  A wave packet undergoes a strong spatial and temporal dispersion while
propagating through a complex medium. This wave scattering is often seen as a
nightmare in wave physics whether it be for focusing, imaging or communication
purposes. Controlling wave propagation through complex systems is thus of
fundamental interest in many areas, ranging from optics or acoustics to medical
imaging or telecommunications. Here, we study the propagation of elastic waves
in a cavity and a disordered waveguide by means of laser interferometry. From
the direct experimental access to the time-delay matrix of these systems, we
demonstrate the existence of particle-like wave packets that remain focused in
time and space throughout their complex trajectory. Due to their limited
dispersion, their selective excitation will be crucially relevant for all
applications involving selective wave focusing and efficient information
transfer through complex media.
