Invariant Mass Dependence of the Entanglement-Enabled Spin Interference effect at STAR

Abstract

In 2023, the STAR collaboration reported a measurement of an azimuthal angular anisotropy in pion pairs produced in ultra-peripheral collisions \cite{STAR:2023}. This anisotropy is attributed to double-slit-like interference between $\rho^0(770)$ meson production channels where opposite nuclei act as the photon emitter. However, the anisotropy has not been measured at pair invariant mass values far from the $\rho^0(770)$ mass. We report the first measurement of the mass dependence of this angular anisotropy, which reveals the presence of contributions from interference between the $\rho^0$ and the continuum. Additionally, by cross-referencing the strength of the anisotropy with the two-pion invariant mass cross section, we obtain additional information on resonant states heavier than the $\rho^0(770)$, such as the $\rho(1700)$.