  We report on the exciton and trion density dynamics in a single layer of
MoSe$_2$, resonantly excited and probed using three-pulse four-wave mixing
(FWM), at temperatures from 300K to 77K . A multi-exponential third-order
response function for amplitude and phase of the heterodyne-detected FWM signal
including four decay processes is used to model the data. We provide a
consistent interpretation within the intrinsic band structure, not requiring
the inclusion of extrinsic effects. We find an exciton radiative lifetime in
the sub-picosecond range consistent to what has been recently reported. After
the dominating radiative decay, the remaining exciton density, which has been
scattered from the initially excited bright radiative state into dark states of
different nature by exciton-phonon scattering or disorder scattering, shows a
slower dynamics, covering 10ps to 10ns timescales. This includes direct bright
transitions with larger in-plane momentum, as well as indirect dark transitions
to indirect dark states. We find that exciton-exciton annihilation is not
relevant in the observed dynamics, in variance from previous finding under
non-resonant excitation. The trion density at 77K reveals a decay of the order
of 1ps, similar to what is observed for the exciton. After few tens of
picoseconds, the trion dynamics resembles the one of the exciton, indicating
that trion ionization occurs on this timescale.
