First observation of the multiplicity dependence of σ_𝜓(2S)/σ_J/𝜓 in pPb collisions with CMS and its implication regarding comover effects in small systems

The substantial masses of charm hadrons make them exceptional tools for studying quantum chromodynamics (QCD), providing quantitative insights into QCD dynamics for systems with high parton densities and/or temperatures. Final-state effects, including interactions with co-moving particles or quark coalescence, may modify the hadronization of heavy quarks. To study these effects, we report the first measurement of prompt and non-prompt separated 𝜓(2S)/J/𝜓 cross-section ratios as a function of the charged-particle multiplicity in proton-lead (pPb) collisions at √s_NN = 8.16 TeV. For the first time in the pPb collision system, a multiplicity dependence of this ratio is observed for prompt charmonium production. The ratio of the nonprompt 𝜓(2S) to J/𝜓 cross sections show no dependency on multiplicity. The rapidity-dependence of this observable is studied, and results are compared with a model incorporating comover interactions. These measurements constrain hadronization models of heavy quarks in nuclear collisions and support a picture in which co-moving particles may dissociate the weakly-bound excited state of charmonia more than the ground state throughout the system’s evolution.