Observation of Two-source Interference in the Photoproduction Reaction $Au Au \to Au Au \rho^0$

TL;DR

This paper reports the observation of two-source quantum interference in rho0 photoproduction during ultra-peripheral gold-gold collisions, demonstrating quantum entanglement and providing limits on decoherence effects.

Contribution

It provides the first experimental evidence of two-source interference in heavy-ion collisions and explores quantum entanglement in a high-energy nuclear environment.

Findings

Observed 87% interference level at low transverse momentum

Demonstrated entanglement of the two-pion system

Set a 23% limit on decoherence effects at 90% confidence level

Abstract

In ultra-peripheral relativistic heavy-ion collisions, a photon from the electromagnetic field of one nucleus can fluctuate to a quark-antiquark pair and scatter from the other nucleus, emerging as a $\rho^0$. The $\rho^0$ production occurs in two well-separated (median impact parameters of 20 and 40 fermi for the cases considered here) nuclei, so the system forms a 2-source interferometer. At low transverse momenta, the two amplitudes interfere destructively, suppressing $\rho^0$ production. Since the $\rho^0$ decay before the production amplitudes from the two sources can overlap, the two-pion system can only be described with an entangled non-local wave function, and is thus an example of the Einstein-Podolsky-Rosen paradox. We observe this suppression in 200 GeV per nucleon-pair gold-gold collisions. The interference is $87% \pm 5% {\rm (stat.)}\pm 8%$ (syst.) of the expected level. This translates into a limit on decoherence due to wave function collapse or other factors, of 23% at the 90% confidence level.