QCD critical surface from constant entropy contours

We provide the first mapping of the critical surface in (2+1)-flavor QCD in the full $(T,\mu_B,\mu_Q,\mu_S)$ space, anchored on lattice QCD results at vanishing chemical potentials and obtained within an expansion along contours of constant entropy density. In the pure $\mu_B$ direction, this framework yields a critical point at $(T_c,\mu_{B,c}) \simeq (114,\, 602)$ MeV. Here we extend the construction to arbitrary directions in the three-dimensional chemical-potential space, parametrized by spherical coordinates $(\mu,\theta,\varphi)$, with the radial expansion truncated at $\mathcal{O}(\mu^2)$. The resulting two-dimensional surface carries a direction-dependent critical temperature $T_c(\theta,\varphi)$ and baryochemical potential $\mu_{B,c}(\theta,\varphi)$, which quantify the shift of the critical point relative to the pure $\mu_B$ direction. We find that $\mu_{B,c}$ increases by 40-100 MeV along the approximately strangeness neutral direction [$\mu_S \approx (0.15$--$0.33)\, \mu_B$, $\mu_Q \approx 0$] relevant for heavy-ion collisions, while the critical temperature stays essentially unchanged. In the charge-neutral, weak-equilibrium direction~[$\mu_Q \approx -(0.05$--$0.1) \,\mu_B$, $\mu_S = 0$] relevant for neutron star mergers, the critical point, and the associated first-order phase transition, remain present at essentially the same location in the $(T,\mu_B)$ plane. We find no evidence for a critical point at large isospin densities, $|\mu_Q| / \mu_B \gtrsim 1$, relevant for cosmic trajectories in the early Universe, nor along the pure electric-charge or strangeness directions, at least outside the regions where pion or kaon condensation may occur.