Observation of the leptonic decay $D^+ \to \tau^+ \nu_\tau$

TL;DR

This paper reports the first observation of the leptonic decay $D^+ o au^+ u_ au$, measuring its branching fraction and testing lepton flavor universality, with results consistent with the Standard Model.

Contribution

It provides the first experimental evidence of $D^+ o au^+ u_ au$ decay and measures its branching fraction, testing lepton flavor universality in charm meson decays.

Findings

First observation of $D^+ o au^+ u_ au$ with 5.1 sigma significance.

Measured branching fraction ${ m B}(D^+ o au^+ u_ au) = (1.20 ext{±}0.24_{ ext{stat}} ext{±}0.12_{ ext{syst}}) imes 10^{-3}$.

Result consistent with Standard Model expectations.

Abstract

We report the first observation of $D^+\to\tau^+\nu_\tau$ with a significance of $5.1\sigma$. We measure ${\cal B}(D^+\to\tau^+\nu_\tau) = (1.20\pm0.24_{\text{stat.}}\pm0.12_{\text{syst.}})\times10^{-3}$. Taking the world average ${\cal B}(D^+\to\mu^+\nu_\mu) = (3.74\pm0.17)\times10^{-4}$, we obtain $R_{\tau/\mu} =\Gamma(D^+\to\tau^+\nu_\tau)/\Gamma(D^+\to\mu^+\nu_\mu) = 3.21\pm0.64_{\text{stat.}}\pm0.43_{\text{syst.}}$, 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.