Global $\mathrm{\ensuremath{\Lambda}}$-hyperon polarization in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{\mathrm{NN}}}=3 \mathrm{GeV}$

Letter

Global $Λ$ -hyperon polarization in $Au + Au$ collisions at $\sqrt{s_{NN}} = 3 GeV$

M. S. Abdallah et al. (STAR Collaboration)

Phys. Rev. C 104, L061901 – Published 21 December 2021

Abstract

Global hyperon polarization, ${\bar{P}}_{H}$ , in $Au + Au$ collisions over a large range of collision energy, $\sqrt{s_{NN}}$ , was recently measured and successfully reproduced by hydrodynamic and transport models with intense fluid vorticity of the quark-gluon plasma. While naïve extrapolation of data trends suggests a large ${\bar{P}}_{H}$ as the collision energy is reduced, the behavior of ${\bar{P}}_{H}$ at small $\sqrt{s_{NN}} < 7.7$ GeV is unknown. Operating the STAR experiment in fixed-target mode, we measured the polarization of $Λ$ hyperons along the direction of global angular momentum in $Au + Au$ collisions at $\sqrt{s_{NN}} = 3$ GeV. The observation of substantial polarization of $4.91 \pm 0.81 (stat .) \pm 0.15 (syst .) %$ in these collisions may require a reexamination of the viscosity of any fluid created in the collision, of the thermalization timescale of rotational modes, and of hadronic mechanisms to produce global polarization.

Received 30 July 2021
Accepted 8 November 2021

DOI:https://doi.org/10.1103/PhysRevC.104.L061901

Physics Subject Headings (PhySH)

Particle correlations & fluctuations Polarization phenomena Relativistic heavy-ion collisions

Accelerators & BeamsNuclear Physics

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Vol. 104, Iss. 6 — December 2021

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Figure 1
The measured $m_{inv}$ distributions of two classes of $Λ$ -hyperon decays: “right” decays in blue, with $({\vec{p}}_{Λ} \times {\vec{p}}_{p}^{*}) \cdot {\vec{B}}_{STAR} > 0$ , and “left” decays in red, with $({\vec{p}}_{Λ} \times {\vec{p}}_{p}^{*}) \cdot {\vec{B}}_{STAR} < 0$ . The “right” decay class has a notably sharper $m_{inv}$ distribution than the “left” decay class, and this is due to the effects of daughter tracks crossing in the STAR TPC with ${\vec{B}}_{STAR} | | - \hat{z}$ . The opposite pattern is obtained by flipping the sign of ${\vec{B}}_{STAR}$ or by reconstructing $\bar{Λ}$ hyperons.
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Figure 2
The signal polarizations extracted according to Eq. (2) as a function of $ϕ_{Λ} - ϕ_{p}^{*}$ , for positive-rapidity $Λ$ hyperons. The sinusoidal behavior is driven by nonzero net $v_{1}$ . The vertical shift corresponds to the vorticity-driven polarization according to Eq. (3); in collider mode, where the net $v_{1}$ is zero, this dependence on $ϕ_{Λ} - ϕ_{p}^{*}$ does not exist. The functional form of Eq. (3) fits the data well, with $χ^{2} / ndf = 1.49$ .
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Figure 3
Global hyperon polarization as a function of $\sqrt{s_{NN}}$ in mid-central heavy-ion collisions. The trend of increasing ${\bar{P}}_{H}$ with decreasing $\sqrt{s_{NN}}$ is maintained at the low energy of $\sqrt{s_{NN}} = 3$ GeV. Statistical uncertainties are represented with lines while systematic uncertainties are represented with boxes. Previous experimental results [9, 10, 11, 17] are scaled [5] using the currently accepted [31] decay parameter $α_{Λ} = 0.732$ . Calculations with a hybrid model ( $UrQMD + vHLLE$ ) [22] and chiral-kinetic transport [21] are compared to the higher- $\sqrt{s_{NN}}$ data only, while others have been extended to lower energy. The AMPT model [20] matches higher-energy data well while dramatically underestimating ${\bar{P}}_{Λ}$ at $\sqrt{s_{NN}} = 3$ GeV. The hydrodynamic 3FD model [40] with two separate equations of state (crossover and first-order phase transition) predicts a sharply rising ${\bar{P}}_{Λ}$ below $\sqrt{s_{NN}} = 7.7$ GeV. The models shown use an impact parameter of 8 fm.
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Figure 4
The centrality dependence of ${\bar{P}}_{Λ}$ is compared with partonic-transport [20] and three-fluid hydrodynamic [40] calculations at impact parameters of 8 fm. Statistical uncertainties are represented with lines while systematic uncertainties are represented with boxes.
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Figure 5
The $p_{T}$ dependence of ${\bar{P}}_{Λ}$ is compared with AMPT [20] at an impact parameter of 8 fm. We observe no dependence within uncertainties. Statistical uncertainties are represented with lines while systematic uncertainties are represented with boxes.
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Figure 6
The $y$ dependence of ${\bar{P}}_{Λ}$ is compared with AMPT [20] at an impact parameter of 8 fm. We observe no dependence within uncertainties, even for the most forward $Λ$ hyperons. Statistical uncertainties are represented with lines while systematic uncertainties are represented with boxes.
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Physical Review C

covering nuclear physics

Global $Λ$ -hyperon polarization in $Au + Au$ collisions at $\sqrt{s_{NN}} = 3 GeV$

M. S. Abdallah et al. (STAR Collaboration)

Phys. Rev. C 104, L061901 – Published 21 December 2021

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Physical Review C

covering nuclear physics

Global Λ-hyperon polarization in Au+Au collisions at sNN=3 GeV

M. S. Abdallah et al. (STAR Collaboration)

Phys. Rev. C 104, L061901 – Published 21 December 2021

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Global $Λ$ -hyperon polarization in $Au + Au$ collisions at $\sqrt{s_{NN}} = 3 GeV$