QCD transition line from strangeness fluctuations

Mapping the QCD phase diagram at large density remains challenging due to the complex action problem in lattice simulations. Using simulations at imaginary baryon chemical potential ($\mu_B$), we observe that, under the strangeness-neutrality condition, both the strangeness chemical potential ($\mu_S / \mu_B$) and the strangeness susceptibility ($\chi_2^S$) take on constant values at the chiral transition across varying $\mu_B$. We present new lattice data to extrapolate contours of constant $\mu_S/\mu_B$ or $\chi_2^S$ to finite baryon chemical potential. These proxies are shown to be only mildly affected by criticality and finite-volume effects, and their continuum limits up to $\mu_B = 400\,\text{MeV}$ agree well with existing determinations of the chiral transition and with the hadron resonance gas model.