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| CMS-PAS-HIN-18-015 | ||
| Measurement of correlated azimuthal anisotropy Fourier harmonics with subevent cumulants in pPb collisions at 8.16 TeV | ||
| CMS Collaboration | ||
| May 2018 | ||
| Abstract: Event-by-event correlations of azimuthal anisotropy Fourier coefficients ($v_n$) in 8.16 TeV pPb data are extracted using a subevent four-particle cumulant technique with the CMS experiment at the LHC. Each combination of four particles is required to fall into two, three or four distinct subevents within the full pseudorapidity range from -2.4 to 2.4 using tracks with transverse momentum between 0.3 and 3.0 GeV/$c$. Using the subevent technique, $v_n$ correlations are measured as a function of particle multiplicity and compared to the standard method without using subevents. At high multiplicity, $v_2$ and $v_3$ anticorrelation exhibits a consistent behavior among the various methods. The $v_2$ and $v_4$ correlation strength is found to depend on the number of subevents used in the calculation. Toward the low-multiplicity region, data from different methods diverge because of different contributions of few-particle correlations. The results presented represent a key step in understanding the onset of collective behavior from low to high multiplicities in small systems. | ||
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These preliminary results are superseded in this paper, PRC 103 (2021) 014902. The superseded preliminary plots can be found here. |
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| Figures | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
$SC(2,3)$ and $SC(2,4)$ as a function of ${N_{\text{trk}}^{\text{offline}}}$ from 2-, 3- and 4-subevents method (blue, red and green full circles respectively). For comparison, published results from the Ref. [31] with no subevents (open black circles) are also shown. Shaded areas are the systematic uncertainties for these measurements. |
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Figure 2:
Left: $SC(2,3)$ Relative difference from 2-, 3-subevents (red) and 2-, 4-subevents (green) as a function of $ {N_{\text{trk}}^{\text{offline}}} $. Right: $SC(2,4)$ Relative difference from 2-, 3-subevents (red) and 2-, 4-subevents (green) as a function of $ {N_{\text{trk}}^{\text{offline}}} $. Shaded areas are the systematic uncertainties for these measurements. |
| Summary |
| The first measurement of event-by-event correlations of different Fourier harmonic orders $SC(2,3)$ and $SC(2,4)$ with 2-, 3-, and 4-subevents in pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 8.16 TeV is presented using data collected by the CMS experiment at CERN. The pPb data analyzed with the subevent method are compared to previously published results using the calculation without subevents. In all cases, an anti-correlation is observed between $v_2$ and $v_3$, while $v_2$ and $v_4$ are positively correlated. For ${N_{\text{trk}}^{\text{offline}}} > $ 100, both, standard and $n$-subevent methods give similar results for $SC(2,3)$ suggesting that non-flow is not the dominant contribution in this region. The $SC(2,4)$ results show a different behavior in the same multiplicity region for different numbers of subevents, which emphasizes the sensitivity of cumulant calculations to other effects such as event-plane decorrelation. At multiplicities ${N_{\text{trk}}^{\text{offline}}} < $ 80, the subevent method suppresses the non-flow contribution and thus reveals the harmonic correlation from collectivity for ${N_{\text{trk}}^{\text{offline}}} > $ 50. The results presented in this note provide further strong evidence of collectivity observed in small systems down to multiplicities ${N_{\text{trk}}^{\text{offline}}} \sim $ 50, and will also impose more constraints on the theoretical interpretations. Future analysis with 4-subevents in pp and PbPb and comparison with present data from pPb collisions will further increase our understanding of how collective flow effects manifest themselves in small collision systems. |
| Additional Figures | |
png pdf |
Additional Figure 1:
$SC(2,3)$ and $SC(2,4)$ as a function of $N_{\mathrm{trk}}^{\mathrm{offline}}$ from 2-, 3- and 4-subevents method (blue, red and green full circles respectively). For comparison, published results from the Ref. [1] with no subevents (open black circles) are also shown. Shaded areas are the systematic uncertainties for these measurements. |
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