Abstract (data abstract)

Measurements of the elliptic flow coefficient relative to the collision plane defined by the spectator neutrons $v_2\{\Psi_\mathrm{SP}\}$ in collisions of Pb ions at center-of-mass energy per nucleon-nucleon pair $\sqrt{s_\mathrm{NN}}=2.76$ TeV and Xe ions at $\sqrt{s_\mathrm{NN}}=5.44$ TeV are reported. The results are presented for charged particles produced at midrapidity as a function of centrality and transverse momentum for the $5-70\%$ and $0.2-6\ \mathrm{GeV}/c$ ranges, respectively. The ratio between $v_2\{\Psi_\mathrm{SP}\}$ and the elliptic flow coefficient relative to the participant plane $v_2\{4\}$, estimated using four-particle correlations, deviates by up to $20\%$ from unity depending on centrality. This observation differs strongly from the magnitude of the corresponding eccentricity ratios predicted by the TRENTo and the elliptic power models of initial state fluctuations that are tuned to describe the participant plane anisotropies. The differences can be interpreted as a decorrelation of the neutron spectator plane and the reaction plane because of fragmentation of the remnants from the colliding nuclei, which points to an incompleteness of current models describing the initial state fluctuations. A significant transverse momentum dependence of the ratio $v_2\{\Psi_\mathrm{SP}\}/v_2\{4\}$ is observed in all but the most central collisions, which may help to understand whether momentum anisotropies at low and intermediate transverse momentum have a common origin in initial state fluctuations. The ratios of $v_2\{\Psi_\mathrm{SP}\}$ and $v_2\{4\}$ to the corresponding initial state eccentricities for Xe--Xe and Pb--Pb collisions at similar initial entropy density show a difference of $(7.0\pm0.9)\%$ with an additional variation of $+1.8\%$ when including RHIC data in the TRENTo parameter extraction. These observations provide new experimental constraints for viscous effects in the hydrodynamic modeling of the expanding quark--gluon plasma produced in heavy-ion collisions at the LHC.

Figure 2.1

Data from Figure 2 upper left panel

10.17182/hepdata.134258.v1/t1

Centrality dependence of $v_2\{\Psi_{\mathrm{SP}}\}$, $v_2\{2,|\Delta\eta|>1\}$, and $v_2\{4\}$ in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76~\mathrm{TeV}$.
Figure 2.2

Data from Figure 2 upper right panel

10.17182/hepdata.134258.v1/t2

Centrality dependence of $v_2\{\Psi_{\mathrm{SP}}\}$, $v_2\{2,|\Delta\eta|>1\}$, and $v_2\{4\}$ in Xe-Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44~\mathrm{TeV}$.
Figure 2.3

Data from Figure 2 lower left panel

10.17182/hepdata.134258.v1/t3

Centrality dependence of $v_2\{\Psi_{\mathrm{SP}}\}/v_2\{4\}$ and $v_2\{2,|\Delta\eta|>1\}/v_2\{4\}$ in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76~\mathrm{TeV}$.
Figure 2.4

Data from Figure 2 lower right panel

10.17182/hepdata.134258.v1/t4

Centrality dependence of $v_2\{\Psi_{\mathrm{SP}}\}/v_2\{4\}$ and $v_2\{2,|\Delta\eta|>1\}/v_2\{4\}$ in Xe-Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44~\mathrm{TeV}$.
Figure 3.1

Data from Figure 3 upper panels

10.17182/hepdata.134258.v1/t5

Transverse momentum dependence of $v_2\{\Psi_{\mathrm{SP}}\}$ for different centrality classes in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76~\mathrm{TeV}$.
Figure 3.2

Data from Figure 3 upper panels

10.17182/hepdata.134258.v1/t6

Transverse momentum dependence of $v_2\{4\}$ for different centrality classes in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76~\mathrm{TeV}$.
Figure 3.3

Data from Figure 3 lower panels

10.17182/hepdata.134258.v1/t7

Transverse momentum dependence of $v_2\{\Psi_{\mathrm{SP}}\}/v_2\{4\}$ for different centrality classes in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76~\mathrm{TeV}$.
Figure 3.4

Slopes of the linear fits from Figure 3 lower panels

10.17182/hepdata.134258.v1/t8

Slopes of the Linear fits to the transverse momentum dependence of $v_2\{\Psi_{\mathrm{SP}}\}/v_2\{4\}$ for different centrality classes in Pb-Pb collisions at...
Figure 4.1

Data from Figure 4 top panel

10.17182/hepdata.134258.v1/t9

Ratios of $v_2/\varepsilon_2$ as a function of $1/S\ \mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$ in Xe--Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44~\mathrm{TeV}$.
Figure 4.2

Data from Figure 4 middle panel

10.17182/hepdata.134258.v1/t10

Ratios of $v_2/\varepsilon_2$ as a function of $1/S\ \mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$ in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76~\mathrm{TeV}$.
Figure 4.3

Data from Figure 4 bottom panel

10.17182/hepdata.134258.v1/t11

Ratios of $v_2/\varepsilon_2$ as a function of $1/S\ \mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$ in Xe--Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44~\mathrm{TeV}$ divided by the linear Pb--Pb fits.
Figure 4.4

Data from Figure 4 bottom panel

10.17182/hepdata.134258.v1/t12

Ratios of $v_2/\varepsilon_2$ as a function of $1/S\ \mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$ for Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76~\mathrm{TeV}$ divided by the linear Pb--Pb fits.
Figure 4.5

Fit from Figure 4 bottom panel

10.17182/hepdata.134258.v1/t13

Relative deviation of $v_2/\varepsilon_2$ as a function of $1/S\ \mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$ in Pb--Pb and Xe--Xe collisions.

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Figure 3.1

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Figure 3.4

Figure 4.1

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Figure 4.3

Figure 4.4

Figure 4.5

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