star focus: Observation of Two-source Interference in STAR
Highlights from the STAR paper: Observation of Two-source Interference in the Photoproduction Reaction Au Au -> Au Au rho0. Submitted for publication to Physical Review Letters.

Posted: Jan 3, 2009

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 Rho0. The Rho0 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 Rho0 production. The produced Rho0s decay almost immediately at two well-separated points, so any interference must develop after the decay, and involve the pi(+) pi(-) final state. Since the pions go in different directions, this requires an entangled pi(+)pi(-) wave function which cannot be factorized into separate pi(+) and pi(-) wave functions; this is an example of the Einstein-Podolsky-Rosen paradox (for more details on this paradox look at reference below).

The figure shows the the uncorrected midrapidity minimum bias Au+Au 200 GeV dN/dt spectra as a function of t(perp) = (pT*pT). These data are compared simulations based with and without interference. The measured dN/dt spectrum is roughly exponential, but with a significant downturn for t(perp) < 0.0015 GeV*GeV, consistent with the predicted interference (dashed histogram). The no-interference histogram is almost exponential (solid histogram), dN/dt ~ exp (-kt(perp)), where k is related to the nuclear radius.

We observe the interference at 87 +/- 5(stat.)+/-8 (syst.)% of the expected level. This shows that the final state wave function retains amplitudes for all possible decays, long after the decay occurs. The maximum decoherence (loss of interference) is less than 23% at the 90% confidence level.

Reference for Einstein-Podolsky-Rosen paradox :
A. Einstein, B. Podolsky and N. Rosen, Phys. Rev. 47, (1935) 777.

Further details can be found in the following STAR paper -
Observation of Two-source Interference in the Photoproduction Reaction Au Au -> Au Au rho0 - arXiv:0812.1063.

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