Balance functions from Au+Au, d+Au, and p+p collisions at root s(NN)=200 GeV
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Balance functions have been measured for charged-particle pairs, identified charged-pion pairs, and identified charged-kaon pairs in Au+Au, d+Au, and p+p collisions at sNN=200 GeV at the Relativistic Heavy Ion Collider using the STAR detector. These balance functions are presented in terms of relative pseudorapidity, , relative rapidity, y, relative azimuthal angle, , and invariant relative momentum, qinv. For charged-particle pairs, the width of the balance function in terms of scales smoothly with the number of participating nucleons, while hijing and urqmd model calculations show no dependence on centrality or system size. For charged-particle and charged-pion pairs, the balance functions widths in terms of and y are narrower in central Au+Au collisions than in peripheral collisions. The width for central collisions is consistent with thermal blast-wave models where the balancing charges are highly correlated in coordinate space at breakup. This strong correlation might be explained by either delayed hadronization or limited diffusion during the reaction. Furthermore, the narrowing trend is consistent with the lower kinetic temperatures inherent to more central collisions. In contrast, the width of the balance function for charged-kaon pairs in terms of y shows little centrality dependence, which may signal a different production mechanism for kaons. The widths of the balance functions for charged pions and kaons in terms of qinv narrow in central collisions compared to peripheral collisions, which may be driven by the change in the kinetic temperature. 2010 The American Physical Society.