First observation of the decay B0
s → D0φ 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 6.5 standard deviations. The branching fraction is measured relative to that
of the decay B0
s → D0K∗0 to be
B(B0
s → D0φ)
B(B0
s → D0K∗0)
= 0.069 ± 0.013 (stat) ± 0.007 (syst).
The first measurement of the ratio of branching fractions for the decays B0
s → D0K∗0 and B0 → D0K∗0
is found to be
B(B0
s → D0K∗0)
B(B0 → D0K∗0)
= 7.8 ± 0.7 (stat) ± 0.3 (syst) ± 0.6 (f s/ fd),
where the last uncertainty is due to the ratio of the B0
s and B0 fragmentation fractions.