Observation of Entanglement Enabled Spin-Interference in the Drell-S\"oding Process in Au+Au Ultraperipheral Collisions at RHIC

Abstract

We report a measurement of the Drell-S\"oding $\pi^+ \pi^-$ 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-S\"oding process through the amplitude $A_{2\Delta\phi}$ of the $\cos(2\Delta\phi)$ modulation. The measured $A_{2\Delta\phi}$ exhibits no significant dependence on the invariant mass $M_{\pi^+ \pi^-}$. An enhancement of three standard deviations is observed for the Drell-S\"oding process compared to $\rho^0$ photoproduction. Furthermore, we measure the $|t|$-dependence of the Drell-S\"oding process and $\rho^0$ photoproduction. The Drell-S\"oding 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.