  The dynamics of exciton formation in transition metal dichalcogenides is
difficult to measure experimentally, since many momentum-indirect exciton
states are not accessible to optical interband spectroscopy. Here, we combine a
tuneable pump, high-harmonic probe laser source with a 3D momentum imaging
technique to map photoemitted electrons from monolayer WS$_2$. This provides
momentum-, energy- and time-resolved access to excited states on an ultrafast
timescale. The high temporal resolution of the setup allows us to trace the
early-stage exciton dynamics on its intrinsic timescale and observe the
formation of a momentum-forbidden dark K$\Sigma$ exciton a few tens of
femtoseconds after optical excitation. By tuning the excitation energy we
manipulate the temporal evolution of the coherent excitonic polarization and
observe its influence on the dark exciton formation. The experimental results
are in excellent agreement with a fully microscopic theory, resolving the
temporal and spectral dynamics of bright and dark excitons in WS$_2$.
