A formula for the block expansion in free CFTs and applications to ${\cal N}=4$ SYM at strong coupling

Abstract

An explicit analytic formula is presented that computes the conformal (super-)block decomposition of any free scalar or half-BPS diagram in 1d, 2d or 4d CFTs, with various supersymmetries, including none. We prove our formula by exploiting a connection between conformal blocks and symmetric polynomials. Then we give a direct application of our result to the study of four-point correlators in ${\cal N}=4$ SYM at strong coupling. In particular, we give a CFT proof of the tree-level Witten diagram representation of $\langle {\cal O}_2^2{\cal O}_2^2 {\cal O}_q{\cal O}_q\rangle$ on AdS$_5\times$S$^5$, providing new and highly non-trivial checks of the AdS/CFT correspondence. Our method works for a more general class of multi-particle correlators and can be used to bootstrap new results at strong coupling.