CMS-PAS-HIN-15-008 | ||

Differential flow harmonic $v_n$ in pPb and PbPb collisions | ||

CMS Collaboration | ||

September 2015 | ||

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Abstract:
Previous CMS measurements have demonstrated the collective nature of multiparticle correlations in high-multiplicity pPb collisions at the LHC. This collectivity is consistent with a hydrodynamic flow origin. However, it can also be interpreted in terms of initial state effects arising from gluon saturation. The pseudorapidity dependence of the azimuthal Fourier coefficients ($v_n$) is expected to be sensitive to the underlying mechanism with, in the hydrodynamic picture, the longer lifetime of the fireball on the Pb-going side expected to lead to a larger flow signal than found on the p-going side. To investigate the detailed properties of the observed collectivity, differential $v_n$ values in transverse momentum ($p_T$) and pseudorapidity ($\eta$) are presented over the full range of the CMS tracker detector ($-2.4 < \eta < 2.4$) for pPb collisions at a nucleon-nucleon center-of mass energy of 5.02 TeV. Results based on multiparticle analyses involving four or more particles are shown. An event plane analysis is presented where the influence of recently demonstrated event-plane decorrelation is considered. Comparisons are made with peripheral PbPb collisions measured at similar mid-rapidity particle multiplicities. The results will be discussed in the context of current models of the longitudinal dependence of the multiparticle correlations.
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Links:
CDS record (PDF) ;
CADI line (restricted) ; Figures are also available from the CDS record.
These preliminary results are superseded in this paper, PRC 98 (2018) 044902.The superseded preliminary plots can be found here. |

Figures | |

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Figure 1-a:
$v_2$ as a function of $p_T$ in pPb (a) and PbPb (b) collisions for different $ {N_\mathrm {trk}^\mathrm {offline}} $ ranges. $v_2\{2, |\Delta \eta |>2\}$ and $v_2\{4\}$ are extracted from Ref [26]. |

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Figure 1-b:
$v_2$ as a function of $p_T$ in pPb (a) and PbPb (b) collisions for different $ {N_\mathrm {trk}^\mathrm {offline}} $ ranges. $v_2\{2, |\Delta \eta |>2\}$ and $v_2\{4\}$ are extracted from Ref [26]. |

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Figure 2:
Top: Comparison of $v_{2}(p_{\rm T})$ distributions based on $HF$ event planes located on the p-going ($v_2^{\rm p}$) and Pb-going ($v_2^{\rm Pb}$) sides of the tracker region, with $\eta _{\rm C} = 0$. Bottom: Same, but with $\eta _{\rm C} = \eta _{\rm POI}$ |

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Figure 3:
$v_2\{EP\}$ as a function of $\eta $ in pPb collisions for different $ {N_\mathrm {trk}^\mathrm {offline}} $ ranges with (top) $\eta _{\rm C}=0$ and (bottom) $\eta _{\rm C} = \eta _{\rm POI}$. The pseudorapidities are given in the laboratory frame. Non-flow effects are evident for points where the pseudorapidity gap between the particles of interest and the respective event plane is small. |

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Figure 4:
$v_2\{EP\}$ as a function of $\eta $ in PbPb collisions for different $ {N_\mathrm {trk}^\mathrm {offline}} $ ranges with (top) $\eta _{\rm C}=0$ and (bottom) $\eta _{\rm C} = \eta _{\rm POI}$. Non-flow effects are evident for points where the pseudorapidity gap between the particles of interest and the respective event plane is small. |

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Figure 5-a:
$v_2$ corresponding to event plane, cumulant, and LYZ methods as a function of $\eta $ in pPb (a) and PbPb (b) collisions for different $ {N_\mathrm {trk}^\mathrm {offline}} $ ranges. The $v_2\{\rm EP\}$ results are based on the furthest $HF$ event plane in pseudorapidity. The pseudorapidities are given in the laboratory frame. |

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Figure 5-b:
$v_2$ corresponding to event plane, cumulant, and LYZ methods as a function of $\eta $ in pPb (a) and PbPb (b) collisions for different $ {N_\mathrm {trk}^\mathrm {offline}} $ ranges. The $v_2\{\rm EP\}$ results are based on the furthest $HF$ event plane in pseudorapidity. The pseudorapidities are given in the laboratory frame. |

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Figure 6:
Role of fluctuations in the pPb and PbPb systems as a function of pseudorapidity for the indicated $ {N_\mathrm {trk}^\mathrm {offline}} $ ranges. |

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Figure 7:
Comparison of event-plane ($v_2\{\rm EP\}$) and cumulant ($v_2\{4\}$) results for the ratio $v_{2}(\eta )/v_{2}(\eta =0)$ with the two-particle correlation results from [49] for pPb collisions at $\sqrt {s_{\rm NN}} = 5.02{\rm TeV}$ and with $220\leq N_{\rm trk}^{\rm offline}< 260$. The [49] results are shown without the peripheral $v_{2}$ component subtraction, a correction for non-flow effects which increases the $v_{2}$ harmonics. The pseudorapidities are given in the laboratory frame. |

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Figure 8-a:
(a) Ratio p-going to Pb-going side $v_2$ coefficients at comparable $\eta _{\rm CM}$ values for pPb collisions. (b) Ratio of $+\eta $ to $-\eta $ $v_2$ values at comparable $\eta _{\rm CM}$ values for PbPb collisions. |

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Figure 8-b:
(a) Ratio p-going to Pb-going side $v_2$ coefficients at comparable $\eta _{\rm CM}$ values for pPb collisions. (b) Ratio of $+\eta $ to $-\eta $ $v_2$ values at comparable $\eta _{\rm CM}$ values for PbPb collisions. |

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Figure 9:
Triangular flow (n=3) harmonic from the event-plane method for pPb collisions at $\sqrt {s_{\rm NN}} =$ 5.02 TeV with (top) $\eta _{\rm C} = 0$ and (bottom) $\eta _{\rm C} = \eta _{\rm POI}$. The legends indicate the direction of the respective HF event planes. Non-flow effects are evident for points where the pseudorapidity gap between the particles of interest and the respective event plane is small. |

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Figure 10:
Triangular flow (n=3) harmonic from the event-plane method for PbPb collisions at $\sqrt {s_{\rm NN}} =$ 2.76 TeV with (top) $\eta _{\rm C} = 0$ and (bottom) $\eta _{\rm C} = \eta _{\rm POI}$. The legends indicate the direction of the respective HF event planes. Non-flow effects are evident for points where the pseudorapidity gap between the particles of interest and the respective event plane is small. |

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Figure 11:
Elliptic (n=2) and triangular flow (n=3) harmonics for pPb collisions at $\sqrt {s_{\rm NN}} =$ 5.02 TeV and PbPb at $\sqrt {s_{\rm NN}} =$ 2.76 TeV with $\eta _{C}=\eta _{POI}$. The $v_2\{\rm EP\}$ results are based on the furthest $HF$ event plane in pseudorapidity. |

Tables | |

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Table 1:
Systematic uncertainty |

Compact Muon Solenoid LHC, CERN |