  Coupling between exciton states across the Brillouin zone in monolayer
transition metal dichalcogenides can lead to ultrafast valley depolarization.
Using time- and angle-resolved photoemission, we present momentum- and
energy-resolved measurements of exciton coupling in monolayer WS$_2$. By
comparing full 4D ($k_x, k_y, E, t$) data sets after both linearly and
circularly polarized excitation, we are able to disentangle intervalley and
intravalley exciton coupling dynamics. Recording in the exciton binding energy
basis instead of excitation energy, we observe strong mixing between the
B$_{1s}$ exciton and A$_{n>1}$ states. The photoelectron energy and momentum
distributions observed from excitons populated via intervalley coupling (e.g.
K$^-$ $\rightarrow$ K$^+$) indicate that the dominant valley depolarization
mechanism conserves the exciton binding energy and center-of-mass momentum,
consistent with intervalley Coulomb exchange. On longer timescales, exciton
relaxation is accompanied by contraction of the momentum space distribution.
