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

TL;DR

This study measures how the ratio of $C92$ to $J/C91$ production varies with multiplicity in proton-lead collisions at 8.16 TeV, revealing different behaviors in prompt and nonprompt charmonium states and suggesting possible quark-gluon plasma effects.

Contribution

It provides the first detailed measurement of the $C92$ to $J/C91$ ratio as a function of multiplicity in $p$Pb collisions at this energy, highlighting system size effects.

Findings

Prompt production ratio depends on multiplicity in $p$Pb collisions.

No multiplicity dependence observed for nonprompt production.

Additional suppression mechanisms may exist in the Pb-going direction.

Abstract

The production ratio of $\psi(2S)$ to $J/\psi$ charmonium states is presented as a function of multiplicity in proton-lead collisions at a centre-of-mass energy of $\sqrt{s_{NN}}=8.16$ TeV, for both prompt and nonprompt sources. The total luminosity recorded by the LHCb experiment corresponds to 13.6 $pb^{-1}$ for $p$Pb collisions and 20.8 $pb^{-1}$ for Pb$p$ collisions, where the first particle indicates the forward direction of the detector. Measurements are performed in the dimuon final state at forward (backward) centre-of-mass rapidity $1.5<y^*<4.0$ ($-5.0<y^*<-2.5$) 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.