Spectral sum rules for conformal field theories in arbitrary dimensions

TL;DR

This paper derives spectral sum rules for conformal field theories in arbitrary dimensions, linking spectral densities to energy density and stress tensor correlators, with implications for theories with gravity duals and causality constraints.

Contribution

It establishes general spectral sum rules in conformal field theories across dimensions, connecting high-frequency OPE behavior and low-frequency hydrodynamics, and relates them to stress tensor three-point functions.

Findings

Sum rules relate spectral integrals to energy density.

Proportionality constants depend on Hofman-Maldacena variables.

Bounds on sum rules derived from causality constraints.

Abstract

We derive spectral sum rules in the shear channel for conformal field theories at finite temperature in general $d\geq 3$ dimensions. The sum rules result from the OPE of the stress tensor at high frequency as well as the hydrodynamic behaviour of the theory at low frequencies. The sum rule states that a weighted integral of the spectral density over frequencies is proportional to the energy density of the theory. We show that the proportionality constant can be written in terms the Hofman-Maldacena variables $t_2, t_4$ which determine the three point function of the stress tensor. For theories which admit a two derivative gravity dual this proportionality constant is given by $\frac{d}{2(d+1)}$. We then use causality constraints and obtain bounds on the sum rule which are valid in any conformal field theory. Finally we demonstrate that the high frequency behaviour of the spectral function in the vector and the tensor channel are also determined by the Hofman-Maldacena variables.