Abstract: | We report the first observation of $D^+ o au^+
u_ au$ with a significance
of $5.1sigma$. We measure ${cal B}(D^+ o au^+
u_ au) =
(1.20pm0.24_{ ext{stat.}}pm0.12_{ ext{syst.}}) imes10^{-3}$. Taking the
world average ${cal B}(D^+ omu^+
u_mu) = (3.74pm0.17) imes10^{-4}$, we
obtain $R_{ au/mu} =Gamma(D^+ o au^+
u_ au)/Gamma(D^+ omu^+
u_mu) =
3.21pm0.64_{ ext{stat.}}pm0.43_{ ext{syst.}}$, which is consistent with the
Standard Model expectation of lepton flavor universality. Using external
inputs, our results give values for the $D^+$ decay constant $f_{D^+}$ and the
CKM matrix element $|V_{cd}|$ that are consistent with, but less precise than,
other determinations. |