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  • Letter
  • Open Access

Speed of sound peak in isospin QCD: A natural prediction of the medium separation scheme

Bruno S. Lopes1,*, Dyana C. Duarte1,†, Ricardo L. S. Farias1,‡, and Rudnei O. Ramos2,§

  • *Contact author: bruno.lopes@acad.ufsm.br
  • †Contact author: dyana.duarte@ufsm.br
  • ‡Contact author: ricardo.farias@ufsm.br
  • §Contact author: rudnei@uerj.br
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Phys. Rev. D 112, L091903 – Published 19 November, 2025

DOI: https://doi.org/10.1103/459l-qqd1

Abstract

We present predictions for the zero-temperature equation of state at finite isospin density using the Nambu-Jona-Lasinio (NJL) model within the medium separation scheme (MSS)—a scheme that explicitly disentangles medium effects from the ultraviolet divergent vacuum terms. Recent lattice QCD results reveal a nonmonotonic speed of sound (cs2) as a function of isospin chemical potential (μI), exhibiting explicit violation of the conformal bound cs2=1/3. These findings have attracted significant theoretical interest, as established models—including the NJL model—failed to anticipate this behavior prior to lattice simulations. Conventional NJL implementations yield unphysical artifacts, often attributed to regularization scale sensitivity stemming from nonrenormalizability. However, in this work, we demonstrate that the standard NJL framework combined with MSS quantitatively reproduces state-of-the-art lattice data for isospin QCD.

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Phys. Rev. D 112, L091903– Published 19 November, 2025
  • Received 23 July 2025
  • Accepted 29 October 2025
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