First Experimental Study of the Purely Leptonic Decay $D_s^{\ast +}\to e^{+}\hat{I}{½}_e$

Abstract

Using $7.33{\mathrm{fb}}^{-1}$ 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 $D_s^{\ast +}\to e^{+}\hat{I}{½}_e$. A signal for the decay $D_s^{\ast +}\to e^{+}\hat{I}{½}_e$ is observed with a statistical significance of $2.9\ddot{I}ƒ$. The branching fraction of $D_s^{\ast +}\to e^{+}\hat{I}{½}_e$ is measured to be $(2.1{{}_{-0.9}^{+1.2}}_{stat.}\pm 0.2_{syst.})\times 10^{-5}$, 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 $f_{D_s^{\ast +}}=(213.6{{}_{-45.8}^{+61.0}}_{stat.}\pm 43.9_{syst.})$ MeV, corresponding to an upper limit of 353.8 MeV at the 90\% confidence level.