Comparing EPOS-4, EPOS-LHC, and SMASH for identified-hadron observables in the NICA energy range

Abstract

We present a systematic simulation study of identified hadron production in minimum bias Au+Au collisions at sqrt(sNN) = 6, 7, and 8 GeV. The event samples were generated with three modern frameworks based on different microscopic pictures: EPOS-LHC, EPOS-4, and the purely hadronic transport model SMASH. We compare observables that probe baryon stopping, transverse dynamics, hadron formation, and strangeness production: rapidity densities dN/dy, transverse momentum spectra dN/dpT, two dimensional pT-y distributions, v2/nq versus pT/nq, and the yield ratios pi-/pi+, K-/K+, pbar/p, K+/pi+, K-/pi-, p/pi+, and Lambda/pi+. For charged and neutral pions, the three models give broadly similar yields and spectral shapes in both dN/dy and dN/dpT. At these energies, resonance decays and isospin constraints reduce the sensitivity to early stage dynamics. In contrast, strange mesons and baryons remain strongly model dependent. EPOS-4 gives the largest midrapidity K+ and Lambda yields and the hardest kaon and strange baryon pT spectra. EPOS-LHC is generally intermediate, while SMASH gives lower strange hadron production. The two dimensional pT-y maps show that the EPOS models populate higher pT over a broader rapidity range. For NCQ scaled elliptic flow, the best approximate scaling is seen in EPOS-LHC, while SMASH and EPOS-4 show only partial scaling. This suggests that EPOS-LHC carries a more coherent partonic anisotropy to the hadronic stage in this energy range. Overall, the model separation grows from 6 to 8 GeV. The clearest discriminators in the NICA domain are intermediate pT baryon to meson ratios, Lambda/pi+, the rapidity dependence of pT, the pT dependence of K-/K+, and NCQ scaled v2.

Figures (14)

• Figure 1: Rapidity distributions (event-normalized $dN/dy$) of identified hadrons at minimum-bias in $Au+Au$ collisions at $\sqrt{s_{NN}}=6~GeV$ forecasted by the three models. The results of EPOS-LHC, EPOS-4, and SMASH for various species of hadrons are presented in panels (a)-(g) to highlight the differences in total yield, width in rapidity, and mid-rapidity behaviour. Model legends are in the panels. Each plot has a ratio panel at its bottom, simplifying the model to model comparison.
• Figure 2: Rapidity distributions (event-scaled $dN/dy$) of hadrons (identified) in minimum-bias $Au+Au$ collisions, on a scale of $\sqrt{s_{NN}}=7~GeV$. The three model predictions (EPOS-LHC, EPOS-4, SMASH) are illustrated to highlight the energy evolution between 6 and 7 GeV and model sensitivity in strange/higher-mass hadrons.
• Figure 3: Rapidity distributions (event-normalized $dN/dy$) of identified hadrons at minimum-bias in $Au+Au$ collisions at $\sqrt{s_{NN}}=8~GeV$ forecasted by the three models. The results of EPOS-LHC, EPOS-4, and SMASH for various species of hadrons are presented in panels (a)-(g) to highlight the differences in total yield, width in rapidity, and mid-rapidity behaviour. Model legends are in the panels.
• Figure 4: Event-normalized $dN/dp_T$ transverse-momentum spectra of identified hadrons in minimum-bias $Au+Au$ collisions at $\sqrt{s_{NN}}=6~GeV$. The EPOS-LHC, EPOS-4, and SMASH model predictions are compared to demonstrate variations in the shape of the spectrum, slope (soft vs. hard components), and relative yield at low and intermediate $p_T$. Each plot has a ratio panel at its bottom, simplifying the model to model comparison.
• Figure 5: Event-normalized $dN/dp_T$ transverse-momentum spectra of identified hadrons in minimum-bias $Au+Au$ collisions at $\sqrt{s_{NN}}=7~GeV$. The EPOS-LHC, EPOS-4, and SMASH model predictions are compared to demonstrate variations in the shape of the spectrum, slope (soft vs. hard components), and relative yield at low and intermediate $p_T$.