Angular analysis of 𝐵0→𝐷∗−𝐷∗+𝑠 with 𝐷∗+𝑠→𝐷+𝑠𝛾 decays
The first full angular analysis of the 𝐵0→𝐷∗−𝐷∗+𝑠B0→D∗−Ds∗+ decay is performed using 6 fb−1 of pp collision data collected with the LHCb experiment at a centre-of-mass energy of 13 TeV. The 𝐷∗+𝑠→𝐷+𝑠𝛾Ds∗+→Ds+γ and D*− → 𝐷⎯⎯⎯⎯⎯0𝜋−D¯0π− vector meson decays are used with the subsequent 𝐷+𝑠Ds+ → K+K−π+ and 𝐷⎯⎯⎯⎯⎯0D¯0 → K+π− decays. All helicity amplitudes and phases are measured, and the longitudinal polarisation fraction is determined to be fL = 0.578 ± 0.010 ± 0.011 with world-best precision, where the first uncertainty is statistical and the second is systematic. The pattern of helicity amplitude magnitudes is found to align with expectations from quark-helicity conservation in B decays. The ratio of branching fractions [ℬ(𝐵0→𝐷∗−𝐷∗+𝑠B0→D∗−Ds∗+) × ℬ(𝐷∗+𝑠→𝐷+𝑠𝛾Ds∗+→Ds+γ)]/ℬ(B0 → D*−𝐷+𝑠Ds+) is measured to be 2.045 ± 0.022 ± 0.071 with world-best precision. In addition, the first observation of the Cabibbo-suppressed Bs → D*−𝐷+𝑠Ds+ decay is made with a significance of seven standard deviations. The branching fraction ratio ℬ(Bs → D*−𝐷+𝑠Ds+)/ℬ(B0 → D*−𝐷+𝑠Ds+) is measured to be 0.049 ± 0.006 ± 0.003 ± 0.002, where the third uncertainty is due to limited knowledge of the ratio of fragmentation fractions.