System-size dependence of charged-particle suppression in ultrarelativistic nucleus-nucleus collisions

Abstract

High-energy partons lose energy while propagating through the hot, strongly interacting medium produced in ultrarelativistic nucleus-nucleus collisions, leading to a suppression of particle production at high transverse momentum ($p_\mathrm{T}$). The dependence of this energy loss on the size of the colliding nuclear system has yet to be firmly established experimentally. This Letter presents a systematic study of charged-particle suppression across four different nucleus-nucleus collision systems using nuclear modification factors ($R_\mathrm{AA}$) measured by the CMS Collaboration at the CERN LHC. Previous CMS measurements of $R_\mathrm{AA}$ in oxygen-oxygen, xenon-xenon, and lead-lead collisions are recast with identical $p_\mathrm{T}$ intervals and are complemented by the first measurement of the charged-particle $R_\mathrm{AA}$ in neon-neon collisions at $\sqrt{s_\mathrm{NN}}$ = 5.36 TeV. The neon-neon data correspond to an integrated luminosity of 0.76 nb$^{-1}$. The $R_\mathrm{AA}$ in all collision systems examined show similar qualitative trends, but have a magnitude which is ordered with the nucleon number A. The $R_\mathrm{AA}$ feature a downward slope at low $p_\mathrm{T}$, a local minimum at around 5$-$7 GeV, and an upward slope with increasing $p_\mathrm{T}$. The $R_\mathrm{AA}$ are also compared in terms of A$^{1/3}$, which is proportional to the nuclear radius. Models including only initial-state nuclear effects fail to reproduce the observed trends, whereas energy loss models reproduce the trends in the region $p_\mathrm{T}$ $\gt$ 9.6 GeV.

Figures (4)

• Figure 1: Charged-particle $R_{\text{AA}}$ values versus $p_{\mathrm{T}}$ measured in $\text{NeNe}$ collisions at 5.36$\,\text{Te\spaceV}$ compared to previous measurements in centrality-integrated $\text{OO}$ collisions at 5.36$\,\text{Te\spaceV}$CMS:2025bta, 0--80% centrality $\text{XeXe}$ at 5.44$\,\text{Te\spaceV}$CMS:2018yyx, and minimum bias $\text{PbPb}$ collisions at 5.02$\,\text{Te\spaceV}$CMS:2016xef. The vertical error bars represent statistical uncertainties and the boxes represent systematic uncertainties. Global normalization uncertainties are not included directly in the data markers error bands, but they are instead represented by the various boxes on the left around unity. For this compilation plot, a uniform $p_{\mathrm{T}}$ binning scheme is adopted to ensure a consistent comparison across collision systems, as described in the text. The same results with finer binning can be found in supplementary material Section 3.
• Figure 2: Charged-particle $R_{\text{AA}}$ values, in intervals of $p_{\mathrm{T}}$, versus $A^{1/3}$ of the AA colliding system. The open markers represent the measured $R_{\text{AA}}$, with the vertical bars representing the statistical uncertainty, and the vertical band representing the experimental uncertainty. Global normalization uncertainties for each data set are shown in the light-gray boxes around unity. For visualization purposes, a subset of four out of the fifteen $p_{\mathrm{T}}$ intervals is presented. A point for ${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$ collisions at $A\xspace= 1$, in which the $R_{\text{AA}}$ is unity by definition, is also included in the upper left.
• Figure 3: Comparison of the charged-particle $R_{\text{AA}}$ measured in $\text{OO}$CMS:2025bta, $\text{NeNe}$, $\text{XeXe}$CMS:2018yyx, and $\text{PbPb}$CMS:2016xef collisions with NLO pQCD calculations that incorporate only initial-state effects through nPDFs and do not include parton energy loss Huss:2020dweMazeliauskas:2025clt. The open boxes with black circular markers represent the data. The box height represents the total experimental uncertainty obtained from the quadratic combination of statistical, systematic, and global normalization uncertainties. The purple markers correspond to calculations using the EPPS21 nPDF set Eskola:2021nhw, while the orange markers represent calculations based on the nNNPDF3.0 nPDF set AbdulKhalek:2022fyi. The vertical uncertainty bands reflect the combined scale-variation and nPDF uncertainties, with the latter providing the dominant contribution.
• Figure 4: Comparison of the charged-particle $R_{\text{AA}}$ measured in $\text{OO}$CMS:2025bta, $\text{NeNe}$, $\text{XeXe}$CMS:2018yyx, and $\text{PbPb}$CMS:2016xef collisions with calculations that incorporate parton energy loss effects Faraday:2025prrFaraday:2025ptoGyulassy:1999zdDjordjevic:2003zkZakharov:2021uzaArleo:2022shsShi:2018izgShi:2018lsfGuo:2025tsf. The open boxes with black circular markers represent the data, with the box height indicating the total experimental uncertainty obtained from the quadratic combination of statistical, systematic, and global normalization uncertainties. The other colored markers represent theoretical predictions based on different approaches, as described in the text. The uncertainty band is smaller than the marker size for some of the calculations.