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 and reproduces the experimental value of
DeltaM_np for ~ 2^2.Comment: Arguments detailed. Two references added. Version to appear in
Physical Review C. DOI:10.1103/PhysRevC.00.00520