  We report a theoretical investigation of the ultrafast dynamics of electrons
and phonons in strained monolayer WS$_2$ following photoexcitation. We show
that strain substantially modifies the phase space for electron-phonon
scattering, unlocking new relaxation pathways that are unavailable in the
pristine monolayer. In particular, strain triggers a transition between
distinct dynamical regimes of the non-equilibrium lattice dynamics
characterized by the emission of chiral phonons under high strain and
linearly-polarized phonons under low strain. For valley-polarized electronic
excitations, this mechanism can be exploited to selectively activate the
emission of chiral phonons -- phonons carrying a net angular momentum. Our
simulations are based on state-of-the-art ab-initio methods and focus
exclusively on realistic excitation and strain conditions that have already
been achieved in recent experimental studies. Overall, strain emerges as a
powerful tool for controlling chiral phonons emission and relaxation pathways
in multivalley quantum materials.
