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Angular analysis of \( {B}^0\to {D}^{\ast -}{D}_s^{\ast +} \) with \( {D}_s^{\ast +}\to {D}_s^{+}\gamma \) decays

A preprint version of the article is available at arXiv.


The first full angular analysis of the \( {B}^0\to {D}^{\ast -}{D}_s^{\ast +} \) decay is performed using 6 fb1 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+Kπ+ 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(B0D*\( {D}_s^{+} \)) is measured to be 2.045 ± 0.022 ± 0.071 with world-best precision. In addition, the first observation of the Cabibbo-suppressed BsD*\( {D}_s^{+} \) decay is made with a significance of seven standard deviations. The branching fraction ratio ℬ(BsD*\( {D}_s^{+} \))/ℬ(B0D*\( {D}_s^{+} \)) 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.


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