We present the first measurement of the azimuthal angular correlation between e^± and the event plane in Ru + Ru and Zr + Zr collisions at √(S_NN) = 200 GeV from STAR. These e^± are produced from the decay of J/ψ mesons with extremely low transverse momentum (p_T < 0.2 GeV/c), originating from photoproduction processes that involve linearly polarized photons generated by intense electromagnetic fields in the initial stage of the collision. The observed anisotropy in the angular distribution of e^± exhibits a significant second-order cosine modulation (cos2φ), with a measured amplitude of -0.28 ± 0.08 (stat.) ± 0.03 (sys.) within the 30%-80% centrality class. This result indicates a strong alignment between photon polarization and the initial collision geometry, offering novel insights into the mechanisms of photoproduction. Moreover, this anisotropy provides a direct probe of the initial geometrical configuration, presenting a promising avenue for deepening our understanding of collective behavior in heavy-ion collisions.

Figures:

Figure 1. Schematic diagram illustrating the polarized photon-nuclear interaction that creates J/Ψ, The electric field vector indicates the polarization direction of the produced J/Ψ, which is aligned with impact parameter.

Figure 2.Invariant mass distribution and fit of J/Ψ particle in 30%-80% Ru+Ru and Zr+Zr collisions, Right: single track efficiency reweighted φe− − ΦEP and Fitted v2 signal

Figure 3. The orange and blue points represent the pT < 0.2 GeV/c and 0.2 GeV/c< pT <5.0 GeV/c , the red points show the photon-induced 〖electron v2 ​ with the hadronic background subtracted, assuming zero electron v2 ​ contribution from hadronic processes. Left: electron v2 vs. Centrallity, Right: electron v2 vs.pT

Summary:

In conclusion, our study presents the first evidence of an azimuthal angular correlation between electrons and the collision geometry induced by electromagnetic fields in Ru + Ru and Zr + Zr collisions at STAR.

These findings reveal a strong correlation between the initial collision geometry and the polarization of photons involved in photon-nucleus interactions, providing a direct probe of photon polarization.

Furthermore, the electrons produced through photoproduction carry the initial geometry information, and their observed anisotropy offers a novel tool for examining the relationship between initial geometric conditions and hadronic collective behaviors. While our current work focuses on the electromagnetic fields induced anisotropy aspects, it opens up possibilities for future studies to take this anisotropy as a probe to investigate the relationship between initial geometry and the collective dynamics of hadrons.

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