The second-order azimuthal anisotropy Fourier harmonics, $v_2$, are obtained in $p\text{−}\mathrm{Pb}$ and PbPb collisions over a wide pseudorapidity ($\eta$) range based on correlations among six or more charged particles. The $p$-Pb data, corresponding to an integrated luminosity of $35{\mathrm{nb}}^{-1}$, were collected during the 2013 LHC $p$-Pb run at a nucleon-nucleon center-of-mass energy of $5.02\mathrm{TeV}$ by the CMS experiment. A sample of semiperipheral PbPb collision data at $\sqrt{s_{\mathrm{NN}}}=2.76\mathrm{TeV}$, corresponding to an integrated luminosity of $2.5\mu {\mathrm{b}}^{-1}$ and covering a similar range of particle multiplicities as the $p\text{−}\mathrm{Pb}$ data, is also analyzed for comparison. The six- and eight-particle cumulant and the Lee-Yang zeros methods are used to extract the $v_2$ coefficients, extending previous studies of two- and four-particle correlations. For both the $p\text{−}\mathrm{Pb}$ and PbPb systems, the $v_2$ values obtained with correlations among more than four particles are consistent with previously published four-particle results. These data support the interpretation of a collective origin for the previously observed long-range (large $\mathrm{\Delta }\eta$) correlations in both systems. The ratios of $v_2$ values corresponding to correlations including different numbers of particles are compared to theoretical predictions that assume a hydrodynamic behavior of a $p\text{−}\mathrm{Pb}$ system dominated by fluctuations in the positions of participant nucleons. These results provide new insights into the multiparticle dynamics of collision systems with a very small overlapping region.

• Received 18 February 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.012301

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© 2015 CERN, for the CMS Collaboration