Measurement of longitudinal flow correlations in Pb+Pb collisions at $\sqrt{s_{\rm{NN}}}$=2.76 and 5.02 TeV with the ATLAS detector

The measurement of longitudinal flow correlations are presented for charged particles in pseudorapidity range $|\eta|<2.4$ using 7 $\mu$b$^{-1}$ and 22 $\mu$b$^{-1}$ Pb+Pb collisions at $\sqrt{s_{\rm{NN}}}=2.76$ and 5.02 TeV, respectively. The correlation of the harmonic flow $v_n$ in two pseudorapidity is found to not factorize into single-particle flow harmonics, and the effects of factorization breaking or flow decorrelation increase linearly with $\eta$ separation between the two particles. The linear coefficient of the breaking effect is found to be nearly independent of transverse momentum $\pT$ and event centrality, except for second order harmonics $n=2$, and they are found to be larger at 2.76 TeV than 5.02 TeV. The higher moments of the $\eta$-dependent flow correlations are also measured, and the corresponding linear coefficients for $k^{\rm{th}}$-moment are found to scale with $k$ for $n>2$, but scale faster than $k$ for $n=2$. The decorrelation effects are separated into contributions from forward-backward asymmetry of the flow magnitude and event-plane twist, which are found to be comparable to each other. The longitudinal flow correlations are also measured between harmonic flow of different order. The longitudinal fluctuations of $v_2$ and $v_3$ are found to be independent of each other, while the longitudinal fluctuations of $v_4$ and $v_5$ are found to be driven by the non-linear contribution from $v_2^2$ and $v_2v_3$, respectively.