Direct measurement of the ${\mathit{D}}_{\mathit{s}}$ branching fraction to φπ

The Beijing Spectrometer (BES) Collaboration has observed exclusive pair production of ${\mathit{D}}_{\mathit{s}}$ mesons at the Beijing Electron-Positron Collider (BEPC) at a center-of-mass energy of 4.03 GeV. The ${\mathit{D}}_{\mathit{s}}$ mesons are detected in the φ${\mathrm{\pi }}^{+}$, K¯ ${}_{}{}^{\mathrm{*}0}{}_{}{}^{}$${\mathit{K}}^{+}$, and K¯ ${}_{}{}^0{}_{}{}^{}$${\mathit{K}}^{+}$ decay modes; two fully reconstructed events yield the value (3.$9_{\mathrm{-}1.9\mathrm{-}1.1}^{+5.1+1.8\mathrm{}}$)% for the ${\mathit{D}}_{\mathit{s}}$ branching fraction to φπ. This is the first direct, model-independent measurement of this quantity.