Observation of the $B_s^0\text{\hspace{0.17em}⁣}\to D^{\ast +}{D}^{\ast -}$ decay

The first observation of the $B_s^0\text{\hspace{0.17em}⁣}\to D^{\ast +}{D}^{\ast -}$ decay and the measurement of its branching ratio relative to the $B^0\text{\hspace{0.17em}⁣}\to D^{\ast +}{D}^{\ast -}$ 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 $f_s\mathrm{/}{f}_d$. The $B_s^0\text{\hspace{0.17em}⁣}\to D^{\ast +}{D}^{\ast -}$ 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 $B^0\text{\hspace{0.17em}⁣}\to D^{\ast +}{D}^{\ast -}$branching fraction. These results are calculated using the average $B_s^0$ meson lifetime in simulation. Correction factors are reported for scenarios where either a purely heavy or a purely light $B_s^0$ eigenstate is considered.