Measurement of the $D_s^+ \to \ell^+\nu_\ell$ branching fractions and the decay constant $f_{D_s^+}$

TL;DR

This paper measures the branching fractions of $D_s^+$ decays to muons and taus, and determines the decay constant $f_{D_s^+}$ using data from the BESIII detector, providing important tests of the Standard Model.

Contribution

The study provides the first precise measurements of $D_s^+$ leptonic decay branching fractions and the decay constant $f_{D_s^+}$ using BESIII data, consistent with Standard Model predictions.

Findings

Branching fraction for $D_s^+ o u_ au$ is (4.83 ± 0.65 ± 0.26)%

Branching fraction for $D_s^+ o u_mu$ is (0.495 ± 0.067 ± 0.026)%

Decay constant $f_{D_s^+}$ is (241.0 ± 16.3 ± 6.5) MeV

Abstract

Using 482~pb$^{-1}$ of $e^+e^-$ collision data collected at a center-of-mass energy of $\sqrt{s} = 4.009$ GeV with the BESIII detector, we measure the branching fractions of the decays $D_s^+\to\mu^+\nu_\mu$ and $D_s^+\to\tau^+\nu_\tau$. By constraining the ratio of decay rates of $D_s^+$ to $\tau^+\nu_\tau$ and to $\mu^+\nu_\mu$ to the Standard Model prediction, the branching fractions are determined to be $\mathcal{B}(D_s^+ \to \mu^+\nu_\mu) = (0.495 \pm 0.067 \pm 0.026)\%$ and $\mathcal{B}(D_s^+ \to \tau^+\nu_\tau) = (4.83 \pm 0.65 \pm 0.26)\%$. Using these branching fractions, we obtain a value for the decay constant $f_{D_s^+}$ of $(241.0 \pm 16.3 \pm 6.5)~\text{MeV}$, where the first error is statistical and the second systematic.