Observation of the decay $B^0\to J\mathrm{/}\psi \omega$ at Belle II

We measure the branching fraction of the decay $B^0\to J\mathrm{/}\psi \omega$ using data collected with the Belle II detector at the SuperKEKB collider. The data contain $(387\pm 6)\times {10}^6$ $B\bar{B}$ meson pairs produced in energy-asymmetric $e^{+}{e}^{-}$ collisions at the $\mathrm{{\rm Y}}(4S)$ resonance. The measured branching fraction $\mathcal{B}(B^0\to J\mathrm{/}\psi \omega )=(2.16\pm 0.30\pm 0.14)\times {10}^{-5}$, where the first uncertainty is statistical and the second is systematic, is more precise than previous results and constitutes the first observation of the decay with a significance of $6.5$ standard deviations.