From conformal correlators to analytic S-matrices: CFT$_1$/QFT$_2$

TL;DR

This paper connects one-dimensional conformal field theories with two-dimensional quantum field theories, demonstrating how S-matrices emerge from CFT correlators and establishing a dispersion relation that links CFT data to S-matrix analyticity and unitarity.

Contribution

It introduces a CFT-based framework for deriving and analyzing S-matrices, including a dispersion formula and the role of spectral gaps, advancing the understanding of unitarity and analyticity in QFT.

Findings

S-matrices can be derived from CFT correlators in the flat space limit.

Positivity of OPE coefficients enforces extended unitarity conditions on S-matrices.

Presence of anomalous thresholds signals unbounded OPEs and spectrum violations.

Abstract

We study families of one-dimensional CFTs relevant for describing gapped QFTs in AdS$_2$. Using the Polyakov bootstrap as our main tool, we explain how S-matrices emerge from the flat space limit of CFT correlators. In this limit we prove that the CFT OPE density matches that of a generalized free field, and that this implies unitarity of the S-matrix. We establish a CFT dispersion formula for the S-matrix, proving its analyticity except for singularities on the real axis which we characterize in terms of the CFT data. In particular positivity of the OPE establishes that any such S-matrix must satisfy extended unitarity conditions. We also carefully prove that for physical kinematics the S-matrix may be more directly described by a phase shift formula. Our results crucially depend on the assumption of a certain gap in the spectrum of operators. We bootstrap perturbative AdS bubble, triangle and box diagrams and find that the presence of anomalous thresholds in S-matrices are precisely signaled by an unbounded OPE arising from violating this assumption. Finally we clarify the relation between unitarity saturating S-matrices and extremal CFTs, establish a mapping between the dual S-matrix and CFT bootstraps, and discuss how our results help understand UV completeness or lack thereof for specific S-matrices.