-level transverse energy of stable particles in a cone of radius ${\ensuremath{\Delta R}\xspace}=0.3$ around each photon candidate, $\text{Iso}_{\text{gen}}^{\gamma}$, is required to be less than 10. This requirement mimics at generator 