\ Inclusive-cluster \sigmalongPb\ distribution (basic cluster selection plus track--cluster association rejection for \PbPb collisions, see Sect. 3.2 of Ref. ) as a function of \pt\ in data for \pp (lower right frame) and \PbPb collisions for five different centrality classes. The dotted line corresponds to the tight value of the upper selection limit for single photon candidate clusters (narrow clusters) at $\sigma_{\rm max}^{2} = 0.3$. The dashed line corresponds to a looser photon upper selection used in \PbPb collisions below 18 \GeVc at $\sigma_{\rm max}^{2}(\pt)=0.6-0.016\times \pt$. The dotted-dashed line corresponds to the narrow cluster's lower limit at $\sigma_{\rm min}^{2} = 0.1$.