Evidence for the decay $B^0\to J\mathrm{/}\psi \omega$ and measurement of the relative branching fractions of $B_s^0$ meson decays to $J\mathrm{/}\psi \eta$ and $J\mathrm{/}\psi {\eta }^{{}^{\mathrm{\prime }}}$

First evidence of the $B^0\to J\mathrm{/}\psi \omega$ decay is found and the $B_s^0\to J\mathrm{/}\psi \eta$ and $B_s^0\to J\mathrm{/}\psi {\eta }^{\mathrm{\prime }}$ decays are studied using a dataset corresponding to an integrated luminosity of 1.0 ${\mathrm{f}\mathrm{b}}^{-1}$ collected by the LHCb experiment in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=7$ TeV. The branching fractions of these decays are measured relative to that of the $$\mathrm{B}$^0\to J/\psi\rho^0$decay:$$\frac{Br(B^0\to J/\psi\omega)}{Br(B^0\to J/\psi\rho^0)} = 0.89 \pm0.19(stat)^{+0.07}{-0.13}(syst)$$ $$\frac{Br(B_s^0\to J\mathrm{/}\psi \eta )}{Br(B^0\to J\mathrm{/}\psi {\rho }^0)}=14.0\pm 1.2(stat{)}_{-1.5}^{+1.1}(syst{)}_{-1.0}^{+1.1}(\frac{f_d}{f_s}),$$ $$\frac{Br(B_s^0\to J\mathrm{/}\psi {\eta }^{\mathrm{\prime }})}{Br(B^0\to J\mathrm{/}\psi {\rho }^0)}=12.7\pm 1.1(stat{)}_{-1.3}^{+0.5}(syst{)}_{-0.9}^{+1.0}(\frac{f_d}{f_s}),$$ where the last uncertainty is due to the knowledge of $f_d\mathrm{/}{f}_s$, the ratio of b-quark hadronization factors that accounts for the different production rate of $B^0$ and $B_s^0$ mesons. The ratio of the branching fractions of $B^0_s\to J/\psi\eta^{\prime}$and$B^0_s\to J/\psi\eta$decaysismeasuredtobe$$ \frac{Br(B^0_s\to J/\psi\eta^{\prime})}{Br(B^0_s\to J/\psi\eta)} = 0.90\pm0.09(stat)^{+0.06}{-0.02}(syst).$$