content_image/298009.jpg content_image/298010.jpg
evaluation of leading order contributions to the Wilson coefficients involves the calculation of multiple two-loop momentum integrals. These loop integrals are often quite difficult to evaluate and constitute a significant technical barrier to extending QSR calculations to higher orders. Chapter 2 discusses techniques for evaluating loop integrals. 

## 1.4.4 Hadronic Spectral Function 

As mentioned previously, the hadronic spectral function can be measured experimentally. For instance, the spectral function for hadronic states with J PC = 1 −− is related to the ratio of the cross sections 

This spectral function is shown in Fig. 1.9. 

<img src='content_image/298009.jpg'>

<img src='content_image/298010.jpg'>

