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| CMS-PAS-HIN-26-005 | ||
| Azimuthal anisotropy of charged particles with transverse momentum up to 50 GeV in oxygen-oxygen, neon-neon, and lead-lead collisions | ||
| CMS Collaboration | ||
| 2026-07-23 | ||
| Abstract: The azimuthal anisotropy of charged particles with transverse momenta ranging from 0.3 to 50 GeV is measured in oxygen-oxygen (OO), neon-neon (NeNe), and lead-lead (PbPb) collisions at a nucleon-nucleon center-of-mass energy of $ \sqrt{s_{_\mathrm{NN}}} = $ 5.36 TeV with the CMS detector at the CERN LHC. The analyzed data samples correspond to integrated luminosities of 8.10 $ \mathrm{nb}^{-1} $ for OO, 0.76 $ \mathrm{nb}^{-1} $ for NeNe, and 1.35 $ \mathrm{nb}^{-1} $ for PbPb collisions. The anisotropy coefficients $ v_{2} $ and $ v_{3} $ are extracted using the scalar-product method for different pseudorapidity separations $ \Delta\eta $ among the produced particles in order to suppress short-range nonflow correlations. These coefficients are measured as functions of $ p_\mathrm{T} $ and collision centrality. To estimate residual nonflow contributions, the same analysis is performed using zero-bias proton-proton collisions at the same center-of-mass energy. Results obtained with and without subtraction of nonflow contributions are presented. Comparing these coefficients across distinct collision systems provides a unique way to probe how the system size and geometry influence the creation, lifetime, and initial-state fluctuations of the quark-gluon plasma. The measured high-$ p_\mathrm{T} $ anisotropies are compared with predictions from parton energy loss models. These measurements provide new insights into the underlying mechanisms of high-$ p_\mathrm{T} $ azimuthal anisotropy observed in small partonic systems. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Scalar product $ v_2(p_{\mathrm{T}}) $ distribution measured in $ \text{OO} $ collisions in six centrality ranges from 0--10% to 50--60%, for $ |\Delta\eta| > $ 3 (red filled circles) and 4 (blue open squares) separations. Vertical bars denote statistical uncertainties and shaded boxes denote systematic uncertainties. |
png pdf |
Figure 2:
Scalar product $ v_3(p_{\mathrm{T}}) $ distribution measured in $ \text{OO} $ collisions in five centrality ranges from 0--10% to 40--50%, for $ |\Delta\eta| > $ 3 (red filled circles) and 4 (blue open squares) separations. Vertical bars denote statistical uncertainties and shaded boxes denote systematic uncertainties. |
png pdf |
Figure 3:
Distribution of azimuthal accumulative correlations $ \left\langle {{q_2}Q_{2A}^*} \right\rangle(p_{\mathrm{T}}) $ measured in $ \text{OO} $ collisions in six centrality ranges from 0--10% to 50--60%, for $ |\Delta\eta| > $ 1 (red filled circles) and 4 (blue open crosses) separations. The distributions measured in pp zero-bias events for $ |\Delta\eta| > $ 1 (black open circles) and 4 (black open squares) separations are added in all plots for comparison. Vertical bars denote statistical uncertainties and shaded boxes denote systematic uncertainties. |
png pdf |
Figure 4:
Distribution of azimuthal accumulative correlations $ \left\langle {{q_3}Q_{3A}^*} \right\rangle(p_{\mathrm{T}}) $ measured in $ \text{OO} $ collisions in six centrality ranges from 0--10% to 50--60%, for $ |\Delta\eta| > $ 1 (red filled circles) and 4 (blue open crosses) separations. The distributions measured in pp zero-bias events for $ |\Delta\eta| > $ 1 (black open circles) and 4 (black open squares) are added in all plots for comparison. Vertical bars denote statistical uncertainties and shaded boxes denote systematic uncertainties. |
png pdf |
Figure 5:
Scalar product $ v_2(p_{\mathrm{T}}) $ distribution measured in $ \text{OO} $ collisions in six centrality ranges from 0--10% to 50--60%. Black open circles and open squares represent results without pp baseline nonflow subtraction for $ |\Delta\eta| > $ 4 and 4.5, respectively. Red filled circles and blue open crosses represent results with pp baseline nonflow subtraction for $ |\Delta\eta| > $ 4 and 4.5, respectively. Vertical bars denote statistical uncertainties and shaded boxes denote systematic uncertainties. |
png pdf |
Figure 6:
Scalar product $ v_2 $ distribution measured in $ \text{OO} $ collisions as a function of $ \Delta\eta $ separation, in six centrality ranges from 0--10% to 50--60%. Black circle markers represent $ v_{2} $ extracted for 10 $ < p_{\mathrm{T}} < $ 12 GeV; black open circles are without pp subtraction, while black filled circles are with pp subtraction. Red square markers represent $ v_{2} $ extracted for 20 $ < p_{\mathrm{T}} < $ 30 GeV; red open squares are without pp subtraction, while red filled squares are with pp subtraction. Vertical bars denote statistical uncertainties and shaded boxes denote systematic uncertainties. |
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Figure 7:
Scalar product $ v_2(p_{\mathrm{T}}) $ distribution (red filled circles) measured in $ \text{OO} $ collisions, after pp baseline nonflow subtraction, in six centrality ranges from 0--10% to 50--60% using the $ |\Delta\eta| > $ 4 separation. Vertical bars denote statistical uncertainties and shaded boxes denote systematic uncertainties. The blue bands represent the NLO pQCD parton model predictions with median value of $ \hat{q}/T^3 $ from Bayesian extraction [54]. The red bands represent a coherent energy loss model within a semi-analytic jet quenching framework incorporating pre-equilibrium effects [55]. The black curves with gray uncertainty bands represent a pQCD-based model including hard-soft geometric decorrelation effects [56]. The dashed and dotted lines represent the calculations from the $ \textrm{Angantyr} $ [57] and HIJING [46] models, respectively. |
png pdf |
Figure 8:
Scalar product $ v_3(p_{\mathrm{T}}) $ distribution measured in $ \text{OO} $ collisions in five centrality ranges from 0--10% to 40--50%. Black open circles and open squares represent results without pp baseline nonflow subtraction for $ |\Delta\eta| > $ 4 and 4.5, respectively. Red filled circles ($ |\Delta\eta| > $ 4) and blue open crosses ($ |\Delta\eta| > $ 4.5) represent results with pp baseline nonflow subtraction. Vertical bars denote statistical uncertainties and shaded boxes denote systematic uncertainties. |
png pdf |
Figure 9:
Scalar product $ v_2(p_{\mathrm{T}}) $ distribution measured in $ \text{OO} $ and $ \text{NeNe} $ collisions in six centrality ranges from 0--10% to 50--60% using a $ |\Delta\eta| > $ 4 separation. Results for $ \text{OO} $ collisions are shown by blue open squares (without pp baseline nonflow subtraction) and blue filled squares (with subtraction). Results for $ \text{NeNe} $ collisions are shown by red open circles (without subtraction) and red filled circles (with subtraction). Vertical bars denote statistical uncertainties and shaded boxes denote systematic uncertainties. |
png pdf |
Figure 10:
Scalar product $ v_2(p_{\mathrm{T}}) $ distribution measured using a $ |\Delta\eta| > $ 4 separation in twelve PbPb centrality ranges from 0--10% to 76--80% and four $ \text{OO} $ centrality ranges from 0--10% to 30--40%. Results for $ \text{OO} $ collisions are shown by blue open squares (without pp baseline nonflow subtraction) and blue filled squares (with subtraction). Results for PbPb collisions are shown by red open circles (without subtraction) and red filled circles (with subtraction). Vertical bars denote statistical uncertainties and shaded boxes denote systematic uncertainties. The blue and red bands represent the NLO pQCD parton model predictions in $ \text{OO} $ and PbPb collisions with median value of $ \hat{q}/T^3 $ from Bayesian extraction [54]. |
| Summary |
| The elliptic ($ v_2 $) and triangular ($ v_3 $) azimuthal anisotropy coefficients of charged particles produced in oxygen-oxygen ($ \text{OO} $), neon-neon ($ \text{NeNe} $), and lead-lead (PbPb) collisions at a nucleon-nucleon center-of-mass energy of $ \sqrt{s_{_\mathrm{NN}}} = 5.36 \text{TeV} $ have been measured by the CMS detector at the CERN LHC, as functions of transverse momentum $ p_{\mathrm{T}} = $ 0.3--50 GeV and collision centrality. The anisotropy coefficients $ v_{2} $ and $ v_{3} $ are extracted using the scalar-product method for different pseudorapidity separations ($ \Delta\eta $) among the produced particles in order to suppress short-range nonflow correlations. Prior to nonflow subtraction, the measurements present a significant sensitivity to the $ \Delta\eta $ separation used for their derivation. For $ \text{OO} $ collisions, increasing the separation from 3 to 4 reduces the $ v_{2} $ values by more than half for $ p_{\mathrm{T}} > $ 20 GeV and also drives $ v_{3}(p_{\mathrm{T}}) $ closer to zero, highlighting the substantial influence of short-range nonflow correlations on high-$ p_{\mathrm{T}} $ anisotropy in light-ion collision systems. To account for nonflow effects, a comparison of the azimuthal accumulative correlations, $ \langle q_{n}Q_{nA}^{*} \rangle $ per trigger particle, has been performed between $ \text{OO} $ and zero-bias pp collisions at the same center-of-mass energy. The correlations from the two systems approach one another at high $ p_{\mathrm{T}} $ across all centrality ranges. In particular, the large negative values observed for $ \langle q_{3}Q_{3A}^{*} \rangle $ in $ \text{OO} $ collisions are well reproduced by the pp baseline, suggesting that per-trigger-particle nonflow effects in light nuclear environments are consistent with those in more elementary pp interactions. Consequently, the final $ v_{n} $ coefficients have been extracted utilizing a pp-baseline nonflow subtraction procedure. While the unsubtracted signals depend heavily on the $ \Delta\eta $ separation size, the results from different separations converge after subtraction, indicating that residual nonflow contributions are greatly reduced. The measured $ v_2(p_{\mathrm{T}}) $ distributions have been compared with perturbative parton energy loss calculations including a jet transport parameter $ \hat{q}/T^{3} $ constrained via Bayesian inference. The extracted anisotropies agree well with the model predictions for $ p_{\mathrm{T}} > $ 10 GeV. Our measurements are further compared with calculations from a coherent energy loss framework incorporating pre-equilibrium effects, which systematically underpredict the observed anisotropy in 0--20% collisions. Furthermore, comparison with a model incorporating hard-soft geometric decorrelation reveals that the data significantly exceed the prediction of a vanishing $ v_2 $, indicating that a distinct azimuthal correlation between high-$ p_{\mathrm{T}} $ partons and the bulk event plane is preserved. Finally, the $ v_2(p_{\mathrm{T}}) $ measurements in $ \text{OO} $ collisions are compared with those in $ \text{NeNe} $ collisions within matching centrality intervals, as well as with PbPb data at equivalent values of average participating nucleons, to systematically investigate the system-size dependence of the high-$ p_{\mathrm{T}} v_2 $ behaviour. Together, these results provide valuable new insights into the underlying mechanisms driving high-$ p_{\mathrm{T}} $ azimuthal anisotropy in small and light-ion nuclear collision systems. |
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