On the parity degeneracy of baryons

The gross features of the observed baryon excitation spectrum below 2 GeV are well explained if the spectrum generating algebra of its intrinsic orbital angular momentum states is o(4) circle times su(2)(I). The spins of the resonances are obtained through the coupling of a Lorentz bi-spinor {1/2, 0} circle plus {0, 1/2} to a multiplet of the type {j,j} in its O(4)/O(3) reduction. The parities of the resonances follow from those of the 0(3) members of the {j,j} multiplets. In this way relativistic SL(2, C) representations are constructed. For example, the first S-11, P-11, and D-13 states with masses around 1500 MeV fit into the {1/2, 1/2} circle times [{1/2, 0} circle plus {0, 1/2}] representation. The observed parities of the resonances correspond to natural parities of the {1/2, 1/2} states. The second P-11, S-11, D-13 - together with the first P-13, P-15, D-15, and (a predicted) F-17-resonances, centered around 1700 MeV, are organized into the {3/2, 3/2} circle times [{1/2, 0} circle plus {0, 1/2}] representation. We argue that the members of the (3/2, 3/2) multiplet carry unnatural parities and that in this region chiral symmetry is restored. In the N(939) --> N(1650) transition the chiral symmetry mode is changed, and therefore, a chiral phase transition is predicted to take place.