Observation of the decays B0 (s) → Ds1(2536)∓K±

This paper reports the observation of the decays B0 (s) → Ds1(2536)∓K± using
proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 9 fb−1. The branching fractions of these decays are measured relative to the normalisation channel B0 → D0K+K−. The Ds1(2536)− meson is reconstructed in the D∗(2007)0K− decay channel and the products of branching fractions are measured to be
B(B0s → Ds1(2536)∓K±) × B(Ds1(2536)− → D∗(2007)0K−)
= (2.49 ± 0.11 ± 0.12 ± 0.25 ± 0.06) × 10−5,
B(B0 → Ds1(2536)∓K±) × B(Ds1(2536)− → D∗(2007)0K−)
= (0.510 ± 0.021 ± 0.036 ± 0.050) × 10−5.
The first uncertainty is statistical, the second systematic, and the third arises from the uncertainty of the branching fraction of the B0 → D0K+K− normalisation channel. The last uncertainty in the B0s result is due to the limited knowledge of the fragmentation fraction ratio, fs/fd. The significance for the B0
s and B0 signals is larger than 10 σ. The ratio of the helicity amplitudes which governs the angular distribution of the Ds1(2536)− → D∗(2007)0K− decay is determined from the data. The ratio of the S- and D-wave amplitudes is found to be 1.11 ± 0.15 ± 0.06 and the phase difference between them
0.70 ± 0.09 ± 0.04 rad, where the first uncertainty is statistical and the second systematic.