QCD Sum Rules: Bridging the Gap between Short and Large Distances

TL;DR

This paper reviews the QCD sum rule method, discussing its limitations, the operator product expansion, and recent holographic approaches, highlighting the agreement and discrepancies in vacuum magnetic susceptibility calculations.

Contribution

It provides a critical analysis of the QCD sum rule method, including recent holographic models and the comparison of different theoretical results for magnetic susceptibility.

Findings

Operator product expansion has intrinsic limitations.

Holographic models offer new insights into QCD at strong coupling.

Vainshtein's formula for magnetic susceptibility matches Son and Yamamoto's result but lacks numerical accuracy.

Abstract

I discuss aspects of the QCD sum rule method which attracted theorists' attention in earnest at a relatively late stage and are not yet fully solved. At first I briefly review such general topics as the structure of the operator product expansion in QCD and intrinsic limitations of the quark-hadron duality concept. In the second part I comment on holographic constructions - a focus of the current efforts to say something new on QCD at strong coupling. Of particular interest to me is the recent derivation of the vacuum magnetic susceptibility due to Son and Yamamoto. Remarkably, their result is the same as that obtained previously by Vainshtein in the field-theoretic framework. For some reasons, the Vainshtein formula, unexpectedly, is not successfull numerically. This is a slightly modified version of the talk delivered at 15th International QCD Conference "QCD 10," June 28 - July 3, 2010, Montpellier, France.