Paper Proposal: Interference in rho production

Proposed for: Physical Review Letters

This paper presents a measurement of inteference in rho^0 production in UPCs. The (still incomplete) supporting documentation is located here. This includes a theoretical introduction.

Abstract

In ultra-peripheral 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 $\rho$ production is well localized at the two nuclei, forming a 2-source interferometer. The two sources interfere, and $\rho^0$ production at low transverse momentum is suppressed. We measure this interference in 200 GeV per nucleon Au-Au collisions, and observe interference at $93\pm 6\pm x\%$ of the expected level, and find a maximum decoherence, due to wave function collapse or other factors, of x\% at the 90\% confidence level.

This interference occurs even though the $\rho^0$ decay before their pre-decay wave functions can overlap. In one interpretation, the interference requires that the post-decay wave functions retain amplitudes for all possible decay modes, long after the decay occurs.

Figures

Figure 1

Fig. 1. A schematic diagram of the two possibilities: a photon from nucleus 1 scattering from nucleus 2, or vice versa.

Figure 2

Fig. 2. The rapidity distribution for exclusive rho^0 (right) and the Coulomb breakup sample (left). The data is in red, while the blue histogram are the result of a simulation based on the soft Pomeron model, after detector acceptance.

Figure 3

Fig. 3. The uncorrected t distribution for rho^0 production with 0.1 < |y| <0.5. The data is red, while the blue and green histogram are simulations with and without interference respectively. The black histogram is the like sign background.

Figure 4

Figure 4a: XnXn 0.1 < |y|<0.5

Figure 4b: XnXn 0.5 < |y| < 1.0

Figure 4c: topology 0.1 < |y| < 0.5