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| CMS-HIN-17-004 ; CERN-EP-2019-075 | ||
| Multiparticle correlation studies in pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ = 8.16 TeV | ||
| CMS Collaboration | ||
| 25 April 2019 | ||
| Phys. Rev. C 101 (2020) 014912 | ||
| Abstract: The second- and third-order azimuthal anisotropy Fourier harmonics of charged particles produced in pPb collisions, at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV, are studied over a wide range of event multiplicities. Multiparticle correlations are used to isolate global properties stemming from the collision overlap geometry. The second-order "elliptic'' harmonic moment is obtained with high precision through four-, six-, and eight-particle correlations and, for the first time, the third-order "triangular'' harmonic moment is studied using four-particle correlations. A sample of peripheral PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV that covers a similar range of event multiplicities as the pPb results is also analyzed. Model calculations of initial-state fluctuations in pPb and PbPb collisions can be directly compared to the high precision experimental results. This work provides new insight into the fluctuation-driven origin of the $v_3$ coefficients in pPb and PbPb collisions, and into the dominating overall collision geometry in PbPb collisions at the earliest stages of heavy ion interactions. | ||
| Links: e-print arXiv:1904.11519 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The multiparticle $v_2\{4,6,8\}$ and $v_3\{4\}$ harmonics are shown for pPb 8.16 TeV (left) and PbPb 5.02 TeV (right) as a function of ${N_\text {trk}^\text {offline}}$. Two-particle results $v_2^{\text {sub}}\{2\}({<} \Delta \eta {>} > 2)$ and $v_3^{\text {sub}}\{2\}({<} \Delta \eta {>} > 2)$ are from Ref. [47]. 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 [53]. |
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Figure 2:
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 ${N_\text {trk}^\text {offline}}$. Error bars and shaded boxes denote statistical and systematic uncertainties, respectively. The dashed curves show a hydrodynamics-motivated initial-state fluctuation calculation of eccentricities $\varepsilon _{n}\{m\}$ for pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV [42]. |
png pdf |
Figure 3:
Cumulant ratios $v_2\{6\}/v_2\{4\}$ (upper) and $v_2\{8\}/v_2\{6\}$ (lower) as a function of $v_2\{4\}/v^{\text {sub}}_2\{2\}$ 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 [40]. |
| 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 functions of the final-state particle multiplicities with 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 the cumulant orders studied, indicating a similar, fluctuation-driven initial-state geometry. 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 expected if the overall collision geometry dominates. The $v_2$ cumulant ratios for pPb collisions are consistent with a collective flow behavior that originates from and is proportional to the initial-state anisotropy. |
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