Charmonium production in $p$Ne collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}=68.5$ GeV

The measurement of charmonium states produced in proton-neon ($p$Ne) collisions by the LHCb experiment in its fixed-target configuration is presented. The production of $J\mathrm{/}\psi$ and $\psi (2S)$ mesons is studied with a beam of 2.5 TeV protons colliding on gaseous neon targets at rest, corresponding to a nucleon-nucleon centre-of-mass energy $\sqrt{s_{\mathrm{N}\mathrm{N}}}=68.5$ GeV. The data sample corresponds to an integrated luminosity of $21.7\pm 1.4$ nb${}^{-1}$. The $J\mathrm{/}\psi$ and $\psi (2S)$ hadrons are reconstructed in ${\mu }^{+}{\mu }^{-}$ final states. The $J\mathrm{/}\psi$ production cross-section per target nucleon in the centre-of-mass rapidity range $y^{\ast }\in [-2.29,0]$ is found to be $506\pm 8\pm 25\text{ nb/nucleon}$. The $\psi (2S)$ to $J\mathrm{/}\psi$ relative production rate is found to be $(1.67\pm 0.27\pm 0.10)\mathrm{\%}$ in good agreement with other measurements involving beam and target nuclei of similar sizes.