Observation of the Bs0 ? D ^*+ D ^*- decay

The first observation of the Bs0 ? D *+ D *- decay and the measurement of its branching ratio relative to the B 0 ? D *+ D *- decay are presented. The data sample used corresponds to an integrated luminosity of 9 fb -1 of proton-proton collisions recorded by the LHCb experiment at centre-of-mass energies of 7, 8 and 13 TeV between 2011 and 2018. The decay is observed with more than 10 standard deviations and the time-integrated ratio of branching fractions is determined to be B(Bs0?D*+D*-)B(B0?D*+D*-)=0.269±0.032±0.011±0.008, where the first uncertainty is statistical, the second systematic and the third due to the uncertainty of the fragmentation fraction ratio fs /fd. The Bs0 ? D *+ D *- branching fraction is calculated to be B(Bs0?D*+D*-)=(2.15±0.26±0.09±0.06±0.16)×10-4, where the fourth uncertainty is due to the B 0 ? D *+ D *- branching fraction. These results are calculated using the average Bs0 meson lifetime in simulation. Correction factors are reported for scenarios where either a purely heavy or a purely light Bs0 eigenstate is considered. [Figure not available: see fulltext.].