Longitudinal flow decorrelations in Xe+Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44$ TeV with the ATLAS detector

First measurement of longitudinal decorrelations of harmonic flow $v_n$ for $n=2$, 3 and 4 in Xe+Xe collisions at \mbox{$\sqrt{s_{\mathrm{NN}}}=5.44$}~TeV is obtained using the ATLAS detector at the LHC. The decorrelation signal for $v_3$ and $v_4$ is found to be nearly independent of collision centrality and transverse momentum, $p_{\mathrm{T}}$, but for $v_2$ a strong centrality and $p_{\mathrm{T}}$ dependence is seen. When compared to the results from Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV, the decorrelation signal in mid-central Xe+Xe collisions is found to be larger for $v_2$, but smaller for $v_3$. Current hydrodynamic models, which reproduce the ratio of $v_n$ between Xe+Xe and Pb+Pb, fail to describe the magnitudes and trends of the ratios of flow decorrelations between Xe+Xe and Pb+Pb. These results provide new insights on the longitudinal structure of the initial-state geometry in heavy-ion collisions.