  While the modification of jets by a medium as created in ultrarelativistic
heavy-ion collisions is often thought of as a phenomenon affecting a hard near
on-shell parton and hence treated as partonic energy loss, a more realistic
view is to think of the medium effect as a relatively small correction to the
Quantum Chromodynamics (QCD) vacuum radiation pattern which reduces the initial
high virtuality of hard partons and leads to the development of a shower. The
uncertainty relation can be used to argue that a significant part of this
shower is developed even before a medium can be formed. The initial virtuality
of a shower-initiating parton is reflected in the final state in the measured
invariant mass of a jet - high mass jets are characterized by more branchings,
higher multiplicity and a wider angular structure. Since most of this structure
is determined before the medium affects the jet, selecting a particular jet
mass range strongly biases the partonic configuration that enters the medium,
and thus the medium is expected to modify high mass jets more strongly than low
mass jets. In this work, this scenario is explored using the in-medium shower
evolution code YaJEM and a possible strategy for a measurement is suggested.
