Observation of the B_s⁰ → D^*+D^*− decay

The first observation of the B_s⁰ → D^∗+D^∗− decay and the measurement of its branching ratio relative to the B⁰ → 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(B_s⁰→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 f_s/f_d. The B_s⁰ → D^*+D^*− branching fraction is calculated to be

B(B_s⁰→D^∗+D^∗−) = (2.15±0.26±0.09±0.06±0.16)×10⁻⁴, where the fourth uncertainty is due to the B⁰ → D^*+ D^*− branching fraction. These results are calculated using the average B_s⁰ meson lifetime in simulation. Correction factors are reported for scenarios where either a purely heavy or a purely light B_s⁰ eigenstate is considered.