Pion Spectra and Entropy per Rapidity in Au–Au Collisions at NICA Energies Using HRG Model with Resonance Decays and Tsallis Distribution

We study transverse momentum spectra and entropy per rapidity of charged pions in central Au–Au collisions at $$\\sqrt {{{s}_{{NN}}}} = 7.7$$ and 11.5 GeV, relevant for the NICA energy range. The low- $${{p}_{{\\text{T}}}}$$ region is fitted with a Tsallis distribution, while the intermediate and high- $${{p}_{{\\text{T}}}}$$ parts are described by a hadron resonance gas (HRG) model including resonance decays. A smooth matching point ensures continuity between the two descriptions. The combined fits allow extrapolation toward $${{p}_{{\\text{T}}}} = 0$$ and large $${{p}_{{\\text{T}}}}$$ , enabling robust estimates of entropy per rapidity. Results show that the hybrid Tsallis + HRG approach reproduces pion spectra across the full $${{p}_{{\\text{T}}}}$$ range with good accuracy, and the extracted entropy provides a sensitive thermodynamic probe of strongly interacting matter at high baryon density.