  CoSn is a prototypical kagome compound showing lattice-born flat bands with
suppressed bandwidth over large parts of the Brillouin zone. Here, by means of
time- and angle-resolved photoelectron spectroscopy, we provide direct evidence
of the response to photoexcitation of the flat bands, that underlie information
about localization in real space. In particular, we detect a sudden shift and
broadening of the flat bands, while after one picosecond only a broadening
survives. We ascribe both these effects to an ultrafast disruption of electron
localization, which renormalizes the effective electron-electron interaction
and affects the flat band dispersion. Since both variations are in the order of
a few meV, our measurements suggest that the flat bands are resilient to
near-infrared photoexcitation.
