Global $Λ$ hyperon polarization in low-energy heavy ion collisions -- a scenario without vorticity

Abstract

Since its discovery, global polarization of the $Λ$ hyperon in heavy-ion collisions has been firmly established and is widely attributed to the large vorticity generated in the rotating quark-gluon plasma. In contrast, nearly fifty years after the first observation of unexpectedly large transverse $Λ$ polarization in unpolarized hadron collisions, its underlying mechanism remains an open and long-standing puzzle, despite being observed across a broad range of collision systems. Although these two phenomena exhibit notable similarities, they are generally regarded as arising from distinct physical origins. In this work, we propose a direct connection between $Λ$ global polarization in heavy-ion collisions and the long-standing transverse polarization observed in unpolarized collision systems. We demonstrate that the alignment between the $Λ$ production plane and the reaction plane, driven by directed flow, can transfer transverse polarization into the measured global polarization signal. Realistic Monte Carlo simulations of Au+Au collisions at $\sqrt{s_{\rm NN}} = 3$ GeV indicate that this mechanism can generate a sizable global polarization, accounting for approximately $23\%\pm6\%$ of the magnitude reported by the STAR Collaboration. Our results establish, for the first time, a quantitative link between these two well-known phenomena and have important implications for the interpretation of $Λ$ global polarization measurements in low-energy heavy-ion collisions.

Figures (6)

• Figure 1: Illustration of heavy ion collisions (top) and two scenarios of $\Lambda$ global polarization: vorticity-induced $\Lambda$ global polarization (bottom left) and that induced by transverse polarization (bottom right). The black cross denotes the orbital angular momentum pointing into the page. Red crosses denote $\Lambda$ spins pointing into the page, while blue dots denote the normal vectors of the reaction and production planes pointing out of the page.
• Figure 2: Distribution of $\cos (\phi_{\text{PP} } - \phi_{\text{EP}} )$ for Au-Au collisions at $\sqrt{s_{\rm{NN}}}= 3~\text{GeV}$ simulated by JAM2 generator.
• Figure 3: The distribution of Feynman-$x$($x_F$) of $\Lambda$ hyperons produced in Au-Au collisions at $\sqrt{s_{\rm{NN}}} = 3~\text{GeV}$. The result is obtained from events generated by JAM2.
• Figure 4: Normalized distribution of $\cos\theta^*$, where $\theta^*$ is the angle between the normal vector of the $\Lambda$ production plane $\hat{n}_{\text{PP}}$ and the momentum of the daughter proton. Red dots show the distribution without polarization, while blue dots correspond to the case with implemented transverse polarization. The dashed lines represent linear fits to the corresponding distributions. The results are obtained from events generated by JAM2.
• Figure 5: The global polarization calculated as a function of $\phi_{\Lambda}-\phi_{p}^*$, where $\phi_{\Lambda}$ is the azimuthal angle of $\Lambda$ hyperon in laboratory frame, and $\phi_p^*$ is the azimuthal angle of daughter proton in $\Lambda$ hyperon's rest frame. Red dots show the distribution without polarization, while blue dots correspond to the case with implemented transverse polarization. The results are obtained from events generated by JAM2.