Observation of the $W$-annihilation decay ${D}_{s}^{+}\to \omega {\pi }^{+}$ and evidence for ${D}_{s}^{+}\to \omega {K}^{+}$

We report the observation of $W$-annihilation decay ${D}_{s}^{+}\to \omega {\pi }^{+}$ and evidence for ${D}_{s}^{+}\to \omega {K}^{+}$ in a data sample corresponding to an integrated luminosity of $3.19\text{}\text{}{\mathrm{fb}}^{-1}$ collected with the BESIII detector at a center-of-mass energy $\sqrt{s}=4.178\text{}\text{}\mathrm{GeV}$. We obtain the branching fractions $B\left({D}_{s}^{+}\to \omega {\pi }^{+}\right)=\phantom{\rule{0ex}{0ex}}\left(1.77±0.3{2}_{\mathrm{stat}}±0.1{3}_{\mathrm{sys}}\right)×{10}^{-3}$ with a significance of $6.7\sigma $ and $B\left({D}_{s}^{+}\to \omega {K}^{+}\right)=\left(0.87±0.2{4}_{\mathrm{stat}}±0.0{8}_{\mathrm{sys}}\right)×{10}^{-3}$ with a significance of $4.4\sigma $. This measurement provides critical information to determine the nonperturbative $W$-annihilation amplitudes and shows the potential of searching for $CP$ asymmetry in ${D}_{s}^{+}\to \omega {K}^{+}$.