\small { (a) Cluster shower shape distribution \sigmalong\ as a function of the cluster \pt\ in data. The dashed lines correspond to the upper and lower selection limit values for single photon candidate clusters used in the analysis. (b) Projected shower shape distribution in two \pt\ intervals in data (black circles) and simulation (red crosses, PYTHIA~8 jet--jet and $\gamma$--jet events). For the simulation, the \sigmalong\ distribution for clusters with different particle origins are also shown as shaded areas except for prompt photons from $\gamma$--jet PYTHIA~8 processes that are presented with a blue histogram line. }.