Observation of Entanglement Enabled Spin-Interference in the Drell-Söding Process in Au+Au Ultraperipheral Collisions at RHIC

Abstract

We report a measurement of the Drell-Söding ${\pi }^{+}{\pi }^{-}$ production in Au + Au ultraperipheral collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{NN}}=200\mathrm{GeV}$ using the STAR detector. For the first time, the Entanglement Enabled Spin-Interference (EESI) effect is observed in the Drell-Söding process through the amplitude $A_{2\Delta \varphi }$ of the $\cos (2\Delta \varphi )$ modulation. The measured $A_{2\Delta \varphi }$ exhibits no significant dependence on the invariant mass $M_{{\pi }^{+}{\pi }^{-}}$. An enhancement of three standard deviations is observed for the Drell-Söding process compared to ${\rho }^0$ photoproduction. Furthermore, we measure the $|t|$-dependence of the Drell-Söding process and ${\rho }^0$ photoproduction. The Drell-Söding spectrum falls more steeply with $|t|$ and exhibits diffractive structures shifted to lower $|t|$ compared to ${\rho }^0$ photoproduction, features that are not captured by theoretical calculations. These results provide new insights into the interplay between quantum interference and photon-nuclear interactions in ultra-peripheral heavy-ion collisions.