  Besides its fundamental interest, understanding the dynamics of pair breaking
in superconducting nanostructures is a central issue to optimize the
performances of superconducting devices such as qubits or photon detectors.
However, despite substantial research efforts, these dynamics are still not
well understood as this requires experiments in which quasiparticles are
injected in a controlled fashion. Until now, such experiments have employed
solid-state tunnel junctions with a fixed tunnel barrier. Here we use instead a
cryogenic scanning tunnelling microscope to tune independently the energy and
the rate of quasiparticle injection through, respectively, the bias voltage and
the tunnelling current. For high energy quasiparticles, we observe the
reduction of the critical current of a nanowire and show it is mainly
controlled by the injected power and, marginally, by the injection rate. Our
results prove a thermal mechanism for the reduction of the critical current and
unveil the rapid dynamics of the generated hot spot.
