What crossing symmetry forbids just above $c=1$

Abstract

What does a unitary two-dimensional conformal field theory look like just above $c=1$? Using the numerical conformal bootstrap, we study the four-point function of a scalar Virasoro primary $ϕ$ such that no primary lighter than $ϕ$ appears in its operator product expansion with itself. Slightly above $c=1$, crossing symmetry excludes the interval $0.17\lesssimΔ_ϕ\lesssim1$, although the compact boson realizes every such $Δ_ϕ$ at $c=1$. Combining this exclusion with results from the modular bootstrap, we show that the lightest primary of a compact unitary conformal field theory at $c=1.0005$ or $1.001$ has dimension $Δ_{\rm gap}<0.17$. Further from $c=1$, the largest excluded central charge at fixed $Δ_ϕ$ has two sharp minima near $Δ_ϕ=1/2$ and $4/7$. At these values it lies within $2\times10^{-7}$ of $c=8/7$, where the $\mathbb{Z}_5$ parafermion and its charge-conjugation orbifold have scalars of exactly these dimensions.