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CMS-HIN-25-009 ; CERN-EP-2025-222
Observation of long-range collective flow in OO and NeNe collisions and implications for nuclear structure studies
Submitted to Phys. Rev. Lett.
Abstract: The long-range collective flow of particles produced in oxygen-oxygen (OO) and neon-neon (NeNe) collisions is measured with the CMS detector at the CERN LHC. The data samples were collected at a center-of-mass energy per nucleon pair of 5.36 TeV, with integrated luminosities of 7 nb$^{-1}$ and 0.8 nb$^{-1}$ for OO and NeNe collisions, respectively. Two- and four-particle azimuthal correlations are measured over nearly five units of pseudorapidity. Significant elliptic ($ v_{2} $) and triangular ($ v_{3} $) flow harmonics are observed in both systems. The ratios of $ v_{n} $ coefficients between NeNe and OO collisions reveal sensitivity to quadrupole correlations in the nuclear wave functions. Hydrodynamic models with ab initio nuclear structure inputs qualitatively reproduce the collision-overlap dependence of both the $ v_{n} $ values and the NeNe to OO ratios. These measurements provide new constraints on hydrodynamic models for small collision systems and offer valuable input on the nuclear structure of $ ^{16} $O and $ ^{20} $Ne.
Figures Summary References CMS Publications
Figures

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Figure 1:
The $ v_{2}\{2,|\Delta\eta| > 2\} $, $ v_{3}\{2,|\Delta\eta| > 2\} $ and $ v_{2}\{4\} $ values for charged particles with 0.3 $ < p_{\mathrm{T}} < $ 3.0 GeV and $ |\eta| < $ 2.4 are shown as functions of centrality in OO (left) and NeNe (right) collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.36 TeV. The $ v_{2}\{2,|\Delta\eta| > 2\} $ and $ v_{3}\{2,|\Delta\eta| > 2\} $ results after subtracting the dijet correlations (denoted as "sub") are indicated by the dot-dashed lines. Statistical uncertainties are smaller than the markers. The shaded boxes denote systematic uncertainties. Model calculations [48,93] are shown for comparison to the two-particle correlation results, where the bands represent the combined statistical and systematic uncertainties.

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Figure 2:
Ratios of $ v_{2}\{2,|\Delta\eta| > 2\} $, $ v_{2}\{4\} $ (left), and $ v_{3}\{2,|\Delta\eta| > 2\} $ (right) of NeNe to OO collisions as functions of centrality. Dot-dashed lines indicate results after subtracting (denoted as "sub") the dijet correlations. Vertical bars denote statistical uncertainties, and shaded boxes denote systematic uncertainties. Model calculations [48,93] are shown for comparison to the two-particle correlation results, where the bands represent the combined statistical and systematic uncertainties.
Summary
In summary, the azimuthal anisotropy of charged-particle emission is measured in oxygen-oxygen (OO) and neon-neon (NeNe) collisions at a center-of-mass energy per nucleon pair of 5.36 TeV with the CMS detector. The elliptic ($ v_{2} $) and triangular ($ v_{3} $) flow harmonics are extracted from two-particle correlations with a large pseudorapidity gap for multiple centrality ranges, as defined by the fraction of the total inelastic cross section, from 50% (peripheral) to 0% (central) collisions. The $ v_{2} $ values are also found using four-particle correlations to highlight how event-by-event geometry fluctuations influence the results. The $ v_{2} $ values rise from peripheral to mid-central events and decrease toward the most central collisions, and the $ v_{3} $ values increase monotonically, reflecting hydrodynamic response to the initial geometry. The ratios of the NeNe and OO $v_{n} $ values further isolate initial-geometry effects, with the $ v_{2} $ ratio increasing strongly toward central collisions, qualitatively consistent with the $ ^{20} $Ne nucleus being deformed. The $ v_{3} $ ratio is found to decrease from peripheral to the most central collisions. Hydrodynamic models with ab initio nuclear structure inputs capture the qualitative trends of both the $ v_{n} $ values and their respective NeNe to OO ratios, although they fail to fully describe the $ v_{n} $ magnitude. These results establish the sensitivity of collective flow in light ion collisions to both the initial geometry and the hydrodynamic medium response. They provide stringent constraints on models of small-system collectivity and offer valuable input to theoretical nuclear structure calculations.
References
1 E. V. Shuryak Theory of hadronic plasma Sov. Phys. JETP 47 (1978) 212
2 J. C. Collins and M. J. Perry Superdense matter: Neutrons or asymptotically free quarks? PRL 34 (1975) 1353
3 B. H. Alver, C. Gombeaud, M. Luzum, and J.-Y. Ollitrault Triangular flow in hydrodynamics and transport theory Phys. Rev. C 82 (2010) 034913 1007.5469
4 B. Schenke, S. Jeon, and C. Gale Elliptic and triangular flow in event-by-event (3+1)D viscous hydrodynamics PRL 106 (2011) 042301 1009.3244
5 Z. Qiu, C. Shen, and U. Heinz Hydrodynamic elliptic and triangular flow in Pb-Pb collisions at $ \sqrt{s}= $ 2.76ATeV PLB 707 (2012) 151 1110.3033
6 STAR Collaboration Distributions of charged hadrons associated with high transverse momentum particles in pp and Au+Au collisions at $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}} = $ 200 GeV PRL 95 (2005) 152301 nucl-ex/0501016
7 STAR Collaboration Long range rapidity correlations and jet production in high energy nuclear collisions Phys. Rev. C 80 (2009) 064912 0909.0191
8 PHOBOS Collaboration High transverse momentum triggered correlations over a large pseudorapidity acceptance in Au+Au collisions at $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}} = $ 200 GeV PRL 104 (2010) 062301 0903.2811
9 CMS Collaboration Long-range and short-range dihadron angular correlations in central PbPb collisions at a nucleon-nucleon center of mass energy of 2.76 TeV JHEP 07 (2011) 076 CMS-HIN-11-001
1105.2438
10 CMS Collaboration Centrality dependence of dihadron correlations and azimuthal anisotropy harmonics in PbPb collisions at $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}}= $ 2.76 TeV EPJC 72 (2012) 2012 CMS-HIN-11-006
1201.3158
11 ALICE Collaboration Elliptic flow of charged particles in Pb-Pb collisions at 2.76 TeV PRL 105 (2010) 252302 1011.3914
12 ATLAS Collaboration Measurement of the azimuthal anisotropy for charged particle production in $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}}= $ 2.76 TeV lead-lead collisions with the ATLAS detector Phys. Rev. C 86 (2012) 014907 1203.3087
13 CMS Collaboration Measurement of the elliptic anisotropy of charged particles produced in PbPb collisions at $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}}= $ 2.76 TeV Phys. Rev. C 87 (2013) 014902 CMS-HIN-10-002
1204.1409
14 CMS Collaboration Studies of azimuthal dihadron correlations in ultra-central PbPb collisions at $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}} = $ 2.76 TeV JHEP 02 (2014) 088 CMS-HIN-12-011
1312.1845
15 CMS Collaboration Overview of high-density QCD studies with the CMS experiment at the LHC Phys. Rept. 1115 (2025) 219 CMS-HIN-23-011
2405.10785
16 J.-Y. Ollitrault Anisotropy as a signature of transverse collective flow PRD 46 (1992) 229
17 U. Heinz and R. Snellings Collective flow and viscosity in relativistic heavy-ion collisions Ann. Rev. Nucl. Part. Sci. 63 (2013) 123 1301.2826
18 C. Gale, S. Jeon, and B. Schenke Hydrodynamic modeling of heavy ion collisions Int. J. Mod. Phys. A 28 (2013) 1340011 1301.5893
19 CMS Collaboration Observation of long-range near-side angular correlations in proton-proton collisions at the LHC JHEP 09 (2010) 091 CMS-QCD-10-002
1009.4122
20 ATLAS Collaboration Observation of long-range elliptic azimuthal anisotropies in $ \sqrt{s}= $ 13 and 2.76 TeV pp collisions with the ATLAS detector PRL 116 (2016) 172301 1509.04776
21 CMS Collaboration Measurement of long-range near-side two-particle angular correlations in pp collisions at $ \sqrt{s}= $ 13 TeV PRL 116 (2016) 172302 CMS-FSQ-15-002
1510.03068
22 CMS Collaboration Evidence for collectivity in pp collisions at the LHC PLB 765 (2017) 193 CMS-HIN-16-010
1606.06198
23 CMS Collaboration Studies of charm and beauty hadron long-range correlations in pp and pPb collisions at LHC energies PLB 813 (2021) 136036 CMS-HIN-19-009
2009.07065
24 CMS Collaboration Observation of long-range near-side angular correlations in proton-lead collisions at the LHC PLB 718 (2013) 795 CMS-HIN-12-015
1210.5482
25 ALICE Collaboration Long-range angular correlations on the near and away side in pPb collisions at $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}}= $ 5.02 TeV PLB 719 (2013) 29 1212.2001
26 ATLAS Collaboration Observation of associated near-side and away-side long-range correlations in $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}}= $ 5.02 TeV proton-lead collisions with the ATLAS detector PRL 110 (2013) 182302 1212.5198
27 LHCb Collaboration Measurements of long-range near-side angular correlations in $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}}= $ 5 TeV proton-lead collisions in the forward region PLB 762 (2016) 473 1512.00439
28 CMS Collaboration Elliptic flow of charm and strange hadrons in high-multiplicity pPb collisions at $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}}= $ 8.16 TeV PRL 121 (2018) 082301 CMS-HIN-17-003
1804.09767
29 CMS Collaboration Observation of prompt J/$ \psi $ meson elliptic flow in high-multiplicity pPb collisions at $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}}= $ 8.16 TeV PLB 791 (2019) 172 CMS-HIN-18-010
1810.01473
30 CMS Collaboration Strange hadron collectivity in pPb and PbPb collisions JHEP 05 (2023) 007 CMS-HIN-19-004
2205.00080
31 PHENIX Collaboration Measurement of long-range angular correlation and quadrupole anisotropy of pions and (anti)protons in central $ d+ $Au collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 200 GeV PRL 114 (2015) 192301 1404.7461
32 PHENIX Collaboration Measurements of elliptic and triangular flow in high-multiplicity $ ^{3} $He$ + $Au collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 200 GeV PRL 115 (2015) 142301 1507.06273
33 PHENIX Collaboration Measurement of long-range angular correlations and azimuthal anisotropies in high-multiplicity $ p+\mathrm{Au} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 200 GeV Phys. Rev. C 95 (2017) 034910 1609.02894
34 PHENIX Collaboration Measurements of mass-dependent azimuthal anisotropy in central $ p+\mathrm{Au} $, $ d+\mathrm{Au} $, and $ ^{3}\mathrm{He}+\mathrm{Au} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 200 GeV Phys. Rev. C 97 (2018) 064904 1710.09736
35 PHENIX Collaboration Creation of quark-gluon plasma droplets with three distinct geometries Nature Phys. 15 (2019) 214 1805.02973
36 PHENIX Collaboration Kinematic dependence of azimuthal anisotropies in $ p+\mathrm{Au} $, $ d+\mathrm{Au} $, and $ ^{3}\mathrm{He}+\mathrm{Au} $ at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 200 GeV Phys. Rev. C 105 (2022) 024901 2107.06634
37 STAR Collaboration Long-range pseudorapidity dihadron correlations in $ d $+Au collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 200 GeV PLB 747 (2015) 265 1502.07652
38 STAR Collaboration Azimuthal harmonics in small and large collision systems at RHIC top energies PRL 122 (2019) 172301 1901.08155
39 STAR Collaboration Measurements of the elliptic and triangular azimuthal anisotropies in central $ ^{3}\mathrm{He}+\mathrm{Au} $, $ d+\mathrm{Au} $ and $ p+\mathrm{Au} $ collisions at $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}}= $ 200 GeV PRL 130 (2023) 242301 2210.11352
40 STAR Collaboration Measurement of flow coefficients in high-multiplicity $ p+\mathrm{Au} $, $ d+\mathrm{Au} $, and $ ^{3}\mathrm{He}+\mathrm{Au} $ collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 200 GeV Phys. Rev. C 110 (2024) 064902 2312.07464
41 J. L. Nagle and W. A. Zajc Small system collectivity in relativistic hadronic and nuclear collisions Ann. Rev. Nucl. Part. Sci. 68 (2018) 211 1801.03477
42 K. Dusling, W. Li, and B. Schenke Novel collective phenomena in high-energy proton-proton and proton-nucleus collisions Int. J. Mod. Phys. E 25 (2016) 1630002 1509.07939
43 CMS Collaboration Evidence for collective multiparticle correlations in pPb collisions PRL 115 (2015) 012301 CMS-HIN-14-006
1502.05382
44 CMS Collaboration Multiparticle correlation studies in pPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $8.16 TeV Phys. Rev. C 101 (2020) 014912 CMS-HIN-17-004
1904.11519
45 B. Schenke The smallest fluid on earth Rept. Prog. Phys. 84 (2021) 082301 2102.11189
46 W. Zhao, S. Ryu, C. Shen, and B. Schenke 3D structure of anisotropic flow in small collision systems at energies available at the BNL Relativistic Heavy Ion Collider Phys. Rev. C 107 (2023) 014904 2211.16376
47 P. Bozek and W. Broniowski The torque effect and fluctuations of entropy deposition in rapidity in ultra-relativistic nuclear collisions PLB 752 (2016) 206 1506.02817
48 G. Giacalone et al. Exploiting $ ^{20}\mathrm{Ne} $ isotopes for precision characterizations of collectivity in small systems PRL 135 (2025) 012302 2402.05995
49 CMS Collaboration Charged-particle angular correlations in XeXe collisions at $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}}= $ 5.44 TeV Phys. Rev. C 100 (2019) 044902 CMS-HIN-18-001
1901.07997
50 H. Li et al. Probing the neutron skin with ultrarelativistic isobaric collisions PRL 125 (2020) 222301 1910.06170
51 H.-j. Xu et al. Probing nuclear structure with mean transverse momentum in relativistic isobar collisions Phys. Rev. C 108 (2023) L011902 2111.14812
52 C. Zhang and J. Jia Evidence of quadrupole and octupole deformations in $ ^{96}\mathrm{Zr} $+$ ^{96}\mathrm{Zr} $ and $ ^{96}\mathrm{Ru} $+$ ^{96}\mathrm{Ru} $ collisions at ultrarelativistic energies PRL 128 (2022) 022301 2109.01631
53 W. Ryssens, G. Giacalone, B. Schenke, and C. Shen Evidence of hexadecapole deformation in uranium-238 at the Relativistic Heavy Ion Collider PRL 130 (2023) 212302 2302.13617
54 H.-j. Xu, J. Zhao, and F. Wang Hexadecapole deformation of $ ^{238}\mathrm{U} $ from relativistic heavy-ion collisions using a nonlinear response coefficient PRL 132 (2024) 262301 2402.16550
55 STAR Collaboration Imaging shapes of atomic nuclei in high-energy nuclear collisions Nature 635 (2024) 67 2401.06625
56 G. Giacalone et al. Anisotropic flow in fixed-target $ ^{208}\mathrm{Pb} $+$ ^{20}\mathrm{Ne} $ collisions as a probe of quark-gluon plasma PRL 134 (2025) 082301 2405.20210
57 D. Robson Evidence for the tetrahedral nature of $ ^{16}\mathrm{O} $ PRL 42 (1979) 876
58 L. Pauling Comment on the test for tetrahedral symmetry in the $ ^{16}\mathrm{O} $ nucleus and its relation to the shell model PRL 49 (1982) 1119
59 R. Bijker and F. Iachello Evidence for tetrahedral symmetry in $ ^{16}\mathrm{O} $ PRL 112 (2014) 152501 1403.6773
60 S. Marcos, H. Flocard, and P. H. Heenen Influence of left-right asymmetry degrees of freedom in self-consistent calculations of $ ^{20}\mathrm{Ne} $ Nucl. Phys. A 410 (1983) 125
61 L. M. Robledo and G. F. Bertsch Global systematics of octupole excitations in even-even nuclei Phys. Rev. C 84 (2011) 054302 1107.3581
62 P. Marevicet al. Quadrupole and octupole collectivity and cluster structures in neon isotopes Phys. Rev. C 97 (2018) 024334 1802.02873
63 S. H. Lim et al. Exploring new small system geometries in heavy ion collisions Phys. Rev. C 99 (2019) 044904 1812.08096
64 E. Epelbaum et al. Ab initio calculation of the spectrum and structure of $ ^{16}\mathrm{O} $ PRL 112 (2014) 102501 1312.7703
65 S. C. Pieper, K. Varga, and R. B. Wiringa Quantum Monte Carlo calculations of A=9, A=10 nuclei Phys. Rev. C 66 (2002) 044310 nucl-th/0206061
66 D. Lee Lattice simulations for few- and many-body systems Prog. Part. Nucl. Phys. 63 (2009) 117 0804.3501
67 CMS Collaboration HEPData record for this analysis link
68 ALICE Collaboration Evidence of nuclear geometry-driven anisotropic flow in OO and Ne-Ne collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.36 TeV Submitted to Phys. Rev. Lett, 2025 2509.06428
69 ATLAS Collaboration Measurement of the azimuthal anisotropy of charged particles in $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.36 TeV $ ^{16} $O$ +^{16} $O and $ ^{20} $Ne$ +^{20} $Ne collisions with the ATLAS detector Submitted to Phys. Rev. C, 2025 2509.05171
70 CMS Collaboration Description and performance of track and primary-vertex reconstruction with the CMS tracker JINST 9 (2014) P10009 CMS-TRK-11-001
1405.6569
71 CMS Collaboration The CMS experiment at the CERN LHC JINST 3 (2008) S08004
72 CMS Collaboration Development of the CMS detector for the CERN LHC Run 3 JINST 19 (2024) P05064 CMS-PRF-21-001
2309.05466
73 CMS Collaboration Observation and studies of jet quenching in PbPb collisions at nucleon-nucleon center-of-mass energy $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}} = $ 2.76 TeV Phys. Rev. C 84 (2011) 024906 CMS-HIN-10-004
1102.1957
74 M. L. Miller, K. Reygers, S. J. Sanders, and P. Steinberg Glauber modeling in high energy nuclear collisions Ann. Rev. Nucl. Part. Sci. 57 (2007) 205 nucl-ex/0701025
75 C. Loizides Glauber predictions for oxygen and neon collisions at LHC Submitted to Phys. Rev. C, 2025 2507.05853
76 CMS Collaboration Multiplicity and transverse momentum dependence of two- and four-particle correlations in pPb and PbPb collisions PLB 724 (2013) 213 CMS-HIN-13-002
1305.0609
77 CMS Collaboration Long-range two-particle correlations of strange hadrons with charged particles in pPb and PbPb collisions at LHC energies PLB 742 (2015) 200 CMS-HIN-14-002
1409.3392
78 X.-N. Wang and M. Gyulassy HIJING: A Monte Carlo model for multiple jet production in pp, $ \mathrm{p}\text{A} $ and $ \text{AA} $ collisions PRD 44 (1991) 3501
79 GEANT4 Collaboration GEANT 4---a simulation toolkit NIM A 506 (2003) 250
80 A. Bilandzic, R. Snellings, and S. Voloshin Flow analysis with cumulants: Direct calculations Phys. Rev. C 83 (2011) 044913 1010.0233
81 Z.-W. Lin et al. A multi-phase transport model for relativistic heavy ion collisions Phys. Rev. C 72 (2005) 064901 nucl-th/0411110
82 I. A. Pshenichnov et al. Nuclear multifragmentation induced by electromagnetic fields of ultrarelativistic heavy ions Phys. Rev. C 57 (1998) 1920 nucl-th/9711030
83 S. J. Das and A. Baty A data-driven method to estimate contamination from light ion beam transmutation at colliders 2509.09736
84 CMS Collaboration Measurement of higher-order harmonic azimuthal anisotropy in PbPb collisions at $ {\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}} = $ 2.76 TeV Phys. Rev. C 89 (2014) 044906 CMS-HIN-11-005
1310.8651
85 J.-Y. Ollitrault, A. M. Poskanzer, and S. A. Voloshin Effect of flow fluctuations and nonflow on elliptic flow methods Phys. Rev. C 80 (2009) 014904 0904.2315
86 G. Nijs, W. van der Schee, U. Gursoy, and R. Snellings Bayesian analysis of heavy ion collisions with the heavy ion computational framework Trajectum Phys. Rev. C 103 (2021) 054909 2010.15134
87 G. Nijs and W. van der Schee Predictions and postdictions for relativistic lead and oxygen collisions with the computational simulation code Trajectum Phys. Rev. C 106 (2022) 044903 2110.13153
88 T. A. Lähde and U.-G. Meißner Nuclear Lattice Effective Field Theory: An introduction volume 957. Springer, ISBN 978-3-030-14187-5, 978-3-030-14189-9, 2019
link
89 D. Lee Recent progress in nuclear lattice simulations Front. Phys. 8 (2020) 174
90 M. Frosini et al. In-medium $ k $-body reduction of $ n $-body operators: A flexible symmetry-conserving approach based on the sole one-body density matrix Eur. Phys. J. A 57 (2021) 151 2102.10120
91 J. M. Yao et al. Ab initio treatment of collective correlations and the neutrinoless double beta decay of $ ^{48} $Ca PRL 124 (2020) 232501 1908.05424
92 J. S. Moreland, J. E. Bernhard, and S. A. Bass Alternative ansatz to wounded nucleon and binary collision scaling in high-energy nuclear collisions Phys. Rev. C 92 (2015) 011901 1412.4708
93 H. Mäntysaari, B. Schenke, C. Shen, and W. Zhao Collision-energy dependence in heavy-ion collisions from nonlinear QCD evolution PRL 135 (2025) 022302 2502.05138
94 B. Schenke, S. Jeon, and C. Gale (3+1)D hydrodynamic simulation of relativistic heavy-ion collisions Phys. Rev. C 82 (2010) 014903 1004.1408
95 S. A. Bass et al. Microscopic models for ultrarelativistic heavy ion collisions Prog. Part. Nucl. Phys. 41 (1998) 255 nucl-th/9803035
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