Experimentally known spectral functions such as that shown in Fig. 1.9 can be related to a theoretically calculated correlation function Π ( Q $^{2}$) via Eq. (1.86). In this way, QSR techniques can be used to extract QCD parameters in terms of experimentally measured quantities. Alternatively, a resonance model can be used to calculate hadron properties in terms of QCD parameters. This must be done in order to study exotic hadrons with QSR. For instance, a single narrow resonance can be parametrized as where f and M are the decay constant and mass of the hadron corresponding to the resonance. It is natural to question accuracy of this admittedly rather simple resonance model. However, it is important to remember that in QCD Laplace sum rules the resonance is multiplied by an exponential factor which tends to obscure any detailed features of the resonance. In addition, methods described in Ref. [46] can be used to estimate resonance width effects. The most basic quantity of interest in any QSR analysis is the hadron mass which can be determined using this model. Inserting Eq. (1.111) into Eq. (1.86) yields The hadron mass M can be isolated and is given by Using this result a hadron mass M can be extracted from the theoretically calculated quantity R$_{k}$ ( τ , s$_{0}$ ). Note that the hadron mass given in Eq. (1.113) is a function of the Borel parameter τ and continuum threshold s$_{0}$ . The Borel parameter τ probes the hadronic spectral function at various energies while the continuum threshold s$_{0}$ is built into the model of the hadronic spectral function 1.84 and is predicted in the QSR analysis. We must first find a region where the mass prediction (1.113) varies little with the Borel parameter. For a given value of s$_{0}$ we 47 M ( τ , s$_{0}$ ) = $^{√}$R$_{1}$ ( τ , s$_{0}$ ) R$_{0}$ ( τ , s$_{0}$ ) . (1.113) R$_{k}$ ( τ , s$_{0}$ ) = f $^{2}$M 2 $^{k}$e − M $^{2}$τ . (1.112) ρ $^{res}$( t ) = πf $^{2}$δ $^{(}$t − M 2 ) (1.111)