  The exceptionally strong Coulomb interaction in semiconducting
transition-metal dichalcogenides (TMDs) gives rise to a rich exciton landscape
consisting of bright and dark exciton states. At elevated densities, excitons
can interact through exciton-exciton annihilation (EEA), an Auger-like
recombination process limiting the efficiency of optoelectronic applications.
Although EEA is a well-known and particularly important process in atomically
thin semiconductors determining exciton lifetimes and affecting transport at
elevated densities, its microscopic origin has remained elusive. In this joint
theory-experiment study combining microscopic and material-specific theory with
time- and temperature-resolved photoluminescence measurements, we demonstrate
the key role of dark intervalley states that are found to dominate the EEA rate
in monolayer WSe$_2$. We reveal an intriguing, characteristic temperature
dependence of Auger scattering in this class of materials with an excellent
agreement between theory and experiment. Our study provides microscopic
insights into the efficiency of technologically relevant Auger scattering
channels within the remarkable exciton landscape of atomically thin
semiconductors.
