Dispersive CFT Sum Rules

TL;DR

This paper develops a unified framework for dispersive sum rules in conformal field theory, connecting different methods and introducing a nonperturbative Polyakov-Regge expansion to analyze correlators and constrain holographic CFTs.

Contribution

It unifies three approaches to dispersive sum rules, introduces a nonperturbative Polyakov-Regge expansion, and constructs sum rules with physical positivity properties.

Findings

Equivalent formulations of dispersive sum rules via three methods.

Construction of non-negative sum rules above the double-twist gap.

Application to extremal functional for the spin-two gap problem.

Abstract

We give a unified treatment of dispersive sum rules for four-point correlators in conformal field theory. We call a sum rule dispersive if it has double zeros at all double-twist operators above a fixed twist gap. Dispersive sum rules have their conceptual origin in Lorentzian kinematics and absorptive physics (the notion of double discontinuity). They have been discussed using three seemingly different methods: analytic functionals dual to double-twist operators, dispersion relations in position space, and dispersion relations in Mellin space. We show that these three approaches can be mapped into one another and lead to completely equivalent sum rules. A central idea of our discussion is a fully nonperturbative expansion of the correlator as a sum over Polyakov-Regge blocks. Unlike the usual OPE sum, the Polyakov-Regge expansion utilizes the data of two separate channels, while having (term by term) good Regge behavior in the third channel. We construct sum rules which are non-negative above the double-twist gap; they have the physical interpretation of a subtracted version of superconvergence sum rules. We expect dispersive sum rules to be a very useful tool to study expansions around mean-field theory, and to constrain the low-energy description of holographic CFTs with a large gap. We give examples of the first kind of applications, notably, we exhibit a candidate extremal functional for the spin-two gap problem.