Quark Model Explanation of the N∗→NηN^*\to N\eta Branching Ratios

The constituent quark model can explain the strong selectivity of the $N\eta$ decay branching ratios of the nucleon resonances if the fine structure interaction between the constituent quarks is described in terms of Goldstone boson exchange. This chiral quark model predicts that the resonances $N(1535)$, $N(1710)$, $\Lambda(1670)$, $\Sigma(1750)$, which have mixed flavor and spin symmetry $[21]_{FS} [21]_F [21]_S$ wavefunctions in lowest order, should have large $N\eta$ branching ratios, while $N\eta$ decay of the other resonances that have different flavor-spin symmetry should be strongly suppressed in agreement with the experimental branching ratios.Comment: Latex 7 p, no figure

Chiral symmetry restoration in hadron spectra

The evidence and the theoretical justification of chiral symmetry restoration in high-lying hadrons is presented.Comment: Invited talk given at Int. School on Nuclear Physics "Quarks in Hadrons and Nuclei", September 2002, Erice/Sicily/Italy; to appear in Progr. Part. Nucl. Phys., vol. 50; 10 page

Chiral symmetry restoration and the string picture of hadrons

QCD string picture of highly excited hadrons very naturally explains parity doubling once the chiral symmetry is restored high in the spectrum. In particular, the spin-orbit and tensor interactions of quarks at the ends of the string, related to dynamics of the string, vanish. High in the spectrum there appears higher degree of degeneracy, namely parity doublets with different angular momentum cluster around energy of the string in the given quantum state.Comment: 7 pages, LaTeX, 2 figs. The paper has been further expanded in order to make the point and physics more clear. To appear in Phys. Lett.

Chiral multiplets of excited mesons

It is shown that experimental meson states with spins J=0,1,2,3 in the energy range 1.9 - 2.4 GeV obtained in recent partial wave analysis of proton-antiproton annihilation at LEAR remarkably confirm all predictions of chiral symmetry restoration. Classification of excited $\bar q q$ mesons according to the representations of chiral $U(2)_L \times U(2)_R$ group is performed. There are two important predictions of chiral symmetry restoration in highly excited mesons: (i) physical states must fill out approximately degenerate parity-chiral multiplets; (ii) some of the physical states with the given $I,J^{PC}$ are members of one parity-chiral multiplet, while the other states with the same $I,J^{PC}$ are members of the other parity-chiral multiplet. For example, while some of the excited $\rho(1,1^{--})$ states are systematically degenerate with $a_1(1,1^{++})$ states forming (0,1)+(1,0) chiral multiplets, the other excited $\rho(1,1^{--})$ states are degenerate with $h_1(0,1^{+-})$ states ((1/2,1/2) chiral multiplets). Hence, one of the predictions of chiral symmetry restoration is that the combined amount of $a_1(1,1^{++})$ and $h_1(0,1^{+-})$ states must coincide with the amount of $\rho(1,1^{--})$ states in the chirally restored regime. It is shown that the same rule applies (and experimentally confirmed) to many other meson states.Comment: 14 pages, discussion and conclusion section is largely extende

Chiral symmetry breaking and the spin content of hadrons

From the parton distributions in the infinite momentum frame one finds that only about 30% of the nucleon spin is carried by spins of the valence quarks, which gave rise to the term "spin crisis". Similar results hold for the lowest mesons, as it follows from the lattice simulations. We define the spin content of a meson in the rest frame and use a complete and orthogonal $\bar q q$ chiral basis and a unitary transformation from the chiral basis to the (2S+1)LJ basis. Then, given a mixture of different allowed chiral representations in the meson wave function at a given resolution scale, one can obtain its spin content at this scale. To obtain the mixture of the chiral representations in the meson we measure in dynamical lattice simulations a ratio of couplings of interpolarors with different chiral structure. For the rho meson we obtain practically the 3S1 state with no trace of the spin crisis. Then a natural question arises: which definition does reflect the spin content of a hadron?Comment: 7 pp, Presented at Int. School of Nuclear Physics: "From Quarks and Gluons to Hadrons and Nuclei", Erice-Sicily, 16 - 24 September, 201

Baryon Spectrum and Chiral Dynamics

New results on baryon structure and spectrum developed in collaboration with Dan Riska [1-4] are reported. The main idea is that beyond the chiral symmetry spontaneous breaking scale light and strange baryons should be considered as systems of three constituent quarks with an effective confining interaction and a chiral interaction that is mediated by the octet of Goldstone bosons (pseudoscalar mesons) between the constituent quarks.Comment: 12 pages + 1 fig., LaTeX, fig. is available from author, to appear in Proceedings of the Int. School of Nucl. Physics: Quarks in Hadrons and Nuclei (Erice, 19-27 September, 1995) - Progr. Part. Nucl. Phys., v. 36 (1996

Baryons, their interactions and the chiral symmetry of QCD

An implication of the spontaneous chiral symmetry breaking in QCD is that at low energy and resolution there appear quasiparticles - constituent quarks and Goldstone bosons. Thus, light and strange baryons should be considered as systems of three constituent quarks with confining interaction and a chiral interaction that is mediated by Goldstone bosons between the constituent quarks. We show how the flavor-spin structure and sign of the short-range part of the Goldstone boson exchange interaction reduces the $SU(6)_{FS}$ symmetry down to $SU(3)_F \times SU(2)_S$, induces hyperfine splittings and provides correct ordering of the lowest states with positive and negative parity. We present a unified description of light and strange baryon spectra calculated in a semirelativistic framework. It is demonstrated that the same short-range part of Goldstone boson exchange also induces strong short-range repulsion in $NN$ system when the latter is treated as $6Q$ system. Thus, all main ingredients of $NN$ interaction are implied by the chiral constituent quark model since the long- and intermediate-range attraction appears in the present framework due to pion and correlated two-pion exchanges between quarks belonging to different nucleons. We also find a very strong short-range repulsion in $\Lambda\Lambda$ system with $J^P=0^+$. It then suggests that the compact H-particle should not exist.Comment: 10 pages, Invited talk given at International Conference on Quark Lepton Nuclear Physics (QULEN97, May 20-23, 1997, Osaka, Japan

Θ+\Theta^+ in a chiral constituent quark model and its interpolating fields

The recently discovered pentaquark $\Theta^+$ is described within the chiral constituent quark model. Within this picture the flavor-spin interaction between valence quarks inverts the $(1s)^4$ and $(1s)^3(1p)$ levels of the four-quark subsystem and consequently the lowest-lying pentaquark is a positive parity, I=0, J=1/2 state of the flavor antidecuplet, similar to the soliton model prediction. Contrary to the soliton model, however, the quark picture predicts its spin-orbit partner with $J=3/2$. Different interpolating fields intended for lattice calculations of $\Theta^+$ are constructed, which have a maximal overlap with this baryon if it is indeed a quark excitation in the 5Q system.Comment: 9 p

Light Baryons in a Constituent Quark Model with Chiral Dynamics

It is shown from rigorous three-body Faddeev calculations that the masses of all 14 lowest states in the $N$ and $\Delta$ spectra can be described within a constituent quark model with a Goldstone-boson-exchange interaction plus linear confinement between the constituent quarks.Comment: 12 pages, submitted to Phys. Lett.

Origins of the baryon spectrum

I begin with a key problem of light and strange baryon spectroscopy which suggests a clue for our understanding of underlying dynamics. Then I discuss spontaneous breaking of chiral symmetry in QCD, which implies that at low momenta there must be quasiparticles - constituent quarks with dynamical mass, which should be coupled to other quasiparticles - Goldstone bosons. Then it is natural to assume that in the low-energy regime the underlying dynamics in baryons is due to Goldstone boson exchange (GBE) between constituent quarks. Using as a prototype of the microscopical quark-gluon degrees of freedom the instanton-induced 't Hooft interaction I show why the GBE is so important. When the 't Hooft interaction is iterated in the qq t-channel it inevitably leads to a pole which corresponds to GBE. This is a typical antiscreening behavior: the interaction is represented by a bare vertex at large momenta, but it blows up at small momenta in the channel with GBE quantum numbers, explaining thus a distinguished role of the latter interaction in the low-energy regime. I show how the explicitly flavour-dependent short-range part of the GBE interaction between quarks, perhaps in combination with the vector-meson exchange interaction, solves a key problem of baryon spectroscopy and present spectra obtained in a simple analytical calculation as well as in exact semirelativistic three-body approach.Comment: Plenary talk given at PANIC 99 (XV Particles and Nuclei International Conference, 10 - 16 June 1999, Uppsala