energy itself because of the stochastic and noise terms in the energy resolution of the ECAL [@Chatrchyan:2013dga; @CMS:EGM-14-001]. The nature of these two terms is such that the energy resolution improves at higher energy. This dependence is propagated to the relative mass resolution estimator [$\sigma_{m}$]{}, which is thus dependent on the mass of the diphoton pair, with events characterized by a larger diphoton mass more likely to have better mass resolution. An event categorization simply based on such a variable would distort the shape of the mass distribution in the different categories and it would make the background distribution more complex to parametrize. In particular, a deficit of low-mass events would be observed in categories corresponding to low values of [$\sigma_{m}$]{}, invalidating the assumption of a smoothly falling mass distribution on which the background model, described in Section \[sec:bgmodel\], is based. To avoid such an effect, the correlation between [$\sigma_{m}$]{}and the diphoton mass is removed, following the methods in Ref. [@Khachatryan2016], and a new relative mass resolution estimator is built, [$\sigma_{m}^{D}$]{}.

The modeling of the decorrelated mass resolution estimator is studied with simulated [$\cPZ\to\Pep\Pem$]{}events, where electrons are reconstructed as photons. The per-photon resolution estimate $\sigma_{E}/E$ is affected by the imperfect 