  In monolayer transition-metal dichalcogenide semiconductors, many-body
correlations can manifest in optical spectra when photoexcited electron-hole
pairs (excitons) are introduced into a 2D Fermi sea of mobile carriers. At low
carrier densities, the formation of positively and negatively charged excitons
($X^\pm$) is well documented. However, in WSe$_2$ monolayers, an additional
absorption resonance, often called $X^{-\prime}$, emerges at high electron
density. Its origin is not understood. Here we investigate the $X^{-\prime}$
state via polarized absorption spectroscopy of electrostatically-gated WSe$_2$
monolayers in high magnetic fields to 60~T. Field-induced filling and emptying
of the lowest optically-active Landau level in the $K'$ valley causes repeated
quenching of the corresponding optical absorption. Surprisingly, however, these
quenchings are accompanied by absorption changes to higher-lying Landau levels
in both $K'$ and $K$ valleys, which are unoccupied. These results cannot be
reconciled within a single-particle picture, and demonstrate the many-body
nature and inter-valley correlations of the $X^{-\prime}$ quasiparticle state.
