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

The first measurement of longitudinal decorrelations of harmonic flow amplitudes $v_n$ for $n=2$, 3 and 4 in Xe+Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44$ TeV is obtained using 3 ${\mu}\textrm{b}^{-1}$ of data with 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 ($p_{\mathrm{T}}$) requirements on final-state particles, but for $v_2$ a strong centrality and $p_{\mathrm{T}}$ dependence is seen. When compared with the results from Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV, the longitudinal decorrelation signal in mid-central Xe+Xe collisions is found to be larger for $v_2$, but smaller for $v_3$. Current hydrodynamic models reproduce the ratios of the $v_n$ 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-time and late-time effects in heavy-ion collisions.