Angular analysis of ${B}^0\to {D}^{\ast -}{D}_s^{\ast +}$ with ${D}_s^{\ast +}\to {D}_s^{+}\gamma$ 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 ${D}_s^{\ast +}\to {D}_s^{+}\gamma$ 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 f_L = 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 B_s → D^*−${D}_s^{+}$ decay is made with a significance of seven standard deviations. The branching fraction ratio ℬ(B_s → 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.