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| CMS-PAS-HIN-17-004 | ||
| Multiparticle correlations and higher order harmonics in pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV | ||
| CMS Collaboration | ||
| May 2018 | ||
| Abstract: The second- and third-order azimuthal anisotropy Fourier harmonics are studied in pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV over a wide range of event multiplicities. The second-order, "elliptic'' harmonic moment is obtained through four-, six-, and eight-particle correlations and the third-order, "triangular'' harmonic moment is studied using four-particle correlations. The data were collected by the CMS experiment during the 2016 LHC run. A sample of peripheral PbPb collisions at $\sqrt{s_{_{\text{NN}}}}= $ 5.02 TeV, covering a similar range of event multiplicities to the pPb results, is also analyzed. Theoretical predictions that assume a hydrodynamic evolution of the created medium with initial-state density fluctuations are compared to the experimental results. | ||
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CDS record (PDF) ;
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These preliminary results are superseded in this paper, PRC 101 (2020) 014912. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
The multiparticle $v_2\{4,6,8\}$ are shown for pPb 8.16 TeV (left) and PbPb 5.02 TeV (right) as a function of ${N_\mathrm {trk}^\mathrm {offline}}$. Two-particle results $v_2^{\text {sub}}\{2\}(|\Delta \eta | > 2)$ are from Ref. [38]. Error bars and shaded boxes denote statistical and systematic uncertainties, respectively. |
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Figure 2:
The multiparticle $v_3\{4\}$ are shown for pPb 8.16 TeV (left) and PbPb 5.02 TeV (right) as a function of ${N_\mathrm {trk}^\mathrm {offline}}$. Two-particle results $v_3^{\text {sub}}\{2\}(|\Delta \eta | > 2)$ are from Ref. [38]. Error bars and shaded boxes denote statistical and systematic uncertainties, respectively. The shaded area shows the hydrodynamic prediction of $v_3\{4\}$ in pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV [44]. |
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Figure 3:
The ratios of four- and two-particle harmonics ($v_2\{4\}/v_2\{2\}$ and $v_3\{4\}/v_3\{2\}$) are shown for pPb ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV (left) and PbPb $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV (right) as a function of ${N_\mathrm {trk}^\mathrm {offline}}$. Error bars and shaded boxes denote statistical and systematic uncertainties, respectively. The dashed curves show a hydrodynamics motivated initial-state fluctuation calculation for pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV [46]. |
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Figure 4:
Cumulant ratios $v_2\{6\}/v_2\{4\}$ (top) and $v_2\{8\}/v_2\{6\}$ (bottom) as a function of $v_2\{4\}/v_2\{2\}$ in pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV and 8.16 TeV. Error bars and shaded areas denote statistical and systematic uncertainties, respectively. The solid curves show the expected behavior based on a hydrodynamics motivated study of the role of initial-state fluctuations [35]. |
| Summary |
| In summary, the azimuthal anisotropy for pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV and PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV are studied as a function of the final-state particle densities by the CMS experiment. The $v_2$ Fourier coefficient is determined using cumulants obtained with four-, six-, and eight-particle correlations with greatly increased precision compared to previous measurements. The higher order $v_3\{4\}$ coefficient is reported for the first time for a small system. For pPb collisions, the ratios $v_2\{4\}/v_2\{2\}$ and $v_3\{4\}/v_3\{2\}$ are comparable, consistent with a purely fluctuation-driven origin for the azimuthal asymmetry. Both the pPb and PbPb systems have very similar $v_3$ coefficients for all cumulant orders, indicating a similar, fluctuation-driven origin. In contrast, both the magnitude of the $v_2$ coefficients and the $v_2\{4\}/v_2\{2\}$ ratio is larger for PbPb collisions, as would be expected if the global collision geometry dominates these results. The present results confirm significant non-Gaussian components to the fluctuation behavior and thereby help to explore the role of fluctuations on the initial-state eccentricities. |
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