Direct Measurement of the Pseudoscalar Decay Constant fD+

Abstract

The absolute branching fraction of $D^{+}\to \hat{I}{¼}^{+}\hat{I}½$ has been directly measured by an analysis of a data sample of about 33 $\mathrm{p}{\mathrm{b}}^{-1}$ collected around $\sqrt{s}=3.773$ GeV with the BES-II at the BEPC. At these energies, $D^{-}$ meson is produced in pair as $e^{+}{e}^{-}\to D^{+}{D}^{-}$. A total of $5321\pm 149\pm 160$ $D^{-}$ mesons are reconstructed from this data set. In the recoil side of the tagged $D^{-}$ mesons, $2.67\pm 1.74$ purely leptonic decay events of $D^{+}\to \hat{I}{¼}^{+}\hat{I}½$ are observed. This yields a branching fraction of $BF(D^{+}\to \hat{I}{¼}^{+}\hat{I}{½}_{\hat{I}}¼)=(0.122_{-0.053}^{+0.111}\pm 0.010)$, and a corresponding pseudoscalar decay constant $f_{D^{+}}=(371_{-119}^{+129}\pm 25)$ MeV.

7 pages, 8 figures, Submitted to Physics Letters B in October, 2004