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) ⊗ su(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 S11, P11, and D13 states with masses around 1500 MeV fit into the {1/2, 1/2} ⊗ [{1/2, 0} e {0, 1/2}] representation. The observed parities of the resonances correspond to natural parities of the {1/2, 1/2} states. The second P11, S11, D13 together with the first P13, F15, D13, and (a predicted) F17-resonances, centered around 1700 MeV, are organized into the {3/2, 3/2} ⊗ [{1/2, 0} ⊕ {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. © World Scientific Publishing Company.

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