QCD Baryons in the 1/N_c Expansion

The 1/N_c expansion provides a theoretical method for analyzing the spin-flavor symmetry properties of baryons in QCD that is quantitative, systematic and predictive. An exact spin-flavor symmetry exists for large-N_c baryons, whereas for QCD baryons, the spin-flavor symmetry is approximate and is broken by corrections proportional to the symmetry-breaking parameter 1/N_c = 1/3. The 1/N_c expansion predicts a hierarchy of spin and flavor symmetry relations for QCD baryons that is observed in nature. It provides a quantitative understanding of why some SU(3) flavor symmetry relations in the baryon sector, such as the Gell-Mann--Okubo mass formula, are satisfied to a greater precision than expected from flavor symmetry-breaking suppression factors alone.Comment: 25 pages, Invited lectures VIII Mexican Workshop on Particles and Fields, Nov. 14-20, 2001, Zacatecas, Mexico, to be published AIP Proceeding

Spin of ground state baryons

We calculate the quark spin contribution to the total angular momentum of flavor octet and flavor decuplet ground state baryons using a spin-flavor symmetry based parametrization method of quantum chromodynamics. We find that third order SU(6) symmetry breaking three-quark operators are necessary to explain the experimental result Sigma_1=0.32(10). For spin 3/2 decuplet baryons we predict that the quark spin contribution is Sigma_3=3.93(22), i.e. considerably larger than their total angular momentum.Comment: 8 page

Baryon Chiral Perturbation Theory in the 1/N Expansion

The chiral Lagrangian for baryons is formulated in an expansion in 1/N_c. The chiral Lagrangian implements the contracted spin-flavor symmetry of large-N_c baryons as well as nonet symmetry of the leading planar diagrams. Large-N_c consistency conditions ensure that chiral loop corrections are suppressed in 1/N_c through exact cancellation of chiral loop graphs to fixed orders in 1/N_c. Application of 1/N_c baryon chiral perturbation theory to the flavor-27 baryon mass splittings and the baryon axial vector currents are considered as examples.Comment: 10 pages, Invited talk, The Phenomenology of Large-N_c QCD, to be published Proceedings of the Institute of Nuclear Theor

Meson-like Baryons and the Spin-Orbit Puzzle

I describe a special class of meson-like \Lambda_Q excited states and present evidence supporting the similarity of their spin-independent spectra to those of mesons. I then examine spin-dependent forces in these baryons, showing that predicted effects of spin-orbit forces are small for them for the same reason they are small for the analogous mesons: a fortuitous cancellation between large spin-orbit forces due to one-gluon-exchange and equally large inverted spin-orbit forces due to Thomas precession in the confining potential. In addition to eliminating the baryon spin-orbit puzzle in these states, this solution provides a new perspective on spin-orbit forces in all baryons.Comment: 24 pages, 4 figure

Gluon Spin in the Nucleon

We study the operator description of the gluon spin contribution ($\Gamma$) to the nucleon's spin as it is measured in deep inelastic processes. $\Gamma$ can be related to the forward matrix element of a local gluon operator in $A^+=0$ gauge. In quark models the nucleon contains ambient color electric and magnetic fields. The latter are thought to be responsible for spin splittings among the light baryons. We show that these fields give rise to a significant {\it negative\/} contribution to $\Gamma$ at the quark model renormalization scale, $\mu_0^2$. The non-Abelian character of QCD is responsible for the sign of $\Gamma$. In a generic non-relativistic quark model $\Gamma_{NQM}=-{8\over 9}{\alpha_{NQM}\over m_q}\langle{1\over r}\rangle$, in the bag model $\Gamma_{bag}=-.1\alpha_{bag}$. These correspond to $\Gamma_{NQM}\approx -0.7$ and $\Gamma_{bag}\approx -0.4$ at $\alpha_{QCD}\approx 1.0$.Comment: 12 pages in REVTeX. The paper has been entirely revise

Baryon Spin and Magnetic Moments in Relativistic Chiral Quark Models

Spin-flavor fractions of quarks in the proton and several hyperons are obtained from their lowest order chiral fluctuations involving Goldstone bosons. SU(3) flavor breaking, relativistic effects and the axial anomaly are included. The validity of the Karl-Sehgal formulas for magnetic moments is studied as well.Comment: 24 pages, 7 tables, no figure