Figure 1: The black histogram shows the pion pair transverse momentum. The peak below 100 MeV/c is from the decay of coherently produced π+π− pairs. The red histogram shows the pair momentum for same-sign pion pairs. Both histograms show pairs that come from vertices with only two tracks.

Figure 3: The shower energy in the West ZDC by neutron produced by mutual dissociation is shown as a distribution of ADC channels. These events had a single neutron detected on the East ZDC. The peaks corresponding to 1 to 4 neutrons are fitted with Gaussian distributions with standard deviations that grow as nσ with n the number of neutrons and σ the standard deviation of the one neutron Gaussian. The red curve is the sum of all Gaussians which are also displayed individually. The quality of fit is χ2/NDF = 498/88. The large χ2/NDF is due to the very small statistical errors and the imperfect descriptions of the neutron peaks. It does not introduce significant errors on the number of neutrons in each peak.

Figure 5: (Top) The ratio |B/A| of amplitudes of non-resonant π+π−and ρ0 mesons. The black points (with shaded blue systematic error band) are from the current analysis, while the previous STAR results are shown with blue-filled circles. The thick black line shows the rapidity-averaged result. In the bottom panel, the black points show the ratio |C/A| of the ω to ρ0 amplitude. The red band shows the systematic errors, while the horizontal blue line shows the STARlight prediction and the most recent branching ratio for ω → π+π− decay [26]. The green band shows the DESY-MIT result for |C/A| [27]. This result was at much lower photon energies leads to a large effective rapidity. For the lower energy photon solution of the two-fold ambiguity, the effective rapidity would be about 2.5.

Figure 6: dσ/dy for exclusively photoproduced ρ0 mesons in (top) XnXn events and (bottom) 1n1n events. The data are shown with red markers. The statistical errors are smaller than the symbols, the orange band shows the quadrature sum of the point-to-point systematic uncertainties. The red box at y ∼ −0.9 shows the quadrature sum of the common systematic uncertainties. The black histograms are the STARlight calculation for ρ0 mesons with mutual dissociation. The blue markers in the top panel show the previous STAR measurement [10].

Figure 7: The −t distribution for exclusive ρ0 mesons in events with 1n,1n mutual dissociation (blue markers) or XnXn (red markers). The high t part of those distributions, which is dominated by the contribution from incoherent interactions is fit to a dipole form factor, shown with a thin line. The STARlight prediction for the incoherent contribution is shown by the histogram with small black markers.

Figure 8: Fully normalized coherent diffraction patterns for ρ0 mesons detected in exclusive XnXn events is shown with red markers. The same distribution but extracted from 1n1n events is shown with black markers. The filled bands shows the sum in quadrature of all systematic uncertainties listed in table 4 and the statistical errors, which are shown as vertical lines.The insert shows, with finer binning at low pT , the effects of the destructive interference between photoproduction with the photon emitted by any of the two ions.

Figure 9: The normalized nucleon distribution in the transverse plane, the result of a two- dimensional Fourier transform (Hankel transform) of the XnXn and 1n1n diffraction patterns shown in Fig. 8. The integration is limited to a region where data is available; in the range 0 < |t| < 0.06 GeV2. The cyan error band shows the effect of changing the maximum t to 0.05, 0.07 and 0.09 GeV2. In order to highlight the similarity of both results at their falling edges, the resulting histograms are scaled by their integrals from -12 to 12 fm. The FWHM of both transforms is 2 × (6.17 ± 0.12) fm consistent with the coherent diffraction of ρ0 mesons off an object as big as the Au nuclei.