Masses and magnetic moments of baryons in a QCD-string model

Abstract

In this thesis we have explored the baryon wave function obtained from the QCD-Lagrangian by the field correlator method. Using this method, the gluonic degrees of freedom have been integrated out and an effective Lagrangian was obtained. In the lowest order of the approximation scheme, neglecting gluon and meson exchanges, the resulting baryon wave function is given by a product of three independent single-quark orbitals, each satisfying the Dirac-like equation with a nonlocal kernel. This kernel is the nonlocal quark mass operator M(x,y), represented by the QCD string and containing both confinement and CSB.

We have used the baryon wave function obtained in this way, to calculate the magnetic moments of the lowest baryon octet and decuplet representation of the SU(3)-flavor group. Reasonable agreement with experimental baryon magnetic moments was obtained for a string tension of while the baryon masses were in the correct region without need for introduction of constituent quark masses.

Since the confining nonlocal operator M(x,y) does not involve any spin-dependent interactions, the first stage of the model does not account for the mass splitting between the baryon octet and decuplet. We have subsequently improved this omission by considering in addition perturbative one-gluon and pseudoscalar meson exchanges. For the interaction of the pseudoscalar meson with the quark both the pseudoscalar and the pseudovector coupling have been investigated.

The effects of excited single-quark orbitals on the baryon ground state are also considered. The calculation is performed by using all possible combinations of four single-quark orbitals for the u,d-quark and the s-quark to build different three-particle states. The resulting states are used as a basis to diagonalize the Hamiltonian. As a result, the predictions on the mass spectrum and the magnetic moments of the quarks improve. The best overall agreement is found when the string tension is lowered to and a pseudovector coupling of the pseudoscalar meson to the quark is used.