Observation of the $B_c^{+}\to J\mathrm{/}\psi {\pi }^{+}{\pi }^0$ decay

The first observation of the $B_c^{+}\to J\mathrm{/}\psi {\pi }^{+}{\pi }^0$ decay is reported with high significance using proton-proton collision data, corresponding to an integrated luminosity of 9fb${}^{-1}$, collected with the LHCb detector at centre-of-mass energies of 7, 8, and 13 TeV. The ratio of its branching fraction relative to the $B_c^{+}\to J\mathrm{/}\psi {\pi }^{+}$ channel is measured to be $$\frac{\mathcal{B}(B_c^{+}\to J\mathrm{/}\psi {\pi }^{+}{\pi }^0)}{\mathcal{B}(B_c^{+}\to J\mathrm{/}\psi {\pi }^{+})}=2.80\pm 0.15\pm 0.11\pm 0.16,$$ where the first uncertainty is statistical, the second systematic and the third related to imprecise knowledge of the branching fractions for $B^{+}\to J\mathrm{/}\psi K^{\ast +}$ and $B^{+}\to J\mathrm{/}\psi K^{+}$ decays, which are used to determine the ${\pi }^0$ detection efficiency. The ${\pi }^{+}{\pi }^0$ mass spectrum is found to be consistent with the dominance of an intermediate ${\rho }^{+}$ contribution in accordance with a model based on QCD factorisation.