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The answer constrains where a critical point can lie in the real phase diagram.","posed_since":"1984","precise":"For $N_{f}=2$ massless flavors (and $2+1$ with physical strange mass) at $\\mu_{B}=0$, determine whether the chiral transition is first or second order and, if second order, whether it is in the $O(4)$ universality class or a larger one because the anomalous $U(1)_{A}$ symmetry is effectively restored at $T_{c}$ (Pisarski and Wilczek, 1984). Answer: order and universality class from continuum-extrapolated lattice QCD, with $T_{c}$ in the chiral limit.","problem_ref":null,"references":"","settled_by":"Lattice QCD with chirally symmetric fermions at several light-quark masses approaching zero, with continuum extrapolation, scaling analysis and $U(1)_{A}$-breaking susceptibilities.","status_note":"Lattice studies found evidence consistent with a second-order transition in the continuum chiral limit for $N_f$ up to 6 (Cuteri, Philipsen, Sciarra, JHEP 11 (2021) 141), while the universality class and the fate of $\\mathrm{U}(1)_A$ remained debated as of 2026.","title":"Order of the chiral transition for massless light 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