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

Target Journal: PRL

PA: Daniel Brandenburg, Sam Corey, Xinbai Li, Lijuan Ruan, Zebo Tang, Kaiyang Wang, Xin Wu, Zhangbu Xu, Wangmei Zha

Abstract

In relativistic heavy-ion collisions, the photoproduction via photon nuclear interaction creates a rich environment to explore quantum interference. For exclusive \( \pi^+ \pi^- \) production, the resonance and continuum production \( \pi^+ \pi^- \) arise from distinct production mechanisms. The continuum \( \pi^+ \pi^- \) production is dominated by Drell-Söding process, in which a virtual \( \pi^+ /\pi^- \) is diffraction-scattered on the nucleus. In contrast to \( \rho^0 \) photoproduction, the short-lived \( \pi^+ \pi^- \) is directly fluctuated from the vacuum rather than from decay of a vector meson which has determined quantum nature. In addition, the elastic scattering cross sections of \( \pi^+ A\) and \( \pi^- A\) have ~5% difference around a \(gamma p\) center-of-mass energy of approximately 12 GeV in photoproduction in 200 GeV Au+Au collisions which might cause destructive interference. The Entanglement Enabled Spin Interference (EESI) of Drell-Söding process and \( \rho^0 \) photoproduction offer a unique opportunity to study the production mechanism, quantum entanglement and interference.

In this paper, we present the first measurement of diffractive \( p_T \) spectrum and EESI through \( A_{2 \Delta \phi} \) for Drell-Söding process. We observe the different interference dynamics between \( \rho^0 \) photoproduction and Drell-Söding process, highlighting the role of production mechanisms in the EESI. Moreover, \( A_{2 \Delta \phi} \) of single production mechanism shows no clear mass dependence for \( p_T < 0.1 \) GeV/c and a notable stronger \( A_{2 \Delta \phi} \) is observed for Drell-Söding process.

Figure 1 :Schematic View of the Production Mechanism for the Drell-Söding Process

Caption

Schematic view of the production mechanism for the Drell-Söding Process. A virtual photon fluctuates into an entangled \( \pi^+ \pi^- \) pair with an extremely short lifetime (~ \( 2 \times 10^{-24} \) s). One single virtual \( \pi^+/\pi^- \) is then diffraction-scattered on the nucleus. The wave and other blue and red entangled pairs represent the vacuum fluctuation.

Figure 2 : Extraction of Drell-Söding

Caption

The differential cross section of exclusive \( \pi^+ \pi^- \) production on mass dependence and the fitting algorithm to separate resonance and continuum components. The black markers show the data, and the lilac markers represent the Drell-Söding term.

Figure 3 : Diffractive \( p_T \) spectra

Caption

(top) The differential cross section of exclusive \( \pi^+ \pi^- \) production on \( p_T \) dependence. The blue and red markers represent \( \rho^0 \) photoproduction and the Drell-Söding process, respectively.(bottom) The ratio for the cross section of Drell-Söding to \( \rho^0 \) photoproduction, which investigates the photon parton distribution and the production mechanism effect.

Figure 4 : Spin interference pattern

Caption

The spin interference dynamics for \( \rho^0 \) photoproduction and the Drell-Söding process, compared with the EPA-VMD model prediction.

Summary & Conclusions

• We present the first measurement of diffractive \( p_T \) spectrum and spin interference measurement for Drell-Söding process.
• The first time to measure \( A_{2 \Delta \phi} \) on mass dependence for \( \rho^0 \) photoproduction and Drell-Söding process. The results shows no clear dependence and an obviously stronger \( A_{2 \Delta \phi} \) for Drell-Söding process.
• From comparison of \( \rho^0 \) photoproduction and Drell-Söding process, results show a notable production mechanism impact and indicate the photon parton distribution.
• Important to EIC \( \phi \) and direct \( K^+ K^- \); Baseline for mc and model (current no channel).