  Excitons and trions (or exciton-polarons) in transition metal dichalcogenides
(TMDs) are known to decay predominantly through intravalley transitions.
Electron-hole recombination across different valleys can also play a
significant role in the excitonic dynamics, but intervalley transitions are
rarely observed in monolayer TMDs, because they violate the conservation of
momentum. Here we reveal the intervalley recombination of dark excitons and
trions through more than one path in monolayer WSe$_2$. We observe the
intervalley dark excitons, which can recombine by the assistance of defect
scattering or chiral-phonon emission. We also reveal that a trion can decay in
two distinct paths - through intravalley or intervalley electron-hole
recombination - into two different final valley states. Although these two
paths are energy degenerate, we can distinguish them by lifting the valley
degeneracy under a magnetic field. In addition, the intra- and inter-valley
trion transitions are coupled to zone-center and zone-corner chiral phonons,
respectively, to produce distinct phonon replicas. The observed multipath
optical decays of dark excitons and trions provide much insight into the
internal quantum structure of trions and the complex excitonic interactions
with defects and chiral phonons in monolayer valley semiconductors.
