$K/\pi$ Fluctuations at Relativistic Energies

We report $K/\pi$ fluctuations from $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=19.6$, 62.4, 130, and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider. $K/\pi$ fluctuations in central collisions show little dependence on incident energy and are on the same order as those from NA49 at the Super Proton Synchrotron in central $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=12.3$ and 17.3 GeV. We report results for the collision centrality dependence of $K/\pi$ fluctuations and results for charge-separated fluctuations. We observe that the $K/\pi$ fluctuations scale with the charged particle multiplicity density.

Figure 1

(a) The event-by-event $K/\pi$ ratio for 200 GeV $\mathrm{Au}+\mathrm{Au}$ central collisions (0–5%) compared with the same quantity calculated from mixed events. (b) The ratio of the distribution from real events to that from mixed events. The errors shown are statistical.

Figure 2

Measured $K/\pi$ fluctuations in terms of ${\sigma }_{\mathrm{dyn}}$ for central $\mathrm{Au}+\mathrm{Au}$ collisions (0–5%) at 19.6, 62.4, 130, and 200 GeV compared with the central $\mathrm{Pb}+\mathrm{Pb}$ collisions (0–3.5%) from NA49 [7] and the statistical hadronization (SH) model of Ref. [11]. The solid line is described in the text. Both statistical (horizontal bar) and systematic (bracket) error bars are shown for the experimental data.

Figure 3

Measured $K/\pi$ fluctuations in terms of ${\nu }_{\mathrm{dyn},K\pi }$ for 62.4 and 200 GeV $\mathrm{Au}+\mathrm{Au}$ compared with ${\sigma }_{\mathrm{dyn}}^2$ from central $\mathrm{Pb}+\mathrm{Pb}$ collisions at 6.3, 7.6, 8.8, 12.3, and 17.3 GeV from NA49 [7]. Statistical errors are shown for the STAR data. Statistical and systematic errors are shown for the NA49 results. The solid line corresponds to a fit to the STAR data of the form $c+d/(d{N}_{\mathrm{ch}}/d\eta )$.

Figure 4

The $d{N}_{\mathrm{ch}}/d\eta$ scaled $K/\pi$ fluctuations for summed charges (stars), same signs (circles), and opposite signs (squares) as a function of $d{N}_{\mathrm{ch}}/d\eta$. The errors shown are statistical. The dash-dotted, dotted, and dashed lines represents HIJING calculations for summed charges, same signs, and opposite signs, respectively.