Angular analysis of $B^0\to D^{\ast -}{D}_s^{\ast +}$ with $D_s^{\ast +}\to D_s^{+}{\hat{I}}^3$ decays

Abstract

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 $D_s^{\ast +}\to D_s^{+}{\hat{I}}^3$ and $D^{\ast -}\to {\bar{D}}^0\ddot{I}{€}^{-}$ vector meson decays are used with the subsequent $D_s^{+}\to K^{+}{K}^{-}\ddot{I}{€}^{+}$ and ${\bar{D}}^0\to K^{+}\ddot{I}{€}^{-}$ decays. All helicity amplitudes and phases are measured, and the longitudinal polarisation fraction is determined to be $f_{\mathrm{L}}=0.578\pm 0.010\pm 0.011$ 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 $[\mathcal{B}(B^0\to D^{\ast -}{D}_s^{\ast +})\times \mathcal{B}(D_s^{\ast +}\to D_s^{+}{\hat{I}}^3)]/\mathcal{B}(B^0\to D^{\ast -}{D}_s^{+})$ is measured to be $2.045\pm 0.022\pm 0.071$ with world-best precision. In addition, the first observation of the Cabibbo-suppressed $B_s^0\to D^{\ast -}{D}_s^{+}$ decay is made with a significance of seven standard deviations. The branching fraction ratio $\mathcal{B}(B_s^0\to D^{\ast -}{D}_s^{+})/\mathcal{B}(B^0\to D^{\ast -}{D}_s^{+})$ is measured to be $0.049\pm 0.006\pm 0.003\pm 0.002$, where the third uncertainty is due to limited knowledge of the ratio of fragmentation fractions.

All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2021-006.html (LHCb public pages)