  We measured the lifetime of optically created valley polarization in single
layer WS2 using transient absorption spectroscopy. The electron valley
relaxation is very short (< 1ps). However the hole valley lifetime is at least
two orders of magnitude longer and exhibits a temperature dependence that
cannot be explained by single carrier spin/valley relaxation mechanisms. Our
theoretical analysis suggests that a collective contribution of two potential
processes may explain the valley relaxation in single layer WS2. One process
involves direct scattering of excitons from K to K' valleys with a spin
flip-flop interaction. The other mechanism involves scattering through spin
degenerate Gamma valley. This second process is thermally activated with an
Arrhenius behavior due to the energy barrier between Gamma and K valleys.
