Angular analysis of B 0 → D ∗ − D s ∗ + $$ {B}^0\to {D}^{\ast -}{D}_s^{\ast +} $$ with D s ∗ + → D s + γ $$ {D}_s^{\ast +}\to {D}_s^{+}\gamma $$ decays

Abstract

Abstract

The first full angular analysis of the

B 0

→

D

∗ −

D s

∗ +

$$ {B}^0\to {D}^{\ast -}{D}_s^{\ast +} $$ 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

∗ +

→

D s +

γ

$$ {D}_s^{\ast +}\to {D}_s^{+}\gamma $$ and D*− →

D ¯

0

π −

$$ {\overline{D}}^0{\pi}^{-} $$ vector meson decays are used with the subsequent

D s +

$$ {D}_s^{+} $$ → K+K−π+ and

D ¯

0

$$ {\overline{D}}^0 $$ → 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

→

D

∗ −

D s

∗ +

$$ {B}^0\to {D}^{\ast -}{D}_s^{\ast +} $$ ) × ℬ(

D s

∗ +

→

D s +

γ

$$ {D}_s^{\ast +}\to {D}_s^{+}\gamma $$ )]/ℬ(B0 → D*−

D s +

$$ {D}_s^{+} $$ ) 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 +

$$ {D}_s^{+} $$ decay is made with a significance of seven standard deviations. The branching fraction ratio ℬ(Bs → D*−

D s +

$$ {D}_s^{+} $$ )/ℬ(B0 → D*−

D s +

$$ {D}_s^{+} $$ ) 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.