\equiv \eta_1 - \eta_2$ to form an autocorrelation [*without loss of information*]{}. The projection is done by averaging bin contents along each diagonal in Fig. \[Figure1\] parallel to the sum axis ([*e.g.*]{} the $\eta_1 = \eta_2$ diagonal) to obtain the bin contents of a 1D autocorrelation on $\eta_{\Delta}$ or $\phi_{\Delta}$ (the difference axes). Autocorrelation details are described in [@inverse; @mitmeths]. If projections are made simultaneously onto both difference variables of Fig. \[Figure1\] the resulting 2D joint autocorrelation on $(\eta_\Delta,\phi_\Delta)$ compactly represents [*all*]{} significant correlation structure on 4D angular subspace $(\eta_1,\eta_2,\phi_1,\phi_2)$.

In Fig. \[Figure2\] perspective views are shown of CD joint autocorrelations for four centrality classes of Au-Au collisions at $\sqrt{s_{NN}} =$ 130 GeV. Quantity $\bar N(\hat r - 1)$[ [@nbar]]{} represents [*per-particle*]{} correlations ([*i.e.,*]{} distribution of average numbers of correlated pairs per final-state particle) and is $O(1)$ for all centralities. Distributions in Fig. \[Figure2\] are dominated by a 2D negative peak which is broader and elliptical for peripheral collisions (d) with major axis along $\phi_\Delta$, transitioning smoothly to a narrower and deeper peak symmetric on $(\eta_\Delta,\phi_\Delta)$ for central collisions (a). The negative peak means that unlike-sign charge pairs are more probable than like-sign pairs for small angular separations on pseudorapidity 