Speed of sound peak in two-color dense QCD: Confronting effective models with lattice data

Lattice simulations of two-color, two-flavor quantum chromodynamics (QCD) at finite quark chemical potential have revealed a distinctive peak structure in the sound velocity. Although chiral perturbation theory (ChPT) and the Nambu-Jona-Lasinio (NJL) model have been employed to explain this phenomenon, neither approach has fully captured the observed behavior. To address this discrepancy, we have extended the NJL framework by incorporating the medium separation scheme (MSS). This approach isolates medium contributions from divergent integrals, allowing for a more accurate treatment of finite-density effects. Our results indicate a clear increase in the diquark gap ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi mathvariant="normal">Δ</a:mi> </a:math> ) with increasing chemical potential, consistent with what is also seen in perturbative QCD predictions at high densities. Furthermore, the MSS-modified NJL model successfully reproduces the observed peak in the sound velocity.