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

Abstract

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∗− → D 0 π − vector meson decays are used with the subsequent D+ s → K+K−π + and 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(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.