CMS-HIN-24-001

CMS-HIN-24-001 ; CERN-EP-2024-332
Observation of the charged-particle multiplicity dependence of $\sigma_{\psi(2\text{S})} /\sigma_{\mathrm{J}/\psi}$ in pPb collisions at 8.16 TeV
CMS Collaboration
3 March 2025
Submitted to Phys. Rev. Lett.
Abstract: Bound states of charm and anticharm quarks, known as charmonia, have a rich spectroscopic structure that can be used to probe the dynamics of hadron production in high-energy hadron collisions. Here, the cross section ratio of excited $(\psi(2\text{S}))$ and ground state $(\mathrm{J}/\psi )$ vector mesons is measured as a function of the charged-particle multiplicity in proton-lead (pPb ) collisions at a center-of-mass (CM) energy per nucleon pair of 8.16 TeV. The data corresponding to an integrated luminosity of 175 nb $^{-1}$ were collected using the CMS detector. The ratio is measured separately for prompt and nonprompt charmonia in the transverse momentum range 6.5 $< p_{\mathrm{T}} <$ 30 GeV and in four rapidity ranges spanning $-$ 2.865 $< y_{\mathrm{CM}} <$ 1.935. For the first time, a statistically significant multiplicity dependence of the prompt cross section ratio is observed in proton-nucleus collisions. There is no clear rapidity dependence in the ratio. The prompt measurements are compared with a theoretical model which includes interactions with nearby particles during the evolution of the system. These results provide additional constraints on hadronization models of heavy quarks in nuclear collisions.
Links: e-print arXiv:2503.02139 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The prompt (red circular points) and nonprompt (blue square points) normalized cross section ratio for $-$ 2.865 $< y_{\mathrm{CM}} < -$ 2 (the leftmost), $-$ 2 $< y_{\mathrm{CM}} < -$ 1 (center-left), $-$ 1 $< y_{\mathrm{CM}} <$ 1 (center-right), and 1 $< y_{\mathrm{CM}} <$ 1.935 (the rightmost) in the range 6.5 $< p_{\mathrm{T}} <$ 30 GeV. The black line with a gray band shows a model incorporating co-mover effects [50] for prompt charmonia. Error bars represent statistical uncertainties, while boxes display systematic uncertainties.
png pdf	Figure 2: (upper) The prompt normalized cross section ratio versus normalized event charged-particle multiplicity for four different rapidity selections. The different sets of colored data points have a slight horizontal offset applied for visual clarity. (lower) The same observable for prompt (red circular points) and nonprompt (blue square points) mesons when combining all four rapidity selections into a single measurement. The black line with a gray band shows a model incorporating co-mover effects [50] for prompt charmonia. Error bars represent statistical uncertainties, while boxes display systematic uncertainties.
png pdf	Figure 3: Dimuon invariant mass distribution showing the $\mathrm{J}/\psi$ and $\psi(2\text{S})$ peaks in pPb data, for $-$ 2.865 $< y_{\mathrm{CM}} < -$ 2 (left) and 1 $< y_{\mathrm{CM}} <$ 1.935 (right) for 3 $< p_{\mathrm{T}} <$ 30 GeV. The total fit is shown with a solid blue line, and the dotted blue line represents the background contribution.
png pdf	Figure 3-a: Dimuon invariant mass distribution showing the $\mathrm{J}/\psi$ and $\psi(2\text{S})$ peaks in pPb data, for $-$ 2.865 $< y_{\mathrm{CM}} < -$ 2 (left) and 1 $< y_{\mathrm{CM}} <$ 1.935 (right) for 3 $< p_{\mathrm{T}} <$ 30 GeV. The total fit is shown with a solid blue line, and the dotted blue line represents the background contribution.
png pdf	Figure 3-b: Dimuon invariant mass distribution showing the $\mathrm{J}/\psi$ and $\psi(2\text{S})$ peaks in pPb data, for $-$ 2.865 $< y_{\mathrm{CM}} < -$ 2 (left) and 1 $< y_{\mathrm{CM}} <$ 1.935 (right) for 3 $< p_{\mathrm{T}} <$ 30 GeV. The total fit is shown with a solid blue line, and the dotted blue line represents the background contribution.
png pdf	Figure 4: The prompt (red circular points) and nonprompt (blue square points) normalized cross section ratio for $-$ 2.865 $< y_{\mathrm{CM}} < -$ 2 (upper), and 1 $< y_{\mathrm{CM}} <$ 1.935 (lower) in the range 3 $< p_{\mathrm{T}} <$ 6.5 GeV. Error bars represent statistical uncertainties, while boxes display systematic uncertainties.

Summary

In summary, this Letter presents a measurement of the normalized production cross section ratio of

$\psi(2\text{S})$ over

$\mathrm{J}/\psi$ mesons as a function of charged-particle multiplicity in proton-lead collisions at a center-of-mass energy per nucleon pair of 8.16 TeV with a data sample collected with the CMS detector. The ratio is measured for both prompt and nonprompt

$\psi(2\text{S})$ and

$\mathrm{J}/\psi$ mesons separately. For the first time in proton-ion collisions, a statistically significant multiplicity-dependent modification of the ratio is observed for prompt charmonia, indicating a decreasing trend with increasing multiplicity. The ratio of the nonprompt cross sections shows no dependency on the multiplicity. No appreciable rapidity dependence is observed within the charmonium rapidity and transverse momentum ranges probed. These observations support the presence of final state effects dissociating weakly bound excited states of charmonia more than the ground state, which can be interpreted as co-mover dissociation or, potentially, the formation of quark-gluon plasma. These new data provide powerful tests for hadronization models in small collision systems.

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