Hydrodynamic predictions for 5.44 TeV Xe+Xe collisions

We argue that relativistic hydrodynamics is able to make robust predictions for soft particle production in Xe+Xe collisions at the CERN Large Hadron Collider (LHC). The change of system size from Pb+Pb to Xe+Xe provides a unique opportunity to test the scaling laws inherent to fluid dynamics. Using event-by-event hydrodynamic simulations, we make quantitative predictions for several observables: mean transverse momentum, anisotropic flow coefficients, and their fluctuations. Results are shown as a function of collision centrality.

Figure 1

Probability distribution of the V0M amplitude, used by the ALICE Collaboration to sort events into centrality classes [42]. Symbols: ALICE data for $5.02\mathrm{TeV}$ Pb+Pb collisions. Dashed line: Rescaled ${\mathrm{T}}_{\mathrm{R}}\mathrm{ENTo}$ entropy in $5.02\mathrm{TeV}$ Pb+Pb events. Full line: Prediction for $5.44\mathrm{TeV}$ Xe+Xe collisions using the same model. The vertical dotted lines indicate the locations of the knees of the histograms (see text for more details).

Figure 2

(a) Root-mean-square values of the initial eccentricity ${\varepsilon }_2$ and the initial triangularity ${\varepsilon }_3$ [51] in the ${\mathrm{T}}_{\mathrm{R}}\mathrm{ENTo}$ model, as a function of centrality percentile, in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$, and Xe+Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44\mathrm{TeV}$. ${\varepsilon }_{RP}$ denotes the mean value of the eccentricity in the reaction plane [27]. (b) Ratio of rms ${\varepsilon }_2$ and ${\varepsilon }_3$ between Xe+Xe and Pb+Pb collisions. The dashed line in this panel is the result for ${\varepsilon }_2$ obtained by switching off the deformation of the Xe nucleus (see text).

Figure 3

(a) Mean transverse momentum $\langle p_T\rangle$ of identified particles as a function of centrality percentile. Symbols: ALICE results for Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76\mathrm{TeV}$ [72]. Black lines: Our hydrodynamic results for Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. Grey lines: Our hydrodynamic results for Xe+Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44\mathrm{TeV}$. Panel (b) displays the ratio between Xe+Xe and Pb+Pb collisions.

Figure 4

(a) rms values of $v_2, v_3$, and $v_4$ as a function of collision centrality. Open symbols: ALICE data for Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ [78]. Full symbols: our hydrodynamic calculations for Pb+Pb and Xe+Xe collisions. The kinematic cuts are $0.2<p_T<5\mathrm{GeV}/c$ and $\left|\eta \right|<0.8$. (b) Hydrodynamic predictions for the ratios of the rms anisotropies between Xe+Xe and Pb+Pb collisions. The dashed line in this panel is the result for $v_2$ obtained by switching off the deformation of the Xe nucleus.

Figure 5

Ratios of cumulants of anisotropic flow (open symbols) and corresponding ratios for initial eccentricity harmonics (full symbols), in Pb+Pb and Xe+Xe collisions from our hydrodynamic calculation, as a function of centrality percentile. From top to bottom: (a) $v_2\left\{4\right\}/v_2\left\{2\right\}$ and ${\varepsilon }_2\left\{4\right\}/{\varepsilon }_2\left\{2\right\}$. Data are from 5.02 TeV Pb+Pb collisions collected by the ALICE Collaboration [78]. (b) $v_3\left\{4\right\}/v_3\left\{2\right\}$ and ${\varepsilon }_3\left\{4\right\}/{\varepsilon }_3\left\{2\right\}$. Data are from 2.76 TeV Pb+Pb collisions collected by the ATLAS Collaboration [85]. (c) $v_2\left\{6\right\}/v_2\left\{4\right\}$ and ${\varepsilon }_2\left\{6\right\}/{\varepsilon }_2\left\{4\right\}$. Data are from 2.76 TeV Pb+Pb collisions collected by the ATLAS Collaboration [85].