content_image/311364.jpg content_image/311363.jpg content_image/311362.jpg content_image/311361.jpg content_image/311360.jpg
Oakes-Renner relation [52]: 

<img src='content_image/311364.jpg'>

where the numerical values have been taken from Ref. [20]. The gluon condensate can be extracted from a QSR analysis of charmonium [90], which yields 

<img src='content_image/311363.jpg'>

Higher-dimensional condensates involving more quark and gluon fields also exist. For in- stance, the mixed condensate has dimension-five and is given by 

<img src='content_image/311362.jpg'>

where M 2 0 = (0 . 8 ± 0 . 1) GeV $^{2}$, which was determined from baryon sum rules [41]. The dimension-six gluon condensate is given by 

<img src='content_image/311361.jpg'>

which was also determined in Ref. [90]. Additional condensates include the dimension-six quark condensate and the dimension-eight gluon condensate, which are given in Ref. [89]. 

In QSR calculations the correlation function (1.97) is evaluated using the OPE (1.99). Contributions that are proportional to the identity operator correspond to purely perturba- tive effects, while contributions from higher dimensional operators in the OPE correspond to non-perturbative effects that are represented through condensates. In practice, the simplest way to calculate these contributions is with the aid of Wick’s theorem (1.16). For instance, consider the calculation of a correlation function of the form 

<img src='content_image/311360.jpg'>

where J ( x ) = ¯ q ( x ) q ( x ). The correlation function can be calculated using the perturbative expansion (1.19). The lowest order term in the perturbative expansion involves the following 

