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| CMS-HIN-25-010 ; CERN-EP-2026-153 | ||
| Centrality dependence of charged-hadron pseudorapidity distributions in oxygen-oxygen collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV | ||
| CMS Collaboration | ||
| 1 June 2026 | ||
| Submitted to Physics Letters B | ||
| Abstract: We report the first measurement of charged-hadron pseudorapidity ($ \eta $) distributions in oxygen-oxygen ($ \mathrm{OO} $) collisions at a nucleon-nucleon center-of-mass energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. The data were recorded by the CMS experiment at the LHC in 2025. Primary charged-hadron yields are measured in the range $ |\eta| < $ 2.4 as a function of centrality (the overlap of the two nuclei). The results are compared with previous measurements in lead-lead ($ \mathrm{PbPb} $) and xenon-xenon collisions at similar energies, as well as predictions from several Monte Carlo event generators and a hydrodynamic model. The charged-hadron pseudorapidity density in the midrapidity region ($ |\eta| < $ 0.5) is $ \langle \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta \rangle = $ 41.8 $ \pm $ 1.1 (syst) integrated over centrality and 135.0 $ \pm $ 4.0 (syst) for the most central (i.e.,, largest nuclear overlap) events. The hydrodynamic model TRAJECTUM provides the best overall description of the data, particularly in central collisions. The particle density at midrapidity divided by the number of nucleons participating in the interaction in central $ \mathrm{OO} $ collisions is consistent with that observed in central $ \mathrm{PbPb} $ collisions at similar collision energy. While the overall energy-scaling behavior observed in other nucleus-nucleus collisions is preserved, the data exhibit deviations from simple participant and system-size scaling, highlighting the role of collision geometry and finite-size effects in light ion collisions. | ||
| Links: e-print arXiv:2606.09567 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
png pdf |
Figure 1:
The $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ distribution in $ \mathrm{OO} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV for the 0--100% centrality class. The result is symmetrized about $ \eta= $ 0. Predictions from the HIJING [39], EPOS LHC [40,25], and AMPT [47] event generators are also shown. The lower panel shows the ratios of the model predictions to the data, normalized to unity at midrapidity. In the upper panel, the gray boxes indicate the total systematic uncertainties, while statistical uncertainties are negligible. In the lower panel, the boxes represent the relative uncertainties of the data. |
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Figure 2:
The $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ distribution in $ \mathrm{OO} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV for several centrality classes. In all cases, the data are symmetrized about $ \eta= $ 0. Boxes indicate the total systematic uncertainties, while statistical uncertainties are negligible. |
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Figure 3:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity in $ \mathrm{OO} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV as a function of event centrality, shown for the measured values (left) and normalized by 2 $ A $ (right), where $ A $ is the mass number of the colliding nuclei. The results are compared with measurements in $ \mathrm{PbPb} $ and $ \mathrm{XeXe} $ collisions at similar collision energies by the CMS [17,22] and ALICE [21,18,23] Collaborations. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 3-a:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity in $ \mathrm{OO} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV as a function of event centrality, shown for the measured values (left) and normalized by 2 $ A $ (right), where $ A $ is the mass number of the colliding nuclei. The results are compared with measurements in $ \mathrm{PbPb} $ and $ \mathrm{XeXe} $ collisions at similar collision energies by the CMS [17,22] and ALICE [21,18,23] Collaborations. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 3-b:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity in $ \mathrm{OO} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV as a function of event centrality, shown for the measured values (left) and normalized by 2 $ A $ (right), where $ A $ is the mass number of the colliding nuclei. The results are compared with measurements in $ \mathrm{PbPb} $ and $ \mathrm{XeXe} $ collisions at similar collision energies by the CMS [17,22] and ALICE [21,18,23] Collaborations. The bands around the data points denote the total systematic uncertainties, while the statistical uncertainties are negligible. |
png pdf |
Figure 4:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity, normalized by $ \langle N_{\text{part}} \rangle $, in $ \mathrm{OO} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV shown as a function of $ \langle N_{\text{part}} \rangle $ (left) and $ \langle N_{\text{part}} \rangle/2A $ (right), where $ A $ is the mass number of the colliding nuclei. The results are compared with measurements in $ \mathrm{PbPb} $ and $ \mathrm{XeXe} $ collisions by the CMS [22,17] and ALICE [23,21,18] Collaborations. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. For the $ \mathrm{OO} $ data, the inner band shows the experimental uncertainties, while the outer band includes both the experimental and $ \langle N_\mathrm{part}\rangle $ uncertainties. |
png pdf |
Figure 4-a:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity, normalized by $ \langle N_{\text{part}} \rangle $, in $ \mathrm{OO} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV shown as a function of $ \langle N_{\text{part}} \rangle $ (left) and $ \langle N_{\text{part}} \rangle/2A $ (right), where $ A $ is the mass number of the colliding nuclei. The results are compared with measurements in $ \mathrm{PbPb} $ and $ \mathrm{XeXe} $ collisions by the CMS [22,17] and ALICE [23,21,18] Collaborations. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. For the $ \mathrm{OO} $ data, the inner band shows the experimental uncertainties, while the outer band includes both the experimental and $ \langle N_\mathrm{part}\rangle $ uncertainties. |
png pdf |
Figure 4-b:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity, normalized by $ \langle N_{\text{part}} \rangle $, in $ \mathrm{OO} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV shown as a function of $ \langle N_{\text{part}} \rangle $ (left) and $ \langle N_{\text{part}} \rangle/2A $ (right), where $ A $ is the mass number of the colliding nuclei. The results are compared with measurements in $ \mathrm{PbPb} $ and $ \mathrm{XeXe} $ collisions by the CMS [22,17] and ALICE [23,21,18] Collaborations. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. For the $ \mathrm{OO} $ data, the inner band shows the experimental uncertainties, while the outer band includes both the experimental and $ \langle N_\mathrm{part}\rangle $ uncertainties. |
png pdf |
Figure 5:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity in $ \mathrm{OO} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV as a function of event centrality (left) and $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ normalized to $ \langle N_{\text{part}} \rangle $ as a function of $ \langle N_{\text{part}} \rangle $ (right). The results are compared to predictions from the HIJING [39], EPOS LHC [40,25], and AMPT [47] event generators, as well as the hydrodynamic model TRAJECTUM [52] using two different nuclear density descriptions for oxygen. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. For the $ \mathrm{OO} $ data, the inner band shows the experimental uncertainties, while the outer band includes both the experimental and $ \langle N_\mathrm{part}\rangle $ uncertainties. |
png pdf |
Figure 5-a:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity in $ \mathrm{OO} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV as a function of event centrality (left) and $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ normalized to $ \langle N_{\text{part}} \rangle $ as a function of $ \langle N_{\text{part}} \rangle $ (right). The results are compared to predictions from the HIJING [39], EPOS LHC [40,25], and AMPT [47] event generators, as well as the hydrodynamic model TRAJECTUM [52] using two different nuclear density descriptions for oxygen. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. For the $ \mathrm{OO} $ data, the inner band shows the experimental uncertainties, while the outer band includes both the experimental and $ \langle N_\mathrm{part}\rangle $ uncertainties. |
png pdf |
Figure 5-b:
Charged-hadron $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity in $ \mathrm{OO} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV as a function of event centrality (left) and $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ normalized to $ \langle N_{\text{part}} \rangle $ as a function of $ \langle N_{\text{part}} \rangle $ (right). The results are compared to predictions from the HIJING [39], EPOS LHC [40,25], and AMPT [47] event generators, as well as the hydrodynamic model TRAJECTUM [52] using two different nuclear density descriptions for oxygen. The bands around the data points denote the systematic uncertainties, while the statistical uncertainties are negligible. For the $ \mathrm{OO} $ data, the inner band shows the experimental uncertainties, while the outer band includes both the experimental and $ \langle N_\mathrm{part}\rangle $ uncertainties. |
png pdf |
Figure 6:
Comparison of average $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ at midrapidity, scaled by $ \langle N_{\text{part}} \rangle $ in $ \mathrm{p} \mathrm{Pb} $ [53,32], $ \mathrm{p} \mathrm{Au} $ [54], $ \mathrm{dAu} $ [55,56,57], and central heavy ion collisions [20,22,17,58,59,60,61,16,62,63,64,65,66,23,56,67,68], as well as nonsingle-diffractive (NSD) [27,28,68,69,70,71] and inelastic [31,58,72,73] pp collisions. The data points for nucleus-nucleus ($ \mathrm{AA} $) collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 TeV have been shifted horizontally by $ \pm $10% for visibility. The dashed curves, reproduced from Ref. [32], are included to guide the eye and correspond to a power law functional form. |
| Tables | |
png pdf |
Table 1:
Centrality intervals and corresponding $ \langle N_{\text{part}} \rangle $ values for 5.36 TeV $ \mathrm{OO} $ collisions, reported up to 70% centrality, while the 0--100% interval corresponds to the fully centrality-integrated event sample. |
png pdf |
Table 2:
Sources of systematic uncertainty affecting the measurement of charged-hadron multiplicities as functions of $ \eta $ and centrality classes in $ \mathrm{OO} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. For each source, the quoted range reflects the spread of the uncertainty across individual $ \eta $ bins and centrality intervals; no averaging over $ \eta $ or centrality is implied. |
| Summary |
| The pseudorapidity ($ \eta $) distributions of charged hadrons are measured for the first time in oxygen-oxygen ($ \mathrm{OO} $) collisions at a center-of-mass collision energy per nucleon pair $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. The measurements are performed in the range $ |\eta| < $ 2.4 for multiple centrality intervals. The charged-hadron pseudorapidity density in the midrapidity region ($ |\eta| < $ 0.5) is measured to be $ \langle \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta \rangle = $ 41.8 $ \pm $ 1.1 (syst) integrated over centrality. The $ \eta $ dependence of the centrality-integrated charged-hadron pseudorapidity density is compared to predictions from the HIJING, EPOS LHC, and AMPT event generators. Among these calculations, AMPT best reproduces the absolute yields, while EPOS LHC provides the best description of the $ \eta $ dependence. For the 5% most central collisions, the charged-hadron density at midrapidity ($ |\eta| < $ 0.5) is measured to be $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta = $ 135.0 $ \pm $ 4.0 (syst). The corresponding per-participant charged-hadron density is consistent with that observed in central $ \mathrm{PbPb} $ collisions at similar collision energies, indicating that particle production per participating nucleon in central collisions remains comparable even in substantially smaller collision systems. The centrality dependence of the midrapidity results is compared with previous measurements in lead-lead ($ \mathrm{PbPb} $) collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 and 5.36 TeV and xenon-xenon ($ \mathrm{XeXe} $) collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.44 TeV. The expected system-size ordering is observed, with $ \mathrm{d} N_{\text{ch}}/\mathrm{d} \eta $ increasing from $ \mathrm{OO} $ to $ \mathrm{XeXe} $ to $ \mathrm{PbPb} $ at fixed centrality. After normalization by the maximum number of nucleons in the colliding system, the $ \mathrm{OO} $ results follow this trend approximately, but exhibit a less steep centrality dependence. At a given participant fraction, the per-participant yields are consistent between $ \mathrm{PbPb} $ and $ \mathrm{XeXe} $ collisions, while those in $ \mathrm{OO} $ collisions show a clear deviation. These observations indicate that, while the overall energy-scaling behavior observed in nucleus-nucleus collisions is preserved, the centrality dependence of normalized particle production deviates from the trends observed in larger collision systems. This behavior highlights the increased importance of collision geometry and finite-size effects in the smallest ion-ion system studied at the LHC. The hydrodynamic model TRAJECTUM provides the best overall description of the midrapidity data, particularly in central collisions. The $ \mathrm{OO} $ data provide new constraints on models of initial-state entropy production and particle production dynamics in light ion-ion collisions. |
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Compact Muon Solenoid LHC, CERN |
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