Understanding the QCD Sum Rule: What is it and How Does it Work?

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i just wonder why we call it "sum rule"?

waht do we sum, and what sum for?
 
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Although 'sum rule' was originally a sum over discrete eigenstates, the term is also used to describe integrals over continuous spectra.
 
In deep inelastic scattering one derives sum rules from structure Functions F(x,Q²) integrating over x where x is the fraction of the nucleon momentum carried by the scattering quark. So sum rules give us e.g. the fraction of energy, spin, etc. of the nucleon carried e.g. by a specific quark flavour.
 
thanks, all guys
 

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