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

The first full angular analysis of the $$ {B}^0\to {D}^{\ast -}{D}_s^{\ast +} $$ 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 $$ {D}_s^{\ast +}\to {D}_s^{+}\gamma $$ and D*− → $$ {\overline{D}}^0{\pi}^{-} $$ vector meson decays are used with the subsequent $$ {D}_s^{+} $$ → K+K−π+ and $$ {\overline{D}}^0 $$ → 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( $$ {B}^0\to {D}^{\ast -}{D}_s^{\ast +} $$ ) × ℬ( $$ {D}_s^{\ast +}\to {D}_s^{+}\gamma $$ )]/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.