Elliptic and triangular flow of (anti)deuterons in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV

The measurements of the (anti)deuteron elliptic flow ($v_2$) and the first measurements of triangular flow ($v_3$) in Pb-Pb collisions at a center-of-mass energy per nucleon-nucleon collision $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV are presented. A mass ordering at low transverse momentum ($p_{\mathrm{T}}$) is observed when comparing these measurements with those of other identified hadrons, as expected from relativistic hydrodynamics. The measured (anti)deuteron $v_2$ lies between the predictions from the simple coalescence and blast-wave models, which provide a good description of the data only for more peripheral and for more central collisions, respectively. The mass number scaling, which is violated for $v_2$, is approximately valid for the (anti)deuterons $v_3$. The measured $v_2$ and $v_3$ are also compared with the predictions from a coalescence approach with phase-space distributions of nucleons generated by iebe-vishnu with ampt initial conditions coupled with urqmd, and from a dynamical model based on relativistic hydrodynamics coupled to the hadronic afterburner smash. The model predictions are consistent with the data within the uncertainties in midcentral collisions, while a deviation is observed in the most central collisions.

Figure 1

Raw yield (left), $v_2$ (middle), and $v_3$ (right) of $d+\overline{d}$ candidates as a function of $\mathrm{\Delta }M$ for $2.2\le p_{\mathrm{T}} <2.4$ GeV/$c$ ($2.0\le p_{\mathrm{T}} <2.4$ GeV/$c$ for $v_3$) and in the centrality interval 20–30%. The data points represent the measurements. The curve on the left panel is the total fit (signal plus background) as described in the text. The curves in the middle and right panel are the fits performed using Eq. (5). Vertical bars represent the statistical uncertainties.

Figure 2

Elliptic ($v_2$, left) and triangular ($v_3$, right) flow of deuterons as a function of $p_{\mathrm{T}}$ for different centrality intervals measured in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV. The horizontal line at zero is to guide the eye. Vertical bars and boxes represent the statistical and systematic uncertainties, respectively.

Figure 3

Deuterons $v_2$ measured in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV (red square) compared to that measured at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV [18] (light blue circles) for two centrality intervals (10–20 and 40–50%). Both protons and deuteron elliptic flow were measured for pseudorapidity gap between the particle of interest and the reference flow particle $\left|\mathrm{\Delta }\eta \right|>$ 0.9. Vertical bars and boxes represent the statistical and systematic uncertainties, respectively.

Figure 4

Ratio of the $v_2$ of deuterons measured in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV to that measured at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV (red circles) compared with the same ratio obtained for protons (blue squares) for two centrality intervals (10–20% on the left panel and 40–50% on the right panel). For a direct comparison of protons and deuterons, the measured $v_2$ and $p_{\mathrm{T}}$ were divided by $A$. Vertical bars and boxes represent the statistical and systematic uncertainties, respectively.

Figure 5

Comparison of the elliptic flow of pions, kaons, protons, deuterons, and (anti)${}^3\mathrm{He}$ in different centrality intervals for Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV. (Anti)${}^3\mathrm{He} v_2$ is measured using the event plane method [19]. Vertical bars and boxes represent the statistical and systematic uncertainties, respectively.

Figure 6

Triangular flow ($v_3$) of deuterons, pions, kaons, and protons [43] as a function of $p_{\mathrm{T}}$ for the centrality intervals 0–20 and $\mathrm{20–40}\%$. Vertical bars and boxes represent the statistical and systematic uncertainties, respectively.

Figure 7

Blast-wave fits to the $v_2$ ($p_{\mathrm{T}}$) of pions, kaons, and protons [43] and predictions of the deuterons $v_2$ ($p_{\mathrm{T}}$) for the centrality intervals $\mathrm{10–20}\%$ (left) and $\mathrm{40–50}\%$ (right). In the lower panels, the data-to-fit ratios are shown for pions, kaons, and protons as well as the ratio of the deuterons $v_2$ to the blast-wave predictions. Vertical bars and boxes represent the statistical and systematic uncertainties, respectively. The dashed line is to guide the eye.

Figure 8

Data-to-model ratios of the deuterons $v_2$ to the blast-wave predictions as a function of $p_{\mathrm{T}}$ for different centrality intervals as indicated in each pad. Vertical bars and boxes represent the statistical and systematic uncertainties, respectively.

Figure 9

Measured deuterons $v_2$ and $v_3$ (red circles) compared with the expectations from simple coalescence [Eq. (6) (blue shaded bands)] for two centrality intervals. In the left panel, the $v_2$ measurements in the 10–20 and $\mathrm{40–50}\%$ centrality intervals are shown. The right panel displays the results of $v_3$ in the 0–20 and $\mathrm{20–40}\%$ centrality intervals. The bottom panels show the ratio between the measured $v_2$ ($v_3$) and the expectations from the coalescence model. In each panel, vertical bars and boxes represent the statistical and systematic uncertainties, respectively. The line is to guide the eye.

Figure 10

Centrality evolution of the deuterons $v_2$ compared with the expectations from the simple coalescence model [Eq. (6)]. Vertical bars and boxes represent the statistical and systematic uncertainties, respectively.

Figure 11

Elliptic (left) and triangular (right) flow of deuterons compared to the predictions of the iebe-vishnu hybrid model with ampt initial conditions [4]. The predictions are shown as bands the widths of which represent the statistical uncertainties associated with the model. The data-to-model differences are shown in the lower panels. Vertical bars and boxes represent the statistical and systematic uncertainties, respectively.

Figure 12

Measured deuterons $v_2$ compared to the predictions from a microscopic model [5] based on the jetscape generator [52]. The model predictions, based on the smash afterburner and which used trento [51] initial conditions, are shown as bands. The width of the band represents the statistical uncertainty associated with the model. In the lower panel the data-to-model differences are shown. Vertical bars and boxes represent the statistical and systematic uncertainties, respectively.