  Many puzzling properties of high-$T_c$ superconducting (HTSC) copper oxides
have deep roots in the nature of the antinodal quasiparticles, the elementary
excitations with wavevector parallel to the Cu-O bonds. These electronic states
are most affected by the onset of antiferromagnetic correlations and charge
instabilities and they host the maximum of the anisotropic superconducting gap
and pseudogap. In this work, we use time-resolved extreme-ultra-violet (EUV)
photoemission with proper photon energy (18 eV) and time-resolution (50 fs) to
disclose the ultrafast dynamics of the antinodal states in a prototypical HTSC
cuprate. After photoinducing a non-thermal charge redistribution within the Cu
and O orbitals, we reveal a dramatic momentum-space differentiation of the
transient electron dynamics. While the nodal quasi-particle distribution is
heated up as in a conventional metal, new quasiparticle states transiently
emerge at the antinodes, similarly to what is expected for a photoexcited Mott
insulator, where the frozen charges can be released by an impulsive excitation.
This transient antinodal metallicity is mapped into the dynamics of the O-2$p$
bands thus directly demonstrating the intertwining between the low- and
high-energy scales that is typical of correlated materials. Our results suggest
that the correlation-driven freezing of the electrons moving along the Cu-O
bonds, analogous to the Mott localization mechanism, constitutes the starting
point for any model of high-$T_c$ superconductivity and other exotic phases of
HTSC cuprates.
