Improved Measurement of $\mathcal{B}(D^{+}\to {\mu }^{+}\nu )$ and the Pseudoscalar Decay Constant $f_{D^{+}}$

We extract a relatively precise value for the decay constant of the $D^{+}$ meson by measuring $\mathcal{B}(D^{+}\to {\mu }^{+}\nu )=(4.40\pm {0.66}_{-0.12}^{+0.09})\times {10}^{-4}$ using $281{\mathrm{pb}}^{-1}$ of data taken on the $\psi (3770)$ resonance with the CLEO-$c$ detector. We find $f_{D^{+}}=(222.6\pm {16.7}_{-3.4}^{+2.8})\mathrm{MeV}$, and compare with current theoretical calculations. We also set a 90% confidence upper limit on $\mathcal{B}(D^{+}\to e^{+}\nu )<2.4\times {10}^{-5}$ which constrains new physics models.

Figure 1

Beam-constrained mass for the sum of fully reconstructed $D^{-}$ decay candidates. The solid curve shows the fit to the sum of signal and background functions, while the dashed curve indicates the background.

Figure 2

${\mathrm{MM}}^2$ using $D^{-}$ tags and one additional opposite sign charged track and no extra energetic clusters (see text). The insert shows the signal region for $D^{+}\to {\mu }^{+}\nu$ enlarged; the defined signal region is between the two arrows.