Beyond the wake: non-hydrodynamic response of an expanding Quark-gluon plasma

We study the response function which describes the evolution of energy density induced by an initial disturbance for a Bjorken-expanding quark-gluon plasma (QGP). We compare the results from solving linearized Boltzmann equation under the relaxation time approximation with those from viscous hydrodynamics. While in long time limit the response becomes hydrodynamics, the non-hydrodynamic response is important at intermediate times when excitations of wavelength larger than the inverse of the relaxation time $1/\tau_{R}$ are not fully damped. Therefore observables sensitive to the jet-induced medium excitations might be employed to explore the properties of QGP at the "mesoscopic scale".