modeling of the electromagnetic shower shape variables in simulation, which are among the inputs of the regression used to estimate $\sigma_{E}/E$, as described in Section \[sec:reco\]. To minimize the disagreement [@CMS-DP-2017-004], the per-photon resolution estimate is recomputed using as input simulated shower shapes corrected to match those observed in data. A systematic uncertainty of 5% is assigned to the value of $\sigma_{E}/E$ for each photon candidate, to cover the residual discrepancy. Figure \[fig:sigmaMoverM\] shows the comparison between data (dots) and simulation (histogram) for the decorrelated mass resolution estimator [$\sigma_{m}^{D}$]{}, with the impact of the systematic uncertainty in the $\sigma_E/E$ (red band). Events with a value of [$\sigma_{m}^{D}$]{}in the region depicted in gray are discarded from the final analysis.

![Comparison of the decorrelated mass resolution estimator [$\sigma_{m}^{D}$]{}distributions in data and simulation for [$\cPZ\to\Pep\Pem$]{}events where both electrons are reconstructed as photons, passing the selection defined in Section \[sec:selection\]. The impact of the systematic uncertainty in the $\sigma_E/E$ is indicated by the red band. The distributions are shown separately for events with both electrons in the EB (left) and the remainder of the events, , events with at least one photon in the EE (right). Events in the shaded gray region are discarded 