QCD sum rule analysis of spin-orbit splitting in baryons.

摘要

The first determination of spin-orbit splitting in N and {dollar}Lambda{dollar} using the QCD sum rule technique of Shifman, Vainshtein, and Zakharov is presented. The QCD sum rules coupling {dollar}Jsp{lcub}P{rcub}{dollar} = {dollar}{lcub}1over2{rcub}sp-{dollar} nucleons, first considered by Belyaev and Ioffe are shown to provide a consistent description of {dollar}N{lcub}1over2{rcub}sp-{dollar}(1535). The QCD sum rules coupling to the low-lying {dollar}Delta{lcub}1over2{rcub}sp+{dollar}, {dollar}Lambda{lcub}1over2{rcub}sp-{dollar}, and {dollar}Lambda{lcub}3over2{rcub}sp-{dollar}, are presented as these sum rules have not been previously considered in the literature. The nonperturbative calculations of spin-orbit splitting and odd parity excitation energies in the low-lying states of N and {dollar}Lambda{dollar} are in agreement with experimental data. This is in contrast to conventional quark models. The large spin-orbit splitting in {dollar}Lambda{dollar} resonances arises without invoking coupling to the =KN scattering channel. Both the relatively large strange quark mass and the reduced quark condensate act to increase the spin-orbit splitting in the low lying {dollar}Lambda{dollar} resonances. The results do not exclude coupling of {dollar}Lambda{dollar}(1405) to the =KN channel provided the spin-orbit splitting arising from the coupling is of the order of 20 to 30 MeV. Finally the {dollar}Delta{lcub}1over2{rcub}sp+{dollar} sum rules cast doubt on the existence of the unconfirmed (one*) {dollar}Delta{lcub}1over2{rcub}sp+{dollar}(1550) resonance.