Speaker
Description
In ultraperipheral collisions, the invariant mass spectrum of $\pi^+ \pi^-$ pairs is very complex due to the numerous production channels and intermediate states. The most dominant component of this spectrum is $\gamma A \rightarrow \rho^0 (770) \rightarrow \pi^+ \pi^-$ production, and the invariant mass region surrounding the $\rho^0 (770)$ is well-described with the inclusion of photonuclear ($\gamma A$) continuum and $\omega$ meson production. Two-photon processes, like those seen in electron-positron collisions, are also present in ultraperipheral collisions, but the $\gamma \gamma \rightarrow \pi^+ \pi^-$ process has not yet been measured in a nuclear environment due to its low cross section relative to the photonuclear channel. Wavefunction-level interference between the $\gamma \gamma $ and $\gamma A$ channels, referred to as Entanglement Enable Spin Interference (EESI), is expected to produce $A_{1\Delta \phi}$ and $A_{3\Delta \phi}$ signals. This represents a unique method to access the $\gamma \gamma \rightarrow \pi^+ \pi^-$ process in the complex environment of ultraperipheral collisions.
In this talk, the first measurement of EESI between photonuclear and light-by-light production of $\pi^+ \pi^-$ pairs, including the strong EESI signal associated with the $f_2(1270)$ resonance, will be presented. The EESI observables are then used to isolate $\gamma \gamma \rightarrow \pi^+ \pi^-$ in ultraperipheral $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}} = 200$ GeV.
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