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Title: Lower edge of the conformal window in SU(3) gauge theory
Plain: QCD with many massless quark species becomes scale invariant at long distances. The smallest number of species for which this happens is unknown.
Precise: $\mathrm{SU}(3)$ gauge theory with $N_{f}$ massless Dirac fermions in the fundamental representation is asymptotically free for $N_{f} < 16.5$ and has a perturbative IR fixed point near that value. Determine $N_{f}^{*}$ such that for $N_{f}^{*} \le N_{f} < 16.5$ the IR is a CFT and for smaller $N_{f}$ chiral symmetry breaks; in particular decide $N_{f} = 8, 10 \text{ and } 12$.
Settled by: Continuum-extrapolated lattice determinations of the step-scaling function or spectrum for $N_f = 8\ \text{to}\ 13$ that agree between independent groups.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: contested
Status note: Lattice groups disagree on $N_{f} = 10\text{ and }12$; estimates of $N_{f}^{*}$ range from about 9 to 13 (2009-2025).
Posed since: (empty)