Phys. Rev. Lett. 132, 091802 (2024) - Observation of ${D}_{s}^{+}\ensuremath{\rightarrow}{\ensuremath{\eta}}^{\ensuremath{'}}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}}$, Precision Test of Lepton Flavor Universality with ${D}_{s}^{+}\ensuremath{\rightarrow}{\ensuremath{\eta}}^{(\ensuremath{'})}{l}^{+}{\ensuremath{\nu}}_{l}$, and First Measurements of ${D}_{s}^{+}\ensuremath{\rightarrow}{\ensuremath{\eta}}^{(\ensuremath{'})}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}

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Observation of $D_{s}^{+} \to η^{'} μ^{+} ν_{μ}$ , Precision Test of Lepton Flavor Universality with $D_{s}^{+} \to η^{(')} l^{+} ν_{l}$ , and First Measurements of $D_{s}^{+} \to η^{(')} μ^{+} ν_{μ}$ Decay Dynamics

M. Ablikim et al. (BESIII Collaboration)

Phys. Rev. Lett. 132, 091802 – Published 28 February 2024

Abstract

By analyzing $7.33 {fb}^{- 1}$ of $e^{+} e^{-}$ annihilation data collected at center-of-mass energies between 4.128 and 4.226 GeV with the BESIII detector, we report the observation of the semileptonic decay $D_{s}^{+} \to η^{'} μ^{+} ν_{μ}$ , with a statistical significance larger than $10 σ$ , and the measurements of the $D_{s}^{+} \to η μ^{+} ν_{μ}$ and $D_{s}^{+} \to η^{'} μ^{+} ν_{μ}$ decay dynamics for the first time. The branching fractions of $D_{s}^{+} \to η μ^{+} ν_{μ}$ and $D_{s}^{+} \to η^{'} μ^{+} ν_{μ}$ are determined to be $(2.235 \pm 0.05 1_{stat} \pm 0.05 2_{syst}) %$ and $(0.801 \pm 0.05 5_{stat} \pm 0.02 8_{syst}) %$ , respectively, with precision improved by factors of 6.0 and 6.6 compared to the previous best measurements. Combined with the results for the decays $D_{s}^{+} \to η e^{+} ν_{e}$ and $D_{s}^{+} \to η^{'} e^{+} ν_{e}$ , the ratios of the decay widths are examined both inclusively and in several $ℓ^{+} ν_{ℓ}$ four-momentum transfer ranges. No evidence for lepton flavor universality violation is found within the current statistics. The products of the hadronic form factors $f_{+, 0}^{η^{(')}} (0)$ and the $c \to s$ Cabibbo-Kobayashi-Maskawa matrix element $| V_{c s} |$ are determined. The results based on the two-parameter series expansion are $f_{+, 0}^{η} (0) | V_{c s} | = 0.452 \pm 0.01 0_{stat} \pm 0.00 7_{syst}$ and $f_{+, 0}^{η^{'}} (0) | V_{c s} | = 0.504 \pm 0.03 7_{stat} \pm 0.01 2_{syst}$ , which help to constrain present models on $f_{+, 0}^{η^{(')}} (0)$ . The forward-backward asymmetries are determined to be $⟨ A_{FB}^{η} ⟩ = - 0.059 \pm 0.03 1_{stat} \pm 0.00 5_{syst}$ and $⟨ A_{FB}^{η^{'}} ⟩ = - 0.064 \pm 0.07 9_{stat} \pm 0.00 6_{syst}$ for the first time, which are consistent with the theoretical calculation.

Received 25 July 2023
Revised 25 January 2024
Accepted 26 January 2024

DOI:https://doi.org/10.1103/PhysRevLett.132.091802

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Published by the American Physical Society