Observation of the B0s → D∗+D∗− decay

The first observation of the B0s → 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(B0s → 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 B0s → D∗+D∗− branching fraction is calculated to be
B(B0s → 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 B0s meson lifetime in simulation. Correction factors are reported for scenarios where either a purely heavy or a purely light B0s eigenstate is considered.