Due to the larger v g of the UP wavepacket, the polariton wavepacket expands rapidly in early time, contributing to a sharp rise in the total transient MSD (Fig ). As the UP expands, it also transfers populations to the dark exciton states (at the location of x n which UP is visiting), but with a slower transfer speed compared to v g . Consequently, the UP wavepacket (blue curve in Fig. ) propagates more rapidly than the dark state wavepacket (black curve in Fig. ). Note that the dark exciton in our model has v g = 0 from the derivative of the dispersion curve. The effective propagation of the dark exciton wavepacket is only due to the transfer of population from UP which propagates ballistically in space at an early time. After t = 108 fs, the UP wavepacket is further spread out and its amplitude also starts to decrease due to cavity loss, resulting in a decrease in its magnitude, as well as its contribution to the transient MSD. In addition, the interference term for UP excitation is constructive, further adding to the contribution of transient MSD at an early time. This constructive interference is fragile to decoherence and eventually disappears after a long time. Thus, a faster v g in the UP band, a relatively slow UP to dark state transition rate, and constructive interference between exciton and pho-ton wavepacket, together explain the MSD behavior as shown in Fig. , where the MSD initially increases to a peak value before declining.