  One of the most successful paradigms of many-body physics is the concept of
quasiparticles: excitations in strongly interacting matter behaving like weakly
interacting particles in free space. Quasiparticles in metals are very robust
objects. Yet, when a system's ground state undergoes a qualitative change at a
quantum critical point (QCP), the quasiparticles may disintegrate and give way
to an exotic quantum-fluid state of matter. The nature of this breakdown is
intensely debated, because the emergent quantum fluid dominates the material
properties up to high temperature and might even be related to the occurence of
superconductivity in some compounds. Here we trace the dynamics of
heavy-fermion quasiparticles in CeCu$_{6-x}$Au$_{x}$ and monitor their
evolution towards the QCP in time-resolved experiments, supported by many-body
calculations. A terahertz pulse disrupts the many-body heavy-fermion state.
Under emission of a delayed, phase-coherent terahertz reflex the heavy-fermion
state recovers, with a coherence time 100 times longer than typically
associated with correlated metals. The quasiparticle weight collapses towards
the QCP, yet its formation temperature remains constant -- phenomena believed
to be mutually exclusive. Coexistence in the same experiment calls for
revisions in our view on quantum criticality.
