to form like-sign (LS: $++,--$), unlike-sign (US: $+-,-+$), and charge-dependent (CD = LS $-$ US) ratios. In this analysis we adopt a CD sign convention compatible with standard particle physics isospin convention and net-charge fluctuation measures [@meanptprl].

Data
====

Data for this analysis were obtained with the STAR detector [@star] using a 0.25 T uniform magnetic field parallel to the beam axis. A minimum-bias event sample required coincidence of two Zero-Degree Calorimeters (ZDC); a 0-15% of total cross section event sample was defined by a threshold on the Central Trigger Barrel (CTB), with ZDC coincidence. Event triggering and charged-particle measurements with the Time Projection Chamber (TPC) are described in [@star]. Tracking efficiencies, event and track quality cuts and primary-particle definition are described in [@meanptprl; @spectra]. Charged particles were accepted in $|\eta| \leq 1.3$, full azimuth and transverse momentum ($p_t$) range $0.15 \leq p_t \leq 2$ GeV/$c$. Particle identification was not implemented but charge sign was determined. Corrections were made to ratio $\hat r$ for two-track inefficiencies due to overlapping space points in the TPC (merging) and intersecting trajectories reconstructed as $>2$ particles (splitting) by applying two-track proximity cuts in the TPC to both $\rho_{sib}$ and $\rho_{mix}$ similar to that done 