Freeze-out temperature from net-kaon fluctuations at energies available at the BNL Relativistic Heavy Ion Collider

We compare the mean-over-variance ratio of the net-kaon distribution calculated within the hadron resonance gas model to the latest experimental data from the Beam Energy Scan at the Relativistic Heavy Ion Collider by the STAR Collaboration. Our analysis indicates that it is not possible to reproduce the experimental results using the freeze-out parameters from the existing combined fit of net-proton and net-electric charge mean over variance. The strange mesons need about 15 MeV higher temperatures than the light hadrons at the highest collision energies. In view of the future $\mathrm{\Lambda }$ fluctuation measurements, we predict the $\mathrm{\Lambda }$ variance over mean and skewness times variance at the light and strange chemical freeze-out parameters. We observe that the $\mathrm{\Lambda }$ fluctuations are sensitive to the difference in the freeze-out temperatures established in this analysis. Our results have implications for other phenomenological models in the field of relativistic heavy ion collisions.

Figure 1

Results for ${\chi }_1^K/{\chi }_2^K$ calculated in the HRG model along the lattice QCD isentropic trajectories (pink, dashed band) compared to ${(M/{\sigma }^2)}_K$ data from [27] (gray, full band) across the Beam Energy Scan at STAR.

Figure 2

Freeze-out parameters across the highest five energies from the Beam Energy Scan. The red points were obtained from the combined fit of ${\chi }_1^p/{\chi }_2^p$ and ${\chi }_1^Q/{\chi }_2^Q$ [28], while the gray bands are obtained from the fit of ${\chi }_1^K/{\chi }_2^K$ in this work. Also shown are the freeze-out parameters obtained by the STAR collaboration at $\sqrt{s}=39$ GeV [10] from thermal fits to all measured ground-state yields (orange triangle) and only to protons, pions, and kaons (blue diamond-shaped symbol).

Figure 3

Upper panel: ${\chi }_2^{\mathrm{\Lambda }}/{\chi }_1^{\mathrm{\Lambda }}$ as a function of $\sqrt{s}$. Lower panel: ${\chi }_3^{\mathrm{\Lambda }}/{\chi }_2^{\mathrm{\Lambda }}$ as a function of $\sqrt{s}$. In both panels, the orange points joined by a full line are calculated at the values of $T_f$ and ${\mu }_{Bf}$ extracted from the fit of ${\chi }_1^K/{\chi }_2^K$, while the blue points joined by a dashed line are calculated at the values of $T_f$ and ${\mu }_{Bf}$ extracted from the combined fit of ${\chi }_1^p/{\chi }_2^p$ and ${\chi }_1^Q/{\chi }_2^Q$ in Ref. [28].