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Finite density QCD phase structure from strangeness fluctuations

Szabolcs Borsányi1, Zoltán Fodor2,3,1,4,5, Jana N. Guenther1, Piyush Kumar1, Paolo Parotto6, Attila Pásztor4,7, and Chik Him Wong1

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Phys. Rev. D 113, 054507 – Published 17 March, 2026

DOI: https://doi.org/10.1103/kb2f-9f8h

Abstract

Charting the phase diagram of quantum chromodynamics (QCD) at large density is a challenging task due to the complex action problem in lattice simulations. Through simulations at imaginary baryon chemical potential μB we observe that, if the strangeness neutrality condition is imposed, both the strangeness chemical potential μS/μB and the strangeness susceptibility χ2S take on constant values at the chiral transition for varying μB. We present new lattice data to extrapolate contours of constant μS/μB or χ2S to finite baryon chemical potential. We argue that they are good proxies for the QCD crossover because, as we show, they are only mildly influenced by criticality and by finite volume effects. We obtain continuum limits for these proxies up to μB=400  MeV, through a next-to-next-to-leading order (N2LO) Taylor expansion based on large-statistics data on 163×8, 203×10 and 243×12 lattices with our 4HEX improved staggered action. We show that these are in excellent agreement with existing results for the chiral transition and, strikingly, also with analogous contours obtained with the hadron resonance gas (HRG) model. On the 163×8 lattice, we carry out the expansion up to next-to-next-to-next-to-next-to-leading order (N4LO), and extend the extrapolation beyond μB=500  MeV, again finding perfect agreement with the HRG model. This suggests that the crossover line constructed from this proxy starts deviating from the chemical freeze-out line near μB≈500  MeV, as expected but not yet observed.

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Phys. Rev. D 113, 054507– Published 17 March, 2026
  • Received 12 November 2025
  • Accepted 18 February 2026
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