The first observation of the $B^0_s\!\to D^{ *+}D^{ *-}$ decay and the measurement of its branching ratio relative to the $B^0\!\to D^{ *+}D^{ *-}$ decay are presented. The data sample used corresponds to an integrated luminosity of $9\,\text{ fb}^{ -1}$ of proton-proton collisions recorded by the LHCb experiment at centre-of-mass energies of 7, 8 and $13\,\text{ TeV}$ between 2011 and 2018. The decay is observed with more than $10$ standard deviations and the time-integrated ratio of branching fractions is determined to be \begin{ align*} \frac{ \mathcal{ B}(B^0_s\!\to D^{ *+}D^{ *-})}{ \mathcal{ B}(B^0\!\to D^{ *+}D^{ *-})} = 0.269 \pm 0.032 \pm 0.011 \pm 0.008\, , \end{ align*} where the first uncertainty is statistical, the second systematic and the third due to the uncertainty of the fragmentation fraction ratio $f_s/f_d$. The $B^0_s\!\to D^{ *+}D^{ *-}$ branching fraction is calculated to be \begin{ align*} \mathcal{ B}(B^0_s\!\to D^{ *+}D^{ *-}) = (2.15 \pm 0.26 \pm 0.09 \pm 0.06 \pm 0.16)\times 10^{ -4} \,, \end{ align*} where the fourth uncertainty is due to the $B^0\!\to D^{ *+}D^{ *-}$branching fraction. These results are calculated using the average $B^0_s$ meson lifetime in simulation. Correction factors are reported for scenarios where either a purely heavy or a purely light $B^0_s$ eigenstate is considered.
Categories: hep-ex