Additional Strange Hadrons from QCD Thermodynamics and Strangeness Freezeout in Heavy Ion Collisions

We compare lattice QCD results for appropriate combinations of net strangeness fluctuations and their correlations with net baryon number fluctuations with predictions from two hadron resonance gas (HRG) models having different strange hadron content. The conventionally used HRG model based on experimentally established strange hadrons fails to describe the lattice QCD results in the hadronic phase close to the QCD crossover. Supplementing the conventional HRG with additional, experimentally uncharted strange hadrons predicted by quark model calculations and observed in lattice QCD spectrum calculations leads to good descriptions of strange hadron thermodynamics below the QCD crossover. We show that the thermodynamic presence of these additional states gets imprinted in the yields of the ground-state strange hadrons leading to a systematic 5–8 MeV decrease of the chemical freeze-out temperatures of ground-state strange baryons.

Figure 1

Ratios of partial pressures of open strange hadrons ($P_{\mathrm{tot}}^{S,X}$), mesons ($P_M^{S,X}$), and baryons ($P_B^{S,X}$) calculated in HRG with particle spectra from the particle data table, $X=\mathrm{PDG}$, and with additional resonances predicted by the relativistic quark model, $X=\mathrm{QM}$, respectively (see text).

Figure 2

Top: $BS$ correlations normalized to the second cumulant of net strangeness fluctuations. Results are from ($2+1$)-flavor lattice QCD calculations performed with a strange to light quark mass ratio $m_s/m_l=20$ (squares) and $m_s/m_l=27$ (diamonds). The band depicts the improved estimate for the continuum result facilitated by the high statistics $N_{\tau }=6$ and 8 data. Bottom: Ratios of baryonic ($B_i^S$) to mesonic ($M_i^S$) pressure observables defined in Eq. (3). The dotted and solid lines show results from PDG-HRG and QM-HRG model calculations, respectively. The shaded region denotes the continuum extrapolated chiral crossover temperature at physical values of quark masses $T_c=(154\pm 9)\mathrm{MeV}$ [9].

Figure 3

The leading-order result for the ratio of strangeness and baryon chemical potentials versus temperature. Data and HRG model results are for a strangeness neutral thermal system having a ratio of net electric charge to net baryon number density $N_Q/N_B=0.4$. The dashed line shows the QM-HRG result for vanishing electric charge chemical potential. All other curves and labels are as in Fig. 2.

Figure 4

Values of (${\mu }_S^f/{\mu }_B^f$, ${\mu }_B^f/T^f$) extracted from fits to multiple strange hadrons yields (see text) are compared to ${\mu }_S/{\mu }_B$ predictions, obtained by imposing strangeness neutrality, from lattice QCD calculations (shaded bands) as well as from QM-HRG (solid lines) and PDG-HRG (dotted lines) models. The predictions are shown for ${\mu }_B/T={\mu }_B^f/T^f$. For each case, the temperature ranges are chosen such that the predicted values reproduce ${\mu }_S^f/{\mu }_B^f$.