Flavor symmetry in the large Nc limit

Abstract The difference between two-flavor and three-flavor descriptions of baryons in large Nc QCD is discussed in detail. For Nc⩾3 a state with the SU(3) flavor quantum numbers of the proton must contain a number of strange quarks n s ⩾ (N c −3) 3 , while a state with no strange quarks must have extra hyperchange Y − 1= 1 3 N c − 1 . The extra strangeness or extra hyperchange which vanishes for Nc = 3 is spurious for the physical proton. This problem does not arise in two-flavor QCD. The flavor-SU(2) skyrmion may give a good approximation for nucleon-pion physics at low energies below strangeness threshold. But any nucleon model with SU(3) flavor symmetry which is interpreted as an approximation to the wave function of the large Nc limit in QCD can lead to erroneous conclusions about the spin and flavor structure of the proton.