Hello! I am reading some group theory applied to QCD and they show how by using the lightest 3 quarks in the ##l=0## state we get 9 pseudoscalar mesons and 9 vector mesons. The difference in masses between the 2 classes is explained by the existence of a spin-spin interaction, which seems like a reasonable argument. However they don't say anything about the fact that the mass of the singlet in each case (the 9 mesons are grouped in an octet and a singlet) is significantly higher than the masses of the mesons in the octet. What is the reason for that? Thank you!