Isospin symmetry breaking and the neutron-proton mass difference
  QCD sum rules using polynomial kernels are used to evaluate the strong part of the proton-neutron mass difference DeltaM_np in a model independent fashion. The result for the mass difference turns out to depend sensitively on the value of the four quark condensate <(qq)^2> and reproduces the experimental value of DeltaM_np for <(qq)^2>~ 2<qq>^2. 
Nasrallah F. Nasrallah
hep-ph