A Note on the External-Field Method in QCD Sum Rules
  The external-field method has been used extensively in the QCD sum-rule approach to explore various hadron static properties. In the traditional formalism of this method, the transitions from the ground state hadron to excited states are not exponentially suppressed relative to the ground state term and thus contaminate the ground state hadron property to be extracted. In this paper, we suggest a modified formalism, in which the transition terms are exponentially suppressed relative to the ground state term. As such, the pole plus continuum spectral model, traditionally invoked in QCD sum-rule approach, can be adopted. Thus, this modified formalism has potential to improve the predictability and reliability of external-field sum-rule calculations, which is illustrated in an explicit example. 
Xuemin Jin