First Experimental Study of the Purely Leptonic Decay $D_s^{\ast +}\to e^{+}{\nu }_e$

Using $7.33{\mathrm{f}\mathrm{b}}^{-1}$ of $e^{+}{e}^{-}$ collision data taken with the BESIII detector at the BEPCII collider, we report the first experimental study of the purely leptonic decay $D_s^{\ast +}\to e^{+}{\nu }_e$. A signal for the decay $D_s^{\ast +}\to e^{+}{\nu }_e$ is observed with a statistical significance of $2.9\sigma$. The branching fraction of $D_s^{\ast +}\to e^{+}{\nu }_e$ is measured to be $(2.1{{}_{-0.9}^{+1.2}}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.2}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\times {10}^{-5}$, corresponding to an upper limit of $4.0\times {10}^{-5}$ at the 90% confidence level. Taking the total width of the $D_s^{\ast +}$~(($0.070\pm 0.028$) keV) predicted by lattice quantum chromodynamics as input, the decay constant of the $D_s^{\ast +}$ is determined to be $f_{D_s^{\ast +}}=(213.6{{}_{-45.8}^{+61.0}}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {43.9}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})$ MeV, corresponding to an upper limit of 353.8 MeV at the 90% confidence level.