CMS-HIN-25-014

CMS-HIN-25-014 ; CERN-EP-2026-032
System-size dependence of charged-particle suppression in ultrarelativistic nucleus-nucleus collisions
CMS Collaboration
24 February 2026
Submitted to Physics Letters B
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_{\text{AA}} $) measured by the CMS Collaboration at the CERN LHC. Previous CMS measurements of $ R_{\text{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_{\text{AA}} $ in neon-neon collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. The neon-neon data correspond to an integrated luminosity of 0.76$ \text{nb}^{-1}$. The $ R_{\text{AA}} $ in all collision systems examined show similar qualitative trends, but have a magnitude which is ordered with the nucleon number $ A $. The $ R_{\text{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_{\text{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}} > $ 9.6 GeV.
Links: e-print arXiv:2602.21325 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Charged-particle $ R_{\text{AA}} $ values versus $ p_{\mathrm{T}} $ measured in $ \text{NeNe} $ collisions at 5.36 TeV compared to previous measurements in centrality-integrated $ \text{OO} $ collisions at 5.36 TeV [22], 0--80% centrality $ \text{XeXe} $ at 5.44 TeV [9], and minimum bias PbPb collisions at 5.02 TeV [12]. 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 \addmatterapp:nominalSection 3.
png pdf	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 pp collisions at $ A= $ 1, in which the $ R_{\text{AA}} $ is unity by definition, is also included in the upper left.
png pdf	Figure 3: Comparison of the charged-particle $ R_{\text{AA}} $ measured in $ \text{OO} $ [22], $ \text{NeNe} $, $ \text{XeXe} $ [9], and PbPb [12] collisions with NLO pQCD calculations that incorporate only initial-state effects through nPDFs and do not include parton energy loss [57,58]. 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 [59], while the orange markers represent calculations based on the nNNPDF3.0 nPDF set [60]. The vertical uncertainty bands reflect the combined scale-variation and nPDF uncertainties, with the latter providing the dominant contribution.
png pdf	Figure 4: Comparison of the charged-particle $ R_{\text{AA}} $ measured in $ \text{OO} $ [22], $ \text{NeNe} $, $ \text{XeXe} $ [9], and PbPb [12] collisions with calculations that incorporate parton energy loss effects [61,62,63,64,65,66,67,68,69]. 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.

Summary

This Letter presents a systematic study of the system-size dependence of the charged-particle nuclear modification factor ($ R_{\text{AA}} $) versus transverse momentum ($ p_{\mathrm{T}} $). The study is based on a cross-system comparison that minimizes centrality selection biases. For this purpose, the analysis reports the first measurement of the charged-particle $ R_{\text{AA}} $ in minimum bias neon-neon (NeNe) collisions at a center-of-mass energy per nucleon pair of 5.36 TeV, based on data collected in 2025 and corresponding to an integrated luminosity of 0.76$ \text{nb}^{-1}$. By comparing existing CMS measurements in oxygen-oxygen, xenon-xenon, and lead-lead collisions with the new $ \text{NeNe} $ results, this analysis samples nuclear systems across a broad range of nucleon number values ($ A $). This enables an evaluation of the role that the system size has on energy loss. The $ R_{\text{AA}} $ is measured for charged particles with pseudorapidity $ |\eta| < $ 1 using identical $ p_{\mathrm{T}} $ intervals across all collision systems, which allows for a direct comparison of nuclear suppression effects with $ A $, which characterizes the system size. The $ R_{\text{AA}} $ at a given $ p_{\mathrm{T}} $ value is found to monotonically decrease as a function of the cubic root of the nucleon number $ A^{1/3} $, a quantity that is related to the nuclear radius. The $ p_{\mathrm{T}} $ dependences of $ R_{\text{AA}} $ in all collision systems have similar qualitative shapes, with a local minimum at about 5--7 GeV. The inclusion of the $ \text{NeNe} $ system extends the coverage in system size and enables a more complete mapping of the evolution of suppression effects across collision systems. Comparisons with theoretical calculations show that baseline perturbative quantum chromodynamics predictions incorporating nuclear parton distribution functions predict only modest modifications relative to $ R_{\text{AA}} = $ 1, with a weak dependence on $ A $. In contrast, models that also include parton energy loss reproduce the observed monotonic evolution of $ R_{\text{AA}} $ with increasing $ A $. Differences among these calculations in the predicted $ R_{\text{AA}} $ value and its detailed system-size dependence reflect variations in the modeling of medium effects. Overall, models incorporating parton energy loss describe the measurements within experimental and theoretical uncertainties. This study constrains the existence and characteristics of any transition towards a deconfined, hot medium as the system size increases. Hence, this measurement offers guidance for the selection of ion species in future nuclear physics programs at the LHC.

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