We study the process e+ e- →(D* D*)± π∓ at a center-of-mass energy of 4.26 GeV using a 827  pb(-1) data sample obtained with the BESIII detector at the Beijing Electron Positron Collider. Based on a partial reconstruction technique, the Born cross section is measured to be (137±9±15)  pb. We observe a structure near the (D* D*)± threshold in the π∓ recoil mass spectrum, which we denote as the Zc±(4025). The measured mass and width of the structure are (4026.3±2.6±3.7)  MeV/c2 and (24.8±5.6±7.7)  MeV, respectively. Its production ratio σ(e+ e- → Zc±(4025)π∓ → (D* D*)± π∓)/σ(e+ e- → (D* D*)± π∓) is determined to be 0.65±0.09±0.06. The first uncertainties are statistical and the second are systematic.

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Using 2.92 fb −1 of electron-positron annihilation data collected at √ s = 3.773 GeV with the BESIII detector, we obtain the first measurements of the absolute branching fractionL e + νe differential decay rate distribution, the product of the hadronic form factor and the magnitude of the CKM matrix element, f K + (0)|Vcs|, is determined 3 to be 0.728 ± 0.006(stat.) ± 0.011(sys.). Using |Vcs| from the SM constrained fit with the measured f K + (0)|Vcs|, f K + (0) = 0.748 ± 0.007(stat.) ± 0.012(sys.) is obtained, and utilizing the unquenched LQCD calculation for f K + (0), |Vcs| = 0.975 ± 0.008(stat.) ± 0.015(sys.) ± 0.025(LQCD).

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