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| CMS-PAS-HIN-25-008 | ||
| Measurement of the charged particle nuclear modification factor in oxygen-oxygen collisions with CMS | ||
| CMS Collaboration | ||
| 2025-08-09 | ||
| Abstract: A hot medium, known as the quark-gluon plasma (QGP), is created in collisions of relativistic heavy nuclei such as lead or gold. Highly energetic quarks and gluons, collectively referred to as partons, lose energy as they travel through the QGP leading to suppressed production of particles with transverse momenta ($ p_{\mathrm{T}} $) of roughly 10-100 GeV. This suppression is typically quantified with the nuclear modification factor ($ R_{\text{AA}} $). Questions regarding what minimum system size is required to see parton energy loss effects remain, as no such suppression has been seen in smaller proton-lead collisions. Experiments involving light nuclei examine a domain that lies between these two extreme cases. Using 6.1 nb$^{-1} $ of oxygen-oxygen (OO) collisions and 1.02 pb$^{-1} $ of proton-proton data collected at a nucleon-nucleon center-of-mass energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.36 TeV by the CMS experiment at the CERN LHC, charged particle invariant cross sections and the OO charged particle $ R_{\text{AA}} $ are measured as a function of particle $ p_{\mathrm{T}} $ for the first time. The $ R_{\text{AA}} $ is notably suppressed, with a local minimum of 0.69 $ \pm $ 0.04 at $ p_{\mathrm{T}}= $ 6 GeV, but increases to a value of 0.97 $ \pm $ 0.06 at $ p_{\mathrm{T}}= $ 100 GeV. To evaluate if parton energy loss effects may be present in OO collisions, the data are compared to previous measurements of other collision systems and a variety of theoretical models. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, Submitted to PRL. |
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| Figures | |
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Figure 1:
Charged particle spectra for pp (black circles) and $ \text{OO} $ collisions (blue squares). Statistical uncertainties are smaller than the markers. The markers show the average cross section across the entire bin width, not the cross section value at the center of each bin. |
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Figure 2:
The charged particle $ R_{\text{AA}} $ for 5.36 TeV $ \text{OO} $ collisions (solid markers) as a function of particle $ p_{\mathrm{T}} $. Error bars represent statistical uncertainties and boxes represent systematic uncertainties not related to the luminosity calibration. The $ R_{\text{AA}} $ for $ \mathrm{p}\text{Pb} $ [19,29], $ \text{XeXe} $ [22], and PbPb [19] collisions (open markers) are also shown. Normalization uncertainties are shown by the bands on the left around unity. |
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Figure 3:
Comparison of the OO $ R_{\text{AA}} $ to various theoretical predictions [33,34,35,73,74,75]. Models in left panel are baselines that do not include parton energy loss, while models on the right include energy loss. |
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Figure 3-a:
Comparison of the OO $ R_{\text{AA}} $ to various theoretical predictions [33,34,35,73,74,75]. Models in left panel are baselines that do not include parton energy loss, while models on the right include energy loss. |
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Figure 3-b:
Comparison of the OO $ R_{\text{AA}} $ to various theoretical predictions [33,34,35,73,74,75]. Models in left panel are baselines that do not include parton energy loss, while models on the right include energy loss. |
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Figure 4:
Systematic uncertainties related to the 5.36 TeV pp spectrum (left) and $ \text{OO} $ spectrum (right). |
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Figure 4-a:
Systematic uncertainties related to the 5.36 TeV pp spectrum (left) and $ \text{OO} $ spectrum (right). |
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Figure 4-b:
Systematic uncertainties related to the 5.36 TeV pp spectrum (left) and $ \text{OO} $ spectrum (right). |
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Figure 5:
Systematic uncertainties related to measurement of $ R_{\text{AA}} $ for 5.36 TeV $ \text{OO} $ collisions. |
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Figure 6:
Comparison of the OO $ R_{\text{AA}} $ to various theoretical predictions [79,81,82,83,84,88]. Models in left panel are baselines that do not include parton energy loss, while models on the right include energy loss. |
png pdf |
Figure 6-a:
Comparison of the OO $ R_{\text{AA}} $ to various theoretical predictions [79,81,82,83,84,88]. Models in left panel are baselines that do not include parton energy loss, while models on the right include energy loss. |
png pdf |
Figure 6-b:
Comparison of the OO $ R_{\text{AA}} $ to various theoretical predictions [79,81,82,83,84,88]. Models in left panel are baselines that do not include parton energy loss, while models on the right include energy loss. |
png pdf |
Figure 7:
Comparison of the OO $ R_{\text{AA}} $ to a theoretical model [33] without energy loss effects that uses three different nPDF hypotheses [69,90,91]. |
| Summary |
| In summary, we report the first measurement of the charged particle nuclear modification factor ($ R_{\text{AA}} $) as a function of transverse momentum ($ p_{\mathrm{T}} $) in oxygen-oxygen collisions at a center of mass energy per nucleon pair of 5.36 TeV. The $ R_{\text{AA}} $ is significantly suppressed with a value of 0.69 $ \pm $ 0.04 around 6 GeV but rises to become consistent with unity at $ p_{\mathrm{T}}= $ 100 GeV. Comparisons with previous measurements of other collision systems reveal a system-size dependence of $ R_{\text{AA}} $ suppression in ion collisions. The data are more consistent with theoretical models that incorporate parton energy loss effects as compared to baseline models without energy loss, supporting the hypothesis that a medium of sufficient size to give rise to observable parton energy loss is created. This measurement constrains models attempting to predict jet quenching in small systems and represents an important first step towards the realization of a physics program examining collisions of light nuclei at TeV-scale energies. |
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