Photon-induced J/ψ production and polarization effects in isobar collisions at STAR

¹ State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China, Hefei 230026, China
² Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China

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Published online: 17 April 2026

Abstract

Photon-induced J/ψ production in heavy-ion collisions offers a unique probe of the nuclear gluon structure and initial geometry. We present differential cross-section measurements of coherent J/ψ photoproduction in Ru+Ru and Zr+Zr ultra-peripheral collisions at √s_NN = 200 GeV. The linear polarization of photons in these processes has been suggested to carry information about the nuclear size and the initial collision geometry. In ultra-peripheral collisions, we observe a clear modulation in the J/ψ decay angular distribution, consistent with expectations from spin interference associated with photon polarization. In non-central collisions, the decay electrons exhibit a pronounced azimuthal anisotropy with respect to the event plane, serving as direct evidence for measuring the photon polarization direction. This novel observation opens new opportunities for mapping the initial state in heavy-ion collisions.