Universität Tübingen
Chiral dynamics of baryons in the perturbative chiral quark model
Abstract
In this work we develop and apply variants of a perturbative chiral quark model (PCQM) to the study of baryonic properties dominantly in the low-energy region. In the PCQM baryons are considered in leading order as bound states of valence quarks with a nontrivial structure, while the sea-quark excitations are contained in a cloud of pseudoscalar mesons as imposed by chiral symmetry requirements. Since the valence quark structure dominates, pseudoscalar or chiral effects are treated perturbatively. In a first step we consider a noncovariant form of the PCQM, where confinement is modelled by a static, effective potential and chiral corrections are treated to second order, in line with similar chiral quark models. We apply the PCQM to the study of the electromagnetic form factors of the baryon octet. We focus in particular on the low-energy observables such as the magnetic moments, the charge and magnetic radii. In addition, the electromagnetic N-delta transition is also studied in the framework of the PCQM, where meson cloud contributions play a decisive role. In the chiral loop calculations we consider a quark propagator, which is restricted to the quark ground state, or in hadronic language to nucleon and delta intermediate states, for simplicity. At this stage reasonable results can be achieved, where the role of the meson cloud, in particular for the N-delta transition, is clearly elaborated. We furthermore include the low-lying excited states to the quark propagator, which influences the result at the level of 15%. In particular, the charge radius of the neutron and the transverse helicity amplitudes of the N-delta transition are considerably improved by this additional effect. In a next step we develop a manifestly Lorentz covariant version of the PCQM, where in addition higher order chiral corrections are included. The full chiral quark Lagrangian is motivated by and in analogy to the one of Chiral Perturbation Theory (ChPT). This Lagrangian contains a set of low energy constants (LECs), which are parameters encoding short distance effects and heavy degrees of freedom. We evaluate the chiral Lagrangian to order O(p^4) and to one loop to generate the dressing of the bare quark operators by pseudoscalar mesons. In addition we include the vector meson degrees of freedom in our study. Projection of the dressed quark operators on the baryonic level serves to calculate the relevant matrix elements. The main result of this technique is that the effects of the meson cloud and of the bare valence quarks factorize in the baryon matrix elements. In a first application of this scheme, we resort to a parameterization of the valence quark form factors in the electromagnetic sector. Constraints on these quark form factors are set by symmetries and by matching to model-independent predictions of ChPT. Physical applications are worked out for the masses and the magnetic moments of the baryon octet, the meson-nucleon sigma terms and the electromagnetic form factors of the nucleon. We demonstrate in particular that the meson cloud plays a vital role to explain the detailed structure of the electromagnetic form factors for momenta transfers up to 0.5 GeV^2.
Author and committee
dc:creator, dc:contributor.*- Author
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- Pumsa-ard, Kem
Identifiers
dc:identifier.*- Identifier
- hdl:10900/48947