Adding vacuum branching to jet evolution in a dense medium

We study the fragmentation of a jet propagating in a dense quark-gluon plasma. We show that the “vacuum-like” emissions triggered by the parton virtualities can be factorized from the medium-induced radiation responsible for the energy loss within a controlled, “double-logarithmic”, approximation in perturbative QCD. We show that the collisions with the plasma constituents modify the vacuum-like parton shower already at leading twist, in two ways: the radiation phase-space is reduced and the first emission outside the medium can violate angular ordering. We compute the jet fragmentation function and find results in qualitative agreement with measurements at the LHC.