  Elucidating the carrier density at which strongly bound excitons dissociate
into a plasma of uncorrelated electron-hole pairs is a central topic in the
many-body physics of semiconductors. However, there is a lack of information on
the high-density response of excitons absorbing in the near-to-mid ultraviolet,
due to the absence of suitable experimental probes in this elusive spectral
range. Here, we present a unique combination of many-body perturbation theory
and state-of-the-art ultrafast broadband ultraviolet spectroscopy to unveil the
interplay between the ultraviolet-absorbing two-dimensional excitons of anatase
TiO$_2$ and a sea of electron-hole pairs. We discover that the critical density
for the exciton Mott transition in this material is the highest ever reported
in semiconductors. These results deepen our knowledge of the exciton Mott
transition and pave the route toward the investigation of the exciton phase
diagram in a variety of wide-gap insulators.
