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

Abstract

The production ratio of $\ddot{I}ˆ(2S)$ to $J/\ddot{I}ˆ$ 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 $p{b}^{-1}$ for $p$Pb collisions and 20.8 $p{b}^{-1}$ 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 $1.5<y^{\ast }<4.0$ ($-5.0<y^{\ast }<-2.5$) 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.

All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://lbfence.cern.ch/alcm/public/analysis/full-details/4177/ (LHCb public pages)