PT Journal AU LHCb Collaboration (Aaij, Rea Jaimes Elles, SJ Jashal, BK Martinez Vidal, F Oyanguren, A Rebollo De Miguel, M Remon Alepuz, C Ruiz Vidal, J Sanderswood, I TI First observation of the B+→D+sD−sK+ decay SO Physical Review D JI Phys. Rev. D PY 2023 BP 034012 EP 14pp VL 108 DI 10.1103/PhysRevD.108.034012 AB The B+→D+sD−sK+ decay is observed for the first time using proton-proton collision data collected by the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9  fb−1. Its branching fraction relative to that of the B+→D+D−K+ decay is measured to be B(B+→D+sD−sK+)B(B+→D+D−K+)=0.525±0.033±0.027±0.034, where the first uncertainty is statistical, the second systematic, and the third is due to the uncertainties on the branching fractions of the D±s→K∓K±π± and D±→K∓π±π± decays. This measurement fills an experimental gap in the knowledge of the family of Cabibbo-favored ¯b→¯cc¯s transitions and opens the path for unique studies of spectroscopy in future. ER