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| CMS-HIN-18-001 ; CERN-EP-2018-345 | ||
| Charged-particle angular correlations in XeXe collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.44 TeV | ||
| CMS Collaboration | ||
| 23 January 2019 | ||
| Phys. Rev. C 100 (2019) 044902 | ||
| Abstract: Azimuthal correlations of charged particles in xenon-xenon collisions at a center-of-mass energy per nucleon pair of $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.44 TeV are studied. The data were collected by the CMS experiment at the LHC with a total integrated luminosity of 3.42 $\mu$b$^{-1}$. The collective motion of the system formed in the collision is parameterized by a Fourier expansion of the azimuthal particle density distribution. The azimuthal anisotropy coefficients $v_{2}$, $v_{3}$, and $v_{4}$ are obtained by the scalar-product, two-particle correlation, and multi-particle correlation methods. Within a hydrodynamic picture, these methods have different sensitivities to non-collective and fluctuation effects. The dependence of the Fourier coefficients on the size of the colliding system is explored by comparing the xenon-xenon results with equivalent lead-lead data. Model calculations that include fluctuation effects are also compared to the experimental results. The observed angular correlations provide new constraints on the hydrodynamic description of heavy ion collisions. | ||
| Links: e-print arXiv:1901.07997 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Elliptic-flow coefficients, $v_2$, based on different analysis techniques, as functions of transverse momentum and in bins of centrality, from the 5% most central (top left) to 60-70% centrality (bottom right). The results for the two-particle and multi-particle correlations correspond to the range $ {| \eta |} < $ 2.4, while the SP results are for $ {| \eta |} < $ 0.8. The bars and the shaded boxes represent statistical and systematic uncertainties, respectively. |
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Figure 2:
Triangular-flow coefficients, $v_3$, based on the different analysis techniques, as functions of transverse momentum and in bins of centrality, from the 5% most central (top left) to 60-70% centrality (bottom right). The results for the two-particle and multi-particle correlations correspond to the range $ {| \eta |} < $ 2.4, while the SP results are for $ {| \eta |} < $ 0.8. The bars and the shaded boxes represent statistical and systematic uncertainties, respectively. |
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Figure 3:
The $v_4$ coefficients, based on the different analysis techniques, as functions of transverse momentum and in bins of centrality, from the 5% most central (top left) to 60-70% centrality (bottom right). The results for the two-particle correlations correspond to the range $ {| \eta |} < $ 2.4, while the SP results are for $ {| \eta |} < $ 0.8. The bars and the shaded boxes represent statistical and systematic uncertainties, respectively. |
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Figure 4:
Centrality dependence of the spectrum-weighted $v_2$, $v_3$, and $v_4$ flow harmonics with 0.3 $ < {p_{\mathrm {T}}} < $ 3.0 GeV/$c$. The $v_2$ results are shown for 2-, 4-, 6-, and 8-particle correlations (left panel). The $v_3$ results are shown for 2- and 4-particle correlations (middle panel), while the $v_4$ values are presented for two-particle correlations technique, only. The magenta line in each panel is the IP-Glasma+music+UrQMD prediction for $v_n\{2, {| \Delta \eta |} > 2\}$. The shaded boxes represent systematic uncertainties. |
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Figure 5:
Centrality dependence of $v_2\{4\}/v_2\{2\}$, $v_2\{6\}/v_2\{4\}$ (left panel) and $v_3\{4\}/v_3\{2\}$ (right panel) ratios. The colored areas represent the theoretical predictions based on the IP-Glasma+music+UrQMD and the relativistic hydrodynamic model from Ref. [35] considering both spherical and deformed xenon nuclei, while the widths of the areas show the statistical uncertainties of the model. The $\mathrm {T_RENTo}$ calculation is done for the ${p_{\mathrm {T}}}$ range 0.2 $ < {p_{\mathrm {T}}} < $ 5.0 GeV/$c$. |
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Figure 6:
Comparison of the $v_2$ results measured with two-particle correlations from two different systems, XeXe collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.44 TeV and PbPb collisions at 5.02 TeV, shown as a function of ${p_{\mathrm {T}}}$ in eleven centrality bins. The bars (smaller than the marker size) and the shaded boxes represent statistical and systematic uncertainties, respectively. |
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Figure 7:
Comparison of the $v_3$ results measured with two-particle correlations from two different systems, XeXe collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.44 TeV and PbPb collisions at 5.02 TeV, shown as a function of ${p_{\mathrm {T}}}$ in eleven centrality bins. The bars (smaller than the marker size) and the shaded boxes represent statistical and systematic uncertainties, respectively. |
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Figure 8:
Comparison of the $v_4$ results measured with two-particle correlations from two different systems, XeXe collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.44 TeV and PbPb collisions at 5.02 TeV, shown as a function of ${p_{\mathrm {T}}}$ in eleven centrality bins. The bars and the shaded boxes represent statistical and systematic uncertainties, respectively. |
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Figure 9:
Centrality dependence of the spectrum-weighted $v_2$, $v_3$, and $v_4$ harmonic coefficients from two-particle correlations method for 0.3 $ < {p_{\mathrm {T}}} < $ 3.0 GeV/$c$ for XeXe collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.44 TeV and PbPb collisions at 5.02 TeV. The lower panels show the ratio of the results for the two systems. The bars and the shaded boxes represent statistical and systematic uncertainties, respectively. Theoretical predictions from Ref. [35] are compared to the data (colored area). The model calculation is done for the ${p_{\mathrm {T}}}$ range 0.2 $ < {p_{\mathrm {T}}} < $ 5.0 GeV/$c$. |
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Figure 10:
Ratios of the $v_2$, $v_3$, and $v_4$ harmonic coefficients from two-particle correlations in XeXe and PbPb collisions as functions of ${p_{\mathrm {T}}}$ in eleven centrality bins. The bars and the shaded boxes represent statistical and systematic uncertainties, respectively. |
| Summary |
| In this paper, the $v_2$, $v_3$, and $v_4$ azimuthal flow harmonics are shown for xenon-xenon (XeXe) collisions at a center-of-mass energy per nucleon pair of ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.44 TeV based on data obtained with the CMS detector. Three analysis techniques with different sensitivities to flow fluctuations, including two-particle correlations, the scalar-product method, and the multi-particle cumulant method, are used to explore the event-by-event fluctuations. The harmonic coefficients are compared to those found with lead-lead (PbPb) collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV to explore the effect of the system size. The magnitude of the $v_2$ coefficients for XeXe collisions are larger than those found in PbPb collisions for the most central collisions. This is attributed to a larger fluctuation component in the lighter colliding system. In more peripheral events, the PbPb $v_n$ coefficients are consistently larger than those found for XeXe collisions. This behavior is qualitatively consistent with expectations from hydrodynamic models. A clear ordering $v_2\{2\} > v_2\{4\} \approx v_2\{6\} \approx v_2\{8\}$ is observed for XeXe collisions, with $v_2\{6\}$ and $v_2\{4\}$ values differing by 2-3%. The $v_3\{4\} / v_3\{2\}$ ratio is found to be significantly smaller than the $v_2\{4\} / v_2\{2\}$ ratio, suggesting a dominant fluctuation component for the $v_3$ harmonic. Hydrodynamic models that consider the xenon nuclear deformation are able to better describe the $v_{2}[{\mathrm{XeXe}}]/v_{2}[{\mathrm{PbPb}}]$ ratio in central collisions than those assuming a spherical Xe shape, although the deformation appears to have little effect on the fluctuation-sensitive ratio of the cumulant orders. These measurements provide new tests of hydrodynamic models and help to constrain hydrodynamic descriptions of the nuclear collisions. |
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