Observation of Two-Source Interference in the Photoproduction Reaction $\mathrm{AuAu}\to \mathrm{AuAu}{\rho }^0$

In ultraperipheral 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 F for the cases considered here) nuclei, so the system forms a two-source interferometer. At low transverse momenta, the two amplitudes interfere destructively, suppressing ${\rho }^0$ production. Since the ${\rho }^0$ decays before the production amplitudes from the two sources can overlap, the two-pion system can only be described with an entangled nonlocal 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 $87\%\pm 5\%(\mathrm{stat}.)\pm 8\%(\mathrm{syst}.)$ 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.

Figure 1

Rapidity (left) and $M_{\pi \pi }$ (right) of the ${\pi }^{+}{\pi }^{-}$ distributions for the topology (exclusive ${\rho }^0$, top) and MB (Coulomb breakup, bottom) samples. The points with statistical error bars are the data, and the histograms are the simulations. The “notch” in the topology data around $y=0$ is due to the explicit rapidity cut to remove cosmic-ray backgrounds.

Figure 2

Raw (uncorrected) ${\rho }^0$ $t_{\perp }$ spectrum in the range $0.0<|y|<0.5$ for the MB data. The points are data, with statistical errors. The dashed (filled) histogram is a simulation with an interference term (“Int”), while the solid histogram is a simulation without interference (“NoInt”). The handful of events histogrammed at the bottom of the plot are the wrong-sign (${\pi }^{+}{\pi }^{+}+{\pi }^{-}{\pi }^{-}$) events, used to estimate the combinatorial background.

Figure 3

Efficiency corrected $t_{\perp }$ spectrum for ${\rho }^0$ from (top) minium bias and (bottom) topology data, for midrapidity (left) and larger rapidity (right) samples. The points are the data, while the solid lines are the results of fits to Eq. (3).