Abstract
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.
Highlights
Longitudinal Flow Decorrelations in Xe + Xe Collisions at pffisffiNffiffiffiNffiffi = 5.44 TeV with the ATLAS Detector
The decorrelation signal for v3 and v4 is found to be nearly independent of collision centrality and transverse momentum
High-energy heavy-ion collisions create a new state of matter known as a quark-gluon plasma (QGP), whose space-time dynamics is well described by relativistic viscous hydrodynamic models [1,2,3]
Summary
Current hydrodynamic models reproduce the ratios of the vn measured in Xe þ Xe collisions to those in Pb þ Pb collisions but fail to describe the magnitudes and trends of the ratios of longitudinal flow decorrelations between Xe þ Xe and Pb þ 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. Hydrodynamic model and transport model calculations [24,25,26,27,28,29] show that the flow decorrelations are driven mostly by longitudinal fluctuation of En in the initial-state geometry.
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