the nonadiabatic regime, the ultrafast photoexcitations lead first to a monotonic growth and then to rapid oscillations in the quasiparticle occupations. Correspondingly, as $t \to \infty$, the order parameter amplitude $| \Delta (t ) |$ first decreases monotonically and then approaches the asymptotic value $\Delta_{\infty} <  | \Delta ( t_i ) |$ in an oscillatory fashion with oscillation frequency [@volkov1974; @amin2004; @barankov2004; @Yuz05; @Yuz06], $$\begin{aligned}
\omega_{\Delta_{\infty}} = 2 \Delta_{\infty} / \hbar ;\   \end{aligned}$$ see Figs. \[FigM1\](d) and \[FigM1\](e). In the collisionless limit, i.e., in the absence of relaxation processes, the oscillations in the quasiparticle densities are undamped, whereas the order parameter oscillations show an algebraic $1 / \sqrt{t}$ decay, due to destructive interference among quasiparticle densities with different momenta \[Fig. \[FigM1\](d)\].

Remarkably, we find that this coherent oscillatory dynamics reveals itself in the pump-probe signal through algebraically decaying oscillations as a function of delay time $\delta t$; see Figs. \[FigM1\](a)-(c). These oscillations are most prominent at the frequency $\omega_{\Delta_{\infty}}$ corresponding to twice the asymptotic gap energy, i.e., at $\hbar \omega_{\Delta_{\infty}} = 2 \Delta_{\infty}   =2.4690$ meV, where $\sigma(\delta t,\omega)$ exhibits a sharp edge as a function of $\omega$; see Figs. \[FigM1\](a) and \[FigM1\](b). The delay-time dependence of $\textrm{Re} [ \sigma(\delta t,\omega_0 