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| CMS-PAS-HIN-16-022 | ||
| Correlations of azimuthal anisotropy harmonics in pp, pPb, and PbPb collisions at the LHC | ||
| CMS Collaboration | ||
| February 2017 | ||
| Abstract: The first measurements of anisotropy harmonics ($v_n$, $n=$ 2-4) in pPb collisions at $\sqrt{s_{\mathrm{NN}}} = $ 8.16 TeV is presented. In addition, measurements of event-by-event correlations of different $v_n$ in pp at $\sqrt{s} = $ 13 TeV, pPb at $\sqrt{s_{\mathrm{NN}}} = $ 5.02 and 8.16 TeV and PbPb at $\sqrt{s_{\mathrm{NN}}} = $ 5.02 TeV collision systems at the LHC are explored. New results of the $v_n$ harmonics in 8.16 TeV pPb collisions are extracted via long-range ($|\Delta\eta|> $ 2) two-particle correlations as a function of event multiplicity. The current measurement is reaching a very-high-multiplicity region beyond that achieved in 5.02 TeV pPb data collected in 2013. Event-by-event correlations among $v_2$, $v_3$ and $v_4$ harmonics are measured using the four-particle symmetric cumulant method in pp and pPb, and compared to the large system of PbPb collisions. For high-multiplicity (more than 100 tracks) pp and pPb events, the $v_2$ harmonic is found to have a negative correlation with the $v_3$ harmonic, while the $v_2$ and $v_4$ harmonics are positively correlated. Normalized by the two-particle $v_n$ harmonics, the correlation coefficients of $v_2$ and $v_3$ harmonics are quantitatively similar for pPb and PbPb data, while a strong system size dependence is observed for correlations of $v_2$ and $v_4$ harmonics. These new data provide important insights to the origin of collectivity observed in small collision systems. | ||
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These preliminary results are superseded in this paper, PRL 120 (2018) 092301. The superseded preliminary plots can be found here. |
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| Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
The $v_2$, $v_3$ and $v_4$ harmonics from long-range two-particle correlations for 0.3 $ < p_{\mathrm{T}} < $ 3 GeV/$c$ as a function of $ N_{text{trk}}^{\text{offline}} $ in pp at ${\sqrt {s}} = $ 13 TeV, pPb at ${\sqrt {s_{\mathrm{NN}}}} = $ 8.16 TeV, and PbPb at $ {\sqrt {s_{\mathrm{NN}}}} = $ 5.02 TeV. The lines show the $v_{n}$ results before subtraction of low-multiplicity correlations. |
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Figure 2:
The SC for the second and third harmonic (red points) and the second and forth harmonic (blue points) as a function of $ N_{text{trk}}^{\text{offline}} $ in pp at $ {\sqrt {s}} = $ 13 TeV, pPb at $ {\sqrt {s_{\mathrm{NN}}}} = $ 5.02 and 8.16 TeV, and PbPb at $ {\sqrt {s_{\mathrm{NN}}}} = $ 5.02 TeV. |
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Figure 3:
The SC for the second and third harmonic (left) and the second and forth harmonic (right) normalized by $ < v_2^{2} > \cdot < v_3^{2} > $ and $ < v_2^{2} > \cdot < v_4^{2} > $ from dihardon correlations respectively. The $ p_{\mathrm{T}} $ range for considered tracks is 0.3 $ < p_{\mathrm{T}} < $ 3 GeV/$c$ and the results are shown as a function of $ N_{text{trk}}^{\text{offline}} $ in pp at $ {\sqrt {s}} = $ 13 TeV, pPb at $ {\sqrt {s_{\mathrm{NN}}}} = $ 8.16 TeV, and PbPb at $ {\sqrt {s_{\mathrm{NN}}}} = $ 5.02 TeV. |
| Tables | |
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Table 1:
Summary of systematic uncertainties for $v_n$ harmonics derived from two-particle correlations in pPb 8.16 TeV and PbPb 5.02 TeV. |
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Table 2:
Summary of systematic uncertainties for SC in pPb 8.16 TeV and PbPb 5.02 TeV. |
| Summary |
| First measurement of azimuthal anisotropy Fourier harmonics and event-by-event correlations of different harmonics orders in pPb collisions at $ \sqrt{ s_{\mathrm{NN}} } = $ 8.16 TeV is presented with data collected by the CMS experiment at CERN. The $v_2$-$v_4$ Fourier harmonics are extracted from long-range two-particle correlations as a function of event multiplicity, and found to be similar to lower energy pPb $ \sqrt{ s_{\mathrm{NN}} } = $ 5.02 TeV data. The pPb data are compared to pp $ \sqrt{ s } = $ 13 TeV and PbPb $ \sqrt{ s_{\mathrm{NN}} } = $ 5.02 TeV systems. Using four-particle cumulant technique, correlations of different harmonic orders are obtained, where an anti-correlation is observed between $v_2$ and $v_3$ in pPb collisions, while $v_2$ and $v_4$ are positively correlated. This behavior is similar between pPb and PbPb systems. Normalized correlation coefficients of the 2$^{\rm nd}$ and 3$^{\rm rd}$ order are found to be quantitatively similar between pPb and PbPb systems. For the 2$^{\rm nd}$ and 4$^{\rm th}$ order, they are slightly higher in pPb than in PbPb. This result may point to a different contribution from the initial-state correlations or different transport properties like the shear viscosity to entropy ratio ($\eta$/s). The pp normalized symetric cumulant result shows a similar trend at high multiplicity compared to pPb and PbPb systems but the statistical uncertainties are too large to make a quantitative statement. The results presented in this paper provide further strong evidence of a similar origin of collectivity observed in small and large hadronic collision systems, and also impose novel constraints on the theoretical calculations. |
| Additional Figures | |
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Additional Figure 1:
Level-1 (L1) trigger efficiency as a function of $ N_{\text{trk}}^{\text{offline}} $ for high-multiplicity event selection in pPb 8.16 TeV. |
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Additional Figure 2:
High Level Trigger (HLT) efficiency as a function of $ N_{\text{trk}}^{\text{offline}} $ for high-multiplicity event selection in pPb 8.16 TeV. |
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Additional Figure 3:
L1+HLT efficiency as a function of $ N_{\text{trk}}^{\text{offline}} $ for high-multiplicity event selection in pPb 8.16 TeV. |
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Additional Figure 4:
2-D two-particle correlation function for 1 $ < p_{\mathrm{T}}^{\text{trig}} <$ 3 GeV/$c$, 1 $ < p_{\mathrm{T}}^{\text{assoc}} < $ 3 GeV/$c$ in 8.16 TeV pPb data with 330 $ < N_{\text{trk}}^{\text{offline}} < $ 350 for charged particles. |
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Additional Figure 5:
1-D two-particle correlation function for 1 $ < p_{\mathrm{T}}^{\text{trig}} < $ 3 GeV/$c$, 1 $ < p_{\mathrm{T}}^{\text{assoc}} < 3 $ GeV/$c$, average over $|\Delta \eta | > $ 2, in 8.16 TeV pPb data with 330 $ < N_{\text{trk}}^{\text{offline}} < $ 350 for charged particles. 2-particle $V_n$ coefficients obtained from a Fourier fit of the distribution are also shown. |
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Additional Figure 6:
SC(2,3) and SC(2,4) as a function of centrality from CMS (full) compared with ALICE data (open) with the same kinematic cuts. |
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Additional Figure 7:
The SC for the second and third harmonic (left) and the second and forth harmonic (right) normalized by unsubtracted $< v_2^{2} >.< v_3^{2} > $ and $< v_2^{2}>.< v_4^{2}> $ from dihadron correlations respectively. The $ p_{\mathrm {T}} $ range for considered tracks is 0.3 $ < p_{\mathrm {T}} < $ 3 GeV/$c$ and the results are shown as a function of $ N_{\text{trk}}^{\text{offline}} $ in pp at ${\sqrt {s}}= $ 13 TeV, pPb at ${\sqrt {s_{\mathrm{NN}}}} =$ 8.16 TeV, and {\text {PbPb}} at ${\sqrt {s_{\mathrm{NN}}}} = $ 5.02 TeV. |
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