  Using a pQCD picture for jet evolution in a dense QCD medium, in which
medium-induced parton branchings are factorized from vacuum-like emissions, we
study two jet substructure observables: the $z_g$ and the Soft Drop
multiplicity distributions. We compute the respective nuclear modification
factors using a Monte-Carlo implementation of the parton showers. Our results
are in qualitative agreement with LHC data for Pb+Pb collisions. We identify
the physical mechanisms explaining our results: incoherent jet energy loss,
semi-hard medium-induced emissions and the bias introduced by the steeply
falling jet spectrum.
