Meson-nucleon Sigma Terms Research Articles

STRANGENESS AND CHIRAL SYMMETRY BREAKING

The implications of chiral symmetry breaking and SU(3) symmetry breaking have been studied in the chiral constituent quark model (χCQM). The role of hidden strangeness component has been investigated for the scalar matrix elements of the nucleon with an emphasis on the meson–nucleon sigma terms. The χCQM is able to give a qualitative and quantitative description of the "quark sea" generation through chiral symmetry breaking. The significant contribution of the strangeness is consistent with the recent available experimental observations.

Phenomenological analysis of nucleon strangeness and meson-nucleon sigma terms

We calculate the nucleon strangeness ${y}_{N}$ in the chiral quark model and the meson cloud model. With the internal relation between the sigma term of $\ensuremath{\pi}N$ (${\ensuremath{\sigma}}_{\ensuremath{\pi}N}$) and ${y}_{N}$, we present the results of ${\ensuremath{\sigma}}_{\ensuremath{\pi}N}$ in these two models. Our calculations show that ${y}_{N}$ from the chiral quark model is significantly larger than that from the meson cloud model, whereas the difference of ${\ensuremath{\sigma}}_{\ensuremath{\pi}N}$ between the two models is relatively small. We also present the results of ${\ensuremath{\sigma}}_{KN}$ and ${\ensuremath{\sigma}}_{\ensuremath{\eta}N}$, which could be determined by ${\ensuremath{\sigma}}_{\ensuremath{\pi}N}$ and ${y}_{N}$ from their definition in the current algebra, and find that these two physical parameters are quite sensitive to ${y}_{N}$. The results indicate the necessity to restrict the parameters of the two models from more precision measurements.

Chiral dynamics of baryons in a Lorentz covariant quark model

We develop a manifestly Lorentz covariant chiral quark model for the study of baryons as bound states of constituent quarks dressed by a cloud of pseudoscalar mesons. The approach is based on a nonlinear chirally symmetric Lagrangian, which involves effective degrees of freedom---constituent quarks and the chiral (pseudoscalar meson) fields. In a first step, this Lagrangian can be used to perform a dressing of the constituent quarks by a cloud of light pseudoscalar mesons and other heavy states using the calculational technique of infrared dimensional regularization of loop diagrams. We calculate the dressed transition operators with a proper chiral expansion which are relevant for the interaction of quarks with external fields in the presence of a virtual meson cloud. In a second step, these dressed operators are used to calculate baryon matrix elements. Applications are worked out for the masses of the baryon octet, the meson-nucleon sigma terms, the magnetic moments of the baryon octet, the nucleon charge radii, the strong vector-meson-nucleon couplings, and the full momentum dependence of the electromagnetic form factors of the nucleon.

Chiral symmetry breaking and meson-nucleon sigma commutators: A review

Chiral symmetries are discussed in considerable detail, and their importance and consequences for the study of elementary particle physics are reviewed. For the study of broken symmetries we concentrate mainly on meson-nucleon sigma terms, $\ensuremath{\pi}\ensuremath{\pi}$ scattering, and ${K}_{l3}$ decays. In addition to giving a critical and detailed review of most of the "experimental" estimates for pion-nucleon and kaon-nucleon sigma terms done so far, we also outline the most common chiral symmetry-breaking schemes at present, such as $(3,\overline{3})+(\overline{3},3)$, $(6,\overline{6})+(\overline{6},6)$, (8,8) and (1, 8) + (8, 1) representations of SU(3) \ifmmode\times\else\texttimes\fi{} SU(3) and possible mixtures of them, and discuss and compare their predictions with experiment. Nonlinear effective Lagrangians are briefly discussed, and the connection between broken scale invariance and chiral symmetry breaking is outlined, with emphasis on our present knowledge of meson-nucleon sigma terms.

Theoretical estimate of the meson nucleon sigma terms

The mechanism of Li and Pagels is used to predict the meson nucleon sigma terms in the framework of the (3,3 X) + (3 X,3) model of chiral symmetry breaking.