Studies on longitudinal fluctuations of anisotropy flow event planes in PbPb and pPb collisions at CMS

Most studies of anisotropy flow phenomena have assumed a global flow phase angle (or event plane angle) that is boost invariant in pseudorapidity ($\eta$). It was realized in recent years that this assumption may not be valid in presence of initial-state fluctuations, especially along the longitudinal direction. The effect of eta-dependent event plane fluctuations would break the factorization relation of Fourier coefficients from two-particle azimuthal correlations into a product of single-particle anisotropy Fourier harmonics as a function of $\eta$. First study of factorization breakdown effect in $\eta$ is carried out using the CMS detector, which covers a wide $\eta$ range of 10 units. A novel method is employed to suppress nonflow correlations at small pseudorapidity gaps of two particles. Significant eta-dependent factorization breakdown is observed in both PbPb and high-multiplicity pPb collisions. The measurements are presented for various orders of flow harmonics as a function of centrality or event multiplicity classes in PbPb and pPb, and are also compared to three-dimensional hydrodynamic calculations with longitudinal fluctuations. The new results presented here provide new insights into the longitudinal dynamics of relativistic heavy ion collisions, and help improve the three-dimensional modeling of the evolution of the strongly-coupled quark gluon medium.