Global polarization of $\mathrm{\Lambda }$ hyperons in Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV

Global polarization of $\mathrm{\Lambda }$ hyperons has been measured to be of the order of a few tenths of a percentage in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV, with no significant difference between $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$. These new results reveal the collision energy dependence of the global polarization together with the results previously observed at $\sqrt{s_{{}_{NN}}}$ = 7.7–62.4 GeV and indicate noticeable vorticity of the medium created in noncentral heavy-ion collisions at the highest Relativistic Heavy Ion Collider collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from a multi-phase transport model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperon's transverse momentum and pseudorapidity ${\eta }^H$ within $|{\eta }^H|<1$. An indication of the polarization dependence on the event-by-event charge asymmetry is observed at the $2\sigma$ level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field.

Figure 1

Resolution of the first-order event plane determined by the ZDC-SMDs [26] in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV; ZDC-SMD E+W denotes the combined plane of ZDC-SMDs in east and west sides and ZDC-SMD E(W) denotes one of the ZDC-SMDs.

Figure 2

Invariant mass distributions of the ($p,{\pi }^{-}$) system for $\mathrm{\Lambda }$ (a) and of the ($\overline{p},{\pi }^{+}$) system for $\overline{\mathrm{\Lambda }}$ (b) in the 30–40% centrality bin for 2014 data. Bold solid lines show the background distribution obtained by a linear fitting function, and dashed lines show the background from mixed events. Shaded areas show the extracted signal after the background subtraction using the fitting function.

Figure 3

$\langle sin({\mathrm{\Psi }}_1-{\varphi }_p^{*})\rangle$ as a function of the invariant mass for $\mathrm{\Lambda }$ (a) and $\overline{\mathrm{\Lambda }}$ (b) in the 10–80% centrality bin for 2014 data. Solid and dashed lines show the fitting function for actual fit range, Eq. (3), with two different background assumptions.

Figure 4

Global polarization of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ as a function of the collision energy $\sqrt{s_{{}_{NN}}}$ for 20–50% centrality Au+Au collisions. Thin lines show calculations from a 3+1D cascade + viscous hydrodynamic model (UrQMD+vHLLE) [15] and bold lines show the AMPT model calculations [16]. In the case of each model, primary $\mathrm{\Lambda }$ with and without the feed-down effect are indicated by dashed and solid lines, respectively. Open boxes and vertical lines show systematic and statistical uncertainties, respectively. Note that the data points at 200 GeV and for $\overline{\mathrm{\Lambda }}$ are slightly horizontally shifted for visibility.

Figure 5

$\mathrm{\Lambda }$ ($\overline{\mathrm{\Lambda }}$) polarization as a function of the collision centrality in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV. Open boxes and vertical lines show systematic and statistical uncertainties. The data points for $\overline{\mathrm{\Lambda }}$ are slightly shifted for visibility.

Figure 6

Polarization of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ as a function of $p_T$ for the 20%–60% centrality bin in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV. Open boxes and vertical lines show systematic and statistical uncertainties, respectively. Hydrodynamic model calculations for $\mathrm{\Lambda }$ with two different IC are compared. Note that the data points for $\overline{\mathrm{\Lambda }}$ are slightly shifted for visibility.

Figure 7

Polarization of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ as a function of $\eta$ for the 20–60% centrality bin in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV. Open boxes and vertical lines show systematic and statistical uncertainties. Note that the data points for $\overline{\mathrm{\Lambda }}$ are slightly shifted for visibility.

Figure 8

Polarization of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ as a function of observed charge asymmetry $A_{\mathrm{ch}}$ normalized with its RMS ${\sigma }_{A_{\mathrm{ch}}}$ for the 20–60% centrality bin in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV. Open boxes and vertical lines show systematic and statistical uncertainties. Solid and dashed lines show linear fitting functions.