Significant experimental and theoretical activity at the LHC is dedicated to the study of a hot and dense state of nuclear matter, called the quark-gluon plasma, created in head-on heavy-ion collisions. This system exhibits collective properties close to the ones expected of a nearly-perfect fluid with strikingly low values of the shear viscosity to entropy ratio. Measurements of fully reconstructed jets in these collisions allow to study new aspects of this exotic state via its coupling to perturbative degrees of freedom. Recently, the question about “jet quenching” has evolved from dealing solely with inclusive jet properties to focussing on substructure observables that, potentially, have a higher sensitivity to medium interactions. These modifications are, in a wider sense, also testing the dynamics underlying the equilibration mechanisms of the quark-gluon plasma. We discuss how energetic colour charges propagate in a medium and extend the previously known formalism of computing energy-loss of independent partons to treating neighbouring partons within the same jet. This extension accounts for the coherent structure of jet fragmentation and implies subtle modifications of the substructure. As a concrete application, we discuss the expected medium-modification of the jet splitting probability that is measured using the SoftDrop procedure.
