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The n-p Mass Ratio in QCD Sum Rules

The QCD sum rules for the proton mass, the n-p mass ratio and the optimal observation point are analysed, which can greatly improve the precision of the theoretical results. If we take =<{\bar d}d>$?> and mu / (mu+md) = 0.35 which agree with the values from SVZ and current algebra[1,2], we obtain Mp = 935.86 MeV and Mp / Mn = 0.99871 which coincide quite well with experimental values. The isospin behavior of light quarks is investigated. It shows that the isospin symmetry of the light quark condensates is very good up to the low-energy region (about 500 MeV), and that the n-p mass difference can be originated from the u-and d-quark mass differences.