QCD SUM-RULE ANALYSIS OF SPIN-ORBIT-SPLITTING IN BARYONS

A determination of spin-orbit splitting in N and Λ using the QCD sum rule technique of Shifman, Vainshtein, and Zakharov is presented. The QCD sum rules coupling to JP = 1 2- nucleons, first considered by Belyaev and Ioffe are shown to provide a consistent description of N 1 2-(1535). The QCD sum rules coupling to the low-lying Δ 1 2+, Λ 1 2-, and Λ 3 2-, are presented as these sum rules have not been previously considered in the literature. The non-perturbative calculations of spin-orbit splitting and odd parity excitation energies in the low-lying states of N and Λ are in agreement with experimental data. This is in contrast to conventional quark models. The large spin-orbit splitting in Λ resonances arises without invoking coupling to the KN scattering channel. The reduction of the strange quark condensate relative to the u or d quark condensate acts to increase the spin-orbit splitting in the low-lying Λ resonances. The results do not exclude coupling of Λ(1405) to the KN channel provided any additional spin-orbit splitting arising from the coupling is of the order of 30 MeV. Finally the Δ 1 2+ sum rules cast doubt on the existence of the unconfirmed (one *) Δ 1 2+(1550) resonance. © 1990.