Observation of the leptonic decay $D^{+}\to {\tau }^{+}{\nu }_{\tau }$

Abstract

We report the first observation of $D^{+}\to {\tau }^{+}{\nu }_{\tau }$ with a significance of $5.1\sigma$. We measure $\mathcal{B}(D^{+}\to {\tau }^{+}{\nu }_{\tau })=(1.20\pm 0.24_{\text{stat.}}\pm 0.12_{\text{syst.}})\times 10^{-3}$. Taking the world average $\mathcal{B}(D^{+}\to {\mu }^{+}{\nu }_{\mu })=(3.74\pm 0.17)\times 10^{-4}$, we obtain $R_{\tau /\mu }=\Gamma (D^{+}\to {\tau }^{+}{\nu }_{\tau })/\Gamma (D^{+}\to {\mu }^{+}{\nu }_{\mu })=3.21\pm 0.64_{\text{stat.}}\pm 0.43_{\text{syst.}}$, 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.

8 pages, 3 figures, as published

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