  As hosts for tightly-bound electron-hole pairs carrying quantized angular
momentum, atomically-thin semiconductors of transition metal dichalcogenides
provide an appealing platform for optically addressing the valley degree of
freedom. In particular, the valleytronic properties of neutral and charged
excitons in these systems have been widely investigated. Meanwhile, correlated
quantum states involving more particles are so far elusive and controversial.
Here we present experimental evidence for valleytronic four-particle biexcitons
and five-particle exciton-trions in high-quality monolayer tungsten diselenide
samples. Through charge doping, thermal activation, and magnetic-field tuning
measurements, we determined that the biexciton and exciton-trion optical
emissions are bound with respect to the bright exciton and the trion
respectively. Further, both the biexciton and the exciton-trion are intervalley
bound states involving dark excitons, giving rise to emissions with large,
negative valley polarizations in contrast to that of the well-known
two-particle excitons. Our studies provide new opportunities for building
valleytronic quantum devices harnessing high-order excitations.
