In 2017, the STAR Collaboration reported the measurements of hyperon global polarization in heavy ion collisions, suggesting the subatomic fireball fluid created in these collisions as the most vortical fluid. The results show a strongly increasing trend of the signal with decreasing beam energy down to $\sqrt{s_{NN}}=7.7\mathrm{GeV}$. There remains an interesting question: How the signal would change toward even lower energy. In this Letter we perform a systematic study on the beam energy dependence of global and local hyperon polarization phenomena especially in the interesting $\widehat{O}(1–10)\mathrm{GeV}$ region. We find a nonmonotonic trend for the global polarization, which first increases and then decreases when beam energy is lowered from 27 GeV down to 3 GeV. The maximum polarization signal has been identified around $\sqrt{s_{NN}}=7.7\mathrm{GeV}$ in this model whereas the averaged thermal vorticity appears to reach a peak around 3 to 4 GeV. The implications of these results are discussed.

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• Collective flow
• Polarization phenomena
• Relativistic heavy-ion collisions