Observation of the decay $B_s^0\to\bar{D}^0\phi$

First observation of the decay $B_s^0\to\overline{D^0}\phi$ is reported using $pp$ collision data, corresponding to an integrated luminosity of 1.0 $fb^{-1}$, collected by the LHCb experiment at a centre-of-mass energy of 7 TeV. The significance of the signal is 6.2 standard deviations. The branching fraction is measured relative to that of the decay $B_s^0\to\overline{D^0}\overline{K^{*0}}$ to be $$\frac{{\cal{B}}( B_s^0\to\overline{D^0}\phi)}{{\cal{B}}(B_s^0\to\overline{D^0}\overline{K^{*0}})} = 0.069 \pm 0.013 ~(\mathrm{stat}) \pm 0.007 ~(\mathrm{syst}).$$ The first measurement of the ratio of branching fractions for the decays $B_s^0\to\overline{D^0}\overline{K^{*0}}$ and $B^0\to\overline{D^0}K^{*0}$ is found to be $$\frac{{\cal{B}}(B_s^0\to\overline{D^0}\overline{K^{*0}})}{{\cal{B}}(B^0\to\overline{D^0}K^{*0})} = 7.8 \pm 0.7 ~(\mathrm{stat}) \pm 0.3 ~(\mathrm{syst}) \pm 0.6 ~(f_s/f_d),$$ where the last uncertainty is due to the ratio of the $B^0_s$ and $B^0$ fragmentation fractions.