Measurement of the branching fractions of the $B^0_s\to J/\psi\eta$, $B^0_s\to J/\psi\eta^{\prime}$ and $B^0\to J\psi\omega$ decays in the LHCb experiment

First evidence of the $B^0\to J/\psi\omega$ decay is found and the $B^0_s\to J/\psi\eta$ and $B^0_s\to J/\psi\eta^{\prime}$ decays are studied using a dataset corresponding to an integrated luminosity of 1.0 $\mathrm{fb}^{-1}$ collected by the LHCb experiment in proton-proton collisions at a center-of-mass energy of $\sqrt{s} = 7$ TeV. Branching fractions of these decays are measured relative to that of the $B^0\to J/\psi\rho^0$ decay. All the channels under study contain photons in the final state. Precise determination of photons energies is essential for accurate measurement of the decays branching fractions. So, an auxiliary task was a regular calibration of the LHCb electromagnetic calorimeter with neutral pions. Values obtained for the $B^0_s\to J/\psi\eta$ and $B^0_s\to J/\psi\eta^{\prime}$ decays branching fractions, as well as a ratio of these values, are found to be in a good agreement with both theoretical predictions and results of previous measurements. The $B^0\to J/\psi\omega$ decay is seen for the first time. The regular precise calibration of the electromagnetic calorimeter with neutral pions was allowing to keep the photon energies measurement accuracy at a level of 2 % during whole period of datataking in 2010-2012.