Charged kaon ratios and yields measured with the STAR detector at the Relativistic Heavy Ion Collider

The mid-rapidity charged kaon ratios and yields are reported for the 200 AGeV Au+Au, 130 AGeV Au+Au, and 200 GeV pp data sets. The K −/K+ ratios are shown to be flat as a function of rapidity, transverse momentum, and centrality for the ranges investigated. The integrated ratios are 0.928 ± 0.0028 (stat.) ± 0.03 (sys.), 0.953 ± 0.0.0012 (stat.) ± 0.01 (sys.), and 0.964 ± 0.0039 (stat.) ± 0.01 (sys.) for 130 AGeV Au+Au, 200 AGeV Au+Au, and 200 GeV pp respectively. Thermal fits are applied to the ratios to extract the baryo-chemical potential and chemical freeze-out temperature. The baryo-chemical potential, as well as the kaon ratio, suggest that the net-baryon density at mid-rapidity is approaching zero at RHIC energies. A quark coalescence model suggests quark degrees of freedom are important in the formation of the ratios. The corrected yields are fit with an exponential in m_t and the dN/dy and inverse slope parameter are extracted. The inverse slope parameter is used along with the average collective flow velocity in a simple relationship to extract the thermal freeze-out temperature. A more sophisticated hydrodynamically motivated fit, using pion, kaon, and proton data, shows agreement with the trend from this simple relationship.