Measurement of the Branching Fraction of $B^0\to J\mathrm{/}\psi {\pi }^0$ Decays

The ratio of branching fractions between $B^0\to J\mathrm{/}\psi {\pi }^0$ and $B^{+}\to J\mathrm{/}\psi K^{\ast +}$ decays is measured with proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 9 fb${}^{-1}$. The measured value is $\frac{{\mathcal{B}}_{B^0\to J\mathrm{/}\psi {\pi }^0}}{{\mathcal{B}}_{B^{+}\to J\mathrm{/}\psi K^{\ast +}}}=(1.153\pm 0.053\pm 0.048)\times {10}^{-2}$, where the first uncertainty is statistical and the second is systematic. The branching fraction for $B^0\to J\mathrm{/}\psi {\pi }^0$ decays is determined using the branching fraction of the normalisation channel, resulting in ${\mathcal{B}}_{B^0\to J\mathrm{/}\psi {\pi }^0}=(1.670\pm 0.077\pm 0.069\pm 0.095)\times {10}^{-5}$, where the last uncertainty corresponds to that of the external input. This result is consistent with the current world average value and competitive with the most precise single measurement to date.