Investigations of anisotropic flow using multi-particle azimuthal correlations in pp, p-Pb, Xe-Xe, and Pb-Pb collisions at the LHC

Measurements of anisotropic flow coefficients ($v_n$) and their cross-correlations using two- and multi-particle cumulant methods are reported in collisions of pp at $\sqrt{s} = 13$ TeV, p-Pb at $\sqrt{s_{_{\rm NN}}} = 5.02$ TeV, Xe-Xe at $\sqrt{s_{_{\rm NN}}} = 5.44$ TeV, and Pb-Pb at $\sqrt{s_{_{\rm NN}}} = 5.02$ TeV recorded with the ALICE detector. The multiplicity dependence of $v_n$ is studied in a very wide range from 20 to 3000 particles produced in the mid-rapidity region $|\eta|<~0.8$ for the transverse momentum range $0.2 <~ p_{\rm T} <~ 3.0$ GeV/$c$. An ordering of the coefficients $v_2 > v_3 > v_4$ is found in pp and p-Pb collisions, similar to that seen in large collision systems, while a weak $v_2$ multiplicity dependence is observed relative to nucleus-nucleus collisions in the same multiplicity range. Using a novel subevent method, $v_{2}$ measured with four-particle cumulants is found to be compatible with that from six-particle cumulants in pp and p-Pb collisions. The magnitude of the correlation between $v_n^2$ and $v_m^2$, evaluated with the symmetric cumulants SC$(m,n)$ is observed to be positive at all multiplicities for $v_2$ and $v_4$, while for $v_2$ and $v_3$ it is negative and changes sign for multiplicities below 100, which may indicate a different $v_n$ fluctuation pattern in this multiplicity range. The observed long-range multi-particle azimuthal correlations in high multiplicity pp and p-Pb collisions can neither be described by PYTHIA 8 nor by IP-Glasma+MUSIC+UrQMD model calculations, and hence provide new insights into the understanding of collective effects in small collision systems.


Phys. Rev. Lett. 123, 142301 (2019)
HEP Data
e-Print: arXiv:1903.01790 | PDF | inSPIRE

Figure 1

(coloured online) Multiplicity dependence of $v_n\{k\}$ for \pp, \pPb, \XeXe\ and \PbPb\ collisions. Statistical uncertainties are shown as vertical lines and systematic uncertainties as filled boxes Data are compared with PYTHIA 8.210 Monash 2013  simulations (solid lines) of pp collisions at $\sqrt{s} = 13$ TeV and IP-Glasma+MUSIC+UrQMD  calculations of pp, \pPb, \PbPb\ collisions at $\snn = 5.02$ TeV and \XeXe\ collisions at $\snn = 5.44$ TeV (filled bands). The width of the band represents the statistical uncertainty of the model (a), (b) and (c): $v_2$, $v_3$ and $v_4$ measured using two-particle cumulants with a pseudorapidity separation $|\Delta\eta| > 1.4$, 1.0 and 1.0, respectively. (d): $v_2$ measured using multi-particle cumulants, with the 3-subevent method for the four-particle cumulant, and 2-subevent method for higher order cumulants in \PbPb\ collisions.

Figure 2

(coloured online) Multiplicity dependence of the (a) and (c) symmetric cumulant $SC(m,n)_{\rm{3-sub}}$ and (b) and (d) normalized ratio SC$(m,n)_{\rm{3-sub}} / \langle v_m^2\rangle\langle v_n^2\rangle$ for \pp, \pPb, \XeXe\ and \PbPb\ collisions. Observables in the denominator are obtained from the $v_2\{2, |\Delta\eta| > 1.4\}$ and $v_n\{2, |\Delta\eta| > 1.0\}$ for higher harmonics. Statistical uncertainties are shown as vertical lines and systematic uncertainties as filled boxes. The measurements in large collision systems are compared with the IP-Glasma+MUSIC+UrQMD  calculations and results in pp collisions are compared with the PYTHIA 8 model .