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

The first full angular analysis of the B0 → D∗−D∗+s 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+s γ and D∗− → D0π− vector meson decays are used with the subsequent D+s → K+K−π+ and D0 → 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 branch-
ing fractions [B(B0 → D∗−D∗+s ) × B(D∗+s → D+s γ)]/B(B0 → D∗−D+s ) 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+s decay is made with a significance of seven standard deviations. The branching fraction ratio B(Bs → D∗−D+s )/B(B0 → D∗−D+s ) 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.