Longitudinal Flow Decorrelations in Xe + Xe Collisions at √_sNN = 5.44 TeV with the ATLAS Detector

The first measurement of longitudinal decorrelations of harmonic flow amplitudes υ_n for n = 2–4 in Xe+Xe collisions at √_sNN =5.44 TeV is obtained using 3 μb⁻¹ of data with the ATLAS detector at the LHC. The decorrelation signal for v3 and v4 is found to be nearly independent of collision centrality and transverse momentum (p_T) requirements on final-state particles, but for υ₂ a strong centrality and p_T dependence is seen. When compared with the results from Pb+Pb collisions at √_sNN = 5.02 TeV, the longitudinal deorrelation signal in midcentral Xe+Xe collisions is found to be larger for υ₂, but smaller for υ₃. Current hydrodynamic models reproduce the ratios of the vn measured in
Xe+Xe collisions to those in Pb+Pb collisions but fail to describe the magnitudes and trends of the ratios of longitudinal flow decorrelations between Xe+Xe and Pb+Pb. The results on the system-size dependence provide new insights and an important lever arm to separate effects of the longitudinal structure of the initial state from other early and late time effects in heavy-ion collisions.