Measurements of longitudinal flow decorrelations in p p and Xe + Xe collisions with the ATLAS detector

Measurements of longitudinal flow decorrelations in 13-TeV p p and 5.44-TeV Xe + Xe collisions with the ATLAS detector are presented. The measurements are performed using the two-particle correlation method, combining charged-particle tracks within | η | < 2.5 with either calorimeter energy clusters or towers within 4.0 < | η | < 4.9 . A template-based subtraction procedure is used to remove nonflow effects in both the p p and the Xe + Xe analyses. The dependence of the longitudinal flow decorrelations on the pseudorapidity separation between the particles is characterized via the slope parameter F n for the elliptic ( n = 2 ) and triangular ( n = 3 ) harmonic moments. The results are reported as a function of charged-particle multiplicity for the p p and Xe + Xe collision systems. Comparing the data to a color string-based model of the initial geometry indicates that in p p and peripheral Xe + Xe collisions, subnucleonic structure, and fluctuations in longitudinal energy deposition are needed to describe the data.