Measurement of the prompt-production cross-section ratio $\sigma ({\chi }_{c2})\mathrm{/}\sigma ({\chi }_{c1})$ in $p$Pb collisions at $\sqrt{s_{NN}}$ = 8.16 TeV

This Letter reports the first measurement of prompt ${\chi }_{c1}$ and ${\chi }_{c2}$ charmonium production in nuclear collisions at Large Hadron Collider energies. The cross-section ratio $\sigma(\chi_{c2}) / \sigma(\chi_{c1})$ismeasuredin$p$Pbcollisionsat$\sqrt{s_{NN}}$ = 8.16 TeV, collected with the LHCb experiment. The ${\chi }_{c1,2}$ states are reconstructed via their decay to a $\mathrm{J}\mathrm{/}\psi$ meson, subsequently decaying into a pair of oppositely charged muons, and a photon, which is reconstructed in the calorimeter or via its conversion in the detector material. The cross-section ratio is consistent with unity in the two considered rapidity regions. Comparison with a corresponding cross-section ratio previously measured by the LHCb collaboration in $pp$ collisions suggests that ${\chi }_{c1}$ and ${\chi }_{c2}$ states are similarly affected by nuclear effects occurring in $p$Pb collisions.