Figure 1.6: Experimental and theoretical predictions for the running QCD coupling α ( µ ). Figure taken from Ref. [20]. energies. There are two broad classes of theoretical techniques that are used to study hadrons: those that are inspired by QCD and those that are based in QCD. The key distinction between these two is that the latter utilize the QCD Lagrangian (1.40) while the former do not. Most QCD-inspired techniques are based on effective field theory methods, such as chiral perturbation theory [45] or heavy quark effective theory [93]. Additional QCD-inspired methods include potential models [74] and techniques based on the AdS/CFT correspondence in string theory [69]. Methods that are based in QCD typically augment perturbation theory in some way or avoid it entirely. In lattice QCD the path integral is calculated numerically in a discretized Euclidean space [73]. The Dyson-Schwinger equations are an infinite set of coupled integral equations relating various correlation functions in the interacting theory. When truncated, the equations can be solved and used to determine hadronic parameters [85]. Another QCD-based approach is QCD sum rules (QSR). The QSR method is based upon the concept of quark-hadron duality and on the operator product expansion (OPE). Refs. [106, 107] are the original papers outlining the QSR tech- 33