Measurement of ψ(2S) to J/ψ cross-section ratio as function of multiplicity in pPb collisions at $ \sqrt{s_{\textrm{NN}}}=8.16 $ TeV

The production ratio of ψ (2 S ) to J/ψ charmonium states is presented as a function of multiplicity in proton-lead collisions at a centre-of-mass energy of $\sqrt{s_{\textrm{NN}}}=8.16$ TeV, for both prompt and nonprompt sources. The total luminosity recorded by the LHCb experiment corresponds to 13.6 nb − 1 for p Pb collisions and 20.8 nb − 1 for Pb p collisions, where the first particle corresponds to the particle traveling towards the detector. Measurements are performed in the dimuon final state at forward (backward) centre-of-mass rapidity, with respect to the proton direction, 1 . 5 < y * < 4 . 0 (−5 . 0 < y * < −2 . 5) for p Pb (Pb p ) collisions. A multiplicity dependence of the prompt production ratio is observed in p Pb collisions, whereas no dependence is found in nonprompt production, nor in either prompt or nonprompt production in Pb p collisions. These results suggest that in the Pb-going direction additional suppression mechanisms beyond comover effects may be present, possibly related to the formation of quark-gluon plasma. This highlights a transition from small to large collision systems and provides important insight into the suppression of charmonia in proton-nucleus collisions.