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

The measurement of longitudinal flow correlations are presented for charged particles in pseudo-rapidity range $| \eta |<2.4$ using 7 $\mu b^{−1}$ and 22 $\mu b^{−1}$ Pb+Pb collisions at $\sqrt{s_{NN}}=$2.76 and 5.02 TeV, respectively. The correlation of the harmonic flow $\boldsymbol{v}_n$ in two pseudo-rapidity is found to not factorize into single-particle flow harmonics, and the effects of factorization breaking or flow de-correlation 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 $p_T$ 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 $k^{th}$-moment are found to scale with k for n$>$2, but scale faster than k for $n=2$. The de-correlation 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 $\boldsymbol{v}_2$ and $\boldsymbol{v}_3$ are found to be independent of each other, while the longitudinal fluctuations of $\boldsymbol{v}_4$ and $\boldsymbol{v}_5$ are found to be driven by the non-linear contribution from $\boldsymbol{v}^2_2$ and $\boldsymbol{v}_2\boldsymbol{v}_3$, respectively.