N=4 Superconformal Bootstrap of the K3 CFT

TL;DR

This paper applies the superconformal bootstrap to N=4 superconformal field theories on K3 surfaces, revealing bounds on the spectrum and moduli dependence, and establishing new relations between superconformal blocks and Virasoro blocks.

Contribution

It introduces a novel relation between N=4 superconformal blocks and Virasoro blocks, and provides bounds on the spectrum and Laplacian eigenvalues in K3 CFTs.

Findings

Bound on the non-BPS spectrum as a function of moduli

Upper bound on the scalar Laplacian eigenvalue on K3

Exact relation between N=2 superconformal blocks and Virasoro blocks

Abstract

We study two-dimensional (4,4) superconformal field theories of central charge c=6, corresponding to nonlinear sigma models on K3 surfaces, using the superconformal bootstrap. This is made possible through a surprising relation between the BPS N=4 superconformal blocks with c=6 and bosonic Virasoro conformal blocks with c=28, and an exact result on the moduli dependence of a certain integrated BPS 4-point function. Nontrivial bounds on the non-BPS spectrum in the K3 CFT are obtained as functions of the CFT moduli, that interpolate between the free orbifold points and singular CFT points. We observe directly from the CFT perspective the signature of a continuous spectrum above a gap at the singular moduli, and find numerically an upper bound on this gap that is saturated by the $A_1$ N=4 cigar CFT. We also derive an analytic upper bound on the first nonzero eigenvalue of the scalar Laplacian on K3 in the large volume regime, that depends on the K3 moduli data. As two byproducts, we find an exact equivalence between a class of BPS N=2 superconformal blocks and Virasoro conformal blocks in two dimensions, and an upper bound on the four-point functions of operators of sufficiently low scaling dimension in three and four dimensional CFTs.