Multiplicity dependence of multiparticle correlations in pPb and PbPb collisions

Measurements of the second-order azimuthal anisotropy Fourier harmonic, $v_2$, using multiparticle correlations for charged particles emitted in pPb and PbPb collisions over a wide pseudorapidity range are presented. The pPb data, corresponding to an integrated luminosity of 35 nb$^{-1}$, were collected during the 2013 LHC pPb run at a nucleon-nucleon center-of-mass energy of 5.02 TeV by the CMS experiment. A sample of 2.76 TeV semi-peripheral PbPb collision data, collected during the 2011 PbPb run, was also analyzed to compare with pPb results covering a similar range of particle multiplicities. Extending previous studies of two- and four-particle correlations in these systems, significant signals corresponding to multiparticle correlations involving six or more charged particles are observed using the six- and eight-particle cumulant and the Lee-Yang Zeros (LYZ) methods. The $v_2$ coefficient is extracted from the cumulant ($v_2\{6\}$, $v_2\{8\}$) and LYZ methods ($v_2\{LYZ\}$). For a given multipicity range in either the PbPb or pPb system, the values of $v_2\{6\}$, $v_2\{8\}$, $v_2\{LYZ\}$, and (previously measured) $v_2\{4\}$ are found to be in agreement within $\pm$10\%, while they are consistently below the $v_2$ values from two-particle correlations ($v_2\{2\}$). The relative ratios of $v_2\{2\}$, $v_2\{4\}$, $v_2\{6\}$ and $v_2\{8\}$ are calculated and compared to theoretical predictions using hydrodynamics for a pPb system dominated by fluctuations in the positions of participant nucleons. The new data further support the multiparticle nature of the observed long-range correlations in pPb collisions, and provide new insights to understand multiparticle dynamics in small collision systems such as pPb.