The BESIII collaboration has reported the observation of D⁺→η^' μ⁺ν_μ and first experimental study of D⁺→η^' l⁺ν_l decay dynamics. These results have been published in Physical Review Letters on 18 March 2025. [Phys. Rev. Lett. 134, 111801 (2025)].
Semileptonic D decays can provide a unique insight for understanding the decay mechanisms of charm quark. In the Standard Model, the semileptonic D decays involve the strong and weak interaction, which can be parameterized by the hadronic form factor and the quark mixing-matrix (CKM matrix) element |V_cd(s)|, respectively. By analyzing the differential decay rates of semileptonic D decays, one can determine the product of form factor and |V_cd(s)|. The obtained form factor and |V_cd(s)| are critical to test the (non-)perturbative quantum chromodynamics effective theory and the unitarity of the CKM matrix, respectively. Additionally, semileptonic D decays involve electron or muon. Precision measurement of the branching fractions of semileptonic D decays are important to test μ-e lepton flavor universality in charm sector.
In the past decades, semileptonic D decays into various ground state mesons have been well-explored. However, little experimental information of D⁺→η^' e⁺ν_e exists and no experimental study of D⁺→η^' μ⁺ν_μ has been reported. Based on 20.3 fb^-1 of e⁺e^- annihilation data collected at center-of-mass energies 3.773 GeV with the BESIII detector, the first observation of D⁺→η^' μ⁺ν_μ and an improved measurement of D⁺→η^' e⁺ν_e are reported. The branching fractions are measured to be B(D⁺→η^' μ⁺ν_μ)=(1.92±0.28±0.08) ×10^-4 and B(D⁺→η^' e⁺ν_e)=(1.79±0.19±0.07) ×10^-4. The ratio of the two branching fractions is determined to be R_μ/e=1.07±0.19±0.03, which agrees with the theoretical expectation of lepton flavor universality within the Standard Model. By analyzing the dynamic of D⁺→η^' l⁺ν_l, the product of the form factor f₊^η'(0) and the CKM matrix element |V_cd| is determined for the first time, giving f₊^η'(0) |V_cd|=(5.92±0.56±0.13) ×10^-2. Figure 1 exhibit the comparison of the branching fractions and form factors measured in this work and those given by different theoretical predictions. The measured branching fraction and form factor can help to rule out some theoretical models.
 

Fig. 1: Comparison of the branching fractions and form factors measured in this work and those given by different theoretical predictions.

Reference: Phys. Rev. Lett. 134, 111801 (2025)
Journal publication: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.134.111801
DOI: 10.1103/PhysRevLett.134.111801