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Observation of D_s^+→η^' μ^+ ν_μ, Precision Test of Lepton Flavor Universality with D_s^+→η^((')) l^+ ν_l, and First Measurements of D_s^+→η^((')) μ^+ ν_μ Decay Dynamics

2024-03-05 Author:
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The BESIII collaboration has reported the observation of Ds+→η'μ+νμ, precision test of lepton flavor universality with Ds+→η(') l+ νl, and first measurements of Ds+→η(') μ+ νμ decay dynamics. These results have been published in Physical Review Letters on 28 February 2024. [Phys. Rev. Lett. 132, 091802 (2024)].

Leptons are the fundamental particles in nature. The couplings between the three generations of leptons (electron, muon and tau) and the gauge bosons are equal in the standard model, known as lepton flavor universality. The discovery of any violation of lepton flavor universality would imply the existence of new physics. Therefore, precision test of lepton flavor universality is a major frontier in particle physics. In recent years, evidences of lepton flavor universality violation have been implied in several B decays, however, it is still unknown whether these evidences are real physical effects or experimental bias. Semileptonic charm decays involve electron or muon, which are ideal window to test μ-e lepton flavor universality. Based on 7.33 fb-1 of e+ e- annihilation data collected at center-of-mass energies between 4.128 and 4.226 GeV, the observation of Ds+→η'μ+νμ and the studies of Ds+→η(')μ+νμ dynamics are presented for the first time. The branching fractions of Ds+→ημ+νμ and Ds+→η'μ+νμ are measured to be (2.235±0.051±0.052)% and (0.801±0.055±0.028)%, respectively, with precision improved by about sixfold over the previous best measurements. Combining our branching fractions or partial decay rates of Ds+→η(')μ+νμ with that of Ds+→η(') e+ νe reported in an early BESIII work, we test the μ-e lepton flavor universality with Ds+→η(') l+ νl. Up to the precision of 3.3%, no evidence for lepton flavor universality violation is found, which is the most precise test of lepton flavor universality in the semileptonic D_s^+ decays to date.

By analyzing the dynamics of Ds+→η(')μ+νμ , the products of the f(+,0)η(') (0) and the CKM matrix element |Vcs| are determined to be f(+,0)η(0)|Vcs|=0.452±0.010±0.007 and f+,0η'(0)|Vcs|= 0.504±0.037±0.012. The forward-backward asymmetries are determined to be <AFBη>=-0.059±0.031±0.005 and <AFBη'>=-0.064±0.079±0.006 for the first time, which are consistent with the theoretical calculation. The fits to the partial decay rates of Ds+→η(')μ+νμ, the test of lepton flavor universalities, and the determined A_FB in different q2 intervals are shown in Fig. 1. The obtained hadronic form factors and |Vcs| are critical to test the (non-)perturbative QCD effective theory and the unitarity of the CKM matrix, respectively. The forward-backward asymmetry parameters are important to validate different theoretical calculations and improve the precision of lattice QCD calculations on the hadronic form factors.
Fig. 1: (a,b) Fits to the partial decay rates. (c,d) The ratios of the partial decay rates of semimuonic decays over their semielectronic counterparts. (e,f) The measured AFB and theoretical predications.