Angular analysis of ^B0→ ^D∗−Ds∗+ with Ds∗+→Ds+γ decays

The first full angular analysis of the B0→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 Ds∗+→Ds+γ and D ^*− → D¯ π ⁻ vector meson decays are used with the subsequent Ds+ → K ⁺K ⁻π ⁺ and D¯ → K ⁺π ⁻ decays. All helicity amplitudes and phases are measured, and the longitudinal polarisation fraction is determined to be f _L = 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∗−Ds∗+) × ℬ(Ds∗+→Ds+γ)]/ℬ(B ⁰ → D ^*−Ds+) is measured to be 2.045 ± 0.022 ± 0.071 with world-best precision. In addition, the first observation of the Cabibbo-suppressed B _s → D ^*−Ds+ decay is made with a significance of seven standard deviations. The branching fraction ratio ℬ(B _s → D ^*−Ds+)/ℬ(B ⁰ → D ^*−Ds+) 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. [Figure not available: see fulltext.]