Study of charmonium and charmonium-like contributions in $B^{+}\to J\mathrm{/}\psi \eta K^{+}$ decays

A study of $B^{+}\to J\mathrm{/}\psi \eta K^{+}$ decays, followed by $J/\psi \rightarrow \mu^+ \mu^-$and$\eta \rightarrow \gamma \gamma$, is performed using a dataset collected with the LHCb detector in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV, corresponding to an integrated luminosity of 9 fb${}^{-1}$. The $J\mathrm{/}\psi \eta$ mass spectrum is investigated for contributions from charmonia and charmonium-like states. Evidence is found for the $B^{+}\to ({\psi }_2(3823)\to J\mathrm{/}\psi \eta )K^{+}$ and $B^{+}\to (\psi (4040)\to J\mathrm{/}\psi \eta )K^{+}$ decays with significance of 3.4 and 4.7~standard deviations, respectively. This constitutes the~first~evidence for the ${\psi }_2(3823)\to J\mathrm{/}\psi \eta$ decay.