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)xsu(2)_I. The spins of the resonances are obtained through the coupling of a Lorentz bi-spinor {1/2,0}+{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 O(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}x[{1/2,0}+{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 $P_{13}$, $F_{15}$, $D_{15}$, and (a predicted) $F_{17}$-resonances, centered around 1700 MeV, are organized into the {3/2,3/2}x[{1/2,0}+{0,1/2}] representation. I 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 $N(939)\to N(1650)$ transition the chiral symmetry mode is changed, and therefore, a chiral phase transition is predicted to take place.