A simple toy model for effective restoration of chiral symmetry in excited hadrons

A simple solvable toy model exhibiting effective restoration of chiral symmetry in excited hadrons is constructed. A salient feature is that while physics of the low-lying states is crucially determined by the spontaneous breaking of chiral symmetry, in the high-lying states the effects of chiral symmetry breaking represent only a small correction. Asymptotically the states approach the regime where their properties are determined by the underlying unbroken chiral symmetry.Comment: This is the published version of this paper. Note that the title has changed from earlier versions as has the abstract. The emphasis is slightly different from previous versions but the essential physical content is the sam

Baryon spectroscopy with spatially improved quark sources

We study baryons on the lattice with a special focus on excited states. For that purpose we construct several interpolators which differ in their Dirac structure. These interpolators are built from Jacobi smeared quarks with different widths in order to allow for operators with improved spatial wavefunctions. We compute all cross correlations and use the variational method to determine which combinations of operators have best overlap with ground and excited states. Our approach yields promising results for the spin-1/2 baryons: nucleon, sigma, xi and lambda. For the spin-3/2 baryons, delta and omega, we obtain results which are consistent with results of other groups.Comment: LaTeX [PoS], 6 pages, 4 figures, talk presented at Lattice 2005 (Hadron spectrum and quark masses

Excited nucleons with chirally improved fermions

We study positive and negative parity nucleons on the lattice using the chirally improved lattice Dirac operator. Our analysis is based on a set of three operators chi_i with the nucleon quantum numbers but in different representations of the chiral group and with different diquark content. We use a variational method to separate ground state and excited states and determine the mixing coefficients for the optimal nucleon operators in terms of the chi_i. We clearly identify the negative parity resonances N(1535) and N(1650) and their masses agree well with experimental data. The mass of the observed excited positive parity state is too high to be interpreted as the Roper state. Our results for the mixing coefficients indicate that chiral symmetry is important for N(1535) and N(1650) states. We confront our data for the mixing coefficients with quark models and provide insights into the physics of the nucleon system and the nature of strong decays.Comment: Tables added, small modifications in the tex

Chiral multiplets versus parity doublets in highly excited baryons

It has recently been suggested that the parity doublet structure seen in the spectrum of highly excited baryons may be due to effective chiral restoration for these states. We argue how the idea of chiral symmetry restoration high in the spectrum is consistent with the concept of quark-hadron duality. If chiral symmetry is effectively restored for highly-lying states, then the baryons should fall into representations of $SU(2)_L\times SU(2)_R$ that are compatible with the given parity of the states - the parity-chiral multiplets. We classify all possible parity-chiral multiplets: (i) $(1/2,0)\oplus(0, 1/2)$ that contain parity doublet for nucleon spectrum;(ii) $(3/2,0) \oplus (0, 3/2)$ consists of the parity doublet for delta spectrum; (iii) $(1/2,1) \oplus (1, 1/2)$ contains one parity doublet in the nucleon spectrum and one parity doublet in the delta spectrum of the same spin that are degenerate in mass. Here we show that the available spectroscopic data for nonstrange baryons in the $\sim$ 2 GeV range is consistent with all possibilities, but the approximate degeneracy of parity doublets in nucleon and delta spectra support the latter possibility with excited baryons approximately falling into $(1/2,1) \oplus (1, 1/2)$ representation of SU(2)_L\timesSU(2)_R with approximate degeneracy between positive and negative parity $N$ and $\Delta$ resonances of the same spin.Comment: RevTeX, 6 pages. The paper has been expanded in order to make the idea of chiral symmetry restoration as it follows from the concept of quark-hadron duality more transparent. To appear in Phys. Rev.

Low-Lying Nucleons from Chirally Improved Fermions

We report on our preliminary results on the low-lying excited nucleon spectra which we obtain through a variational basis formed with three different interpolators.Comment: Contributed to Lattice 2003(spectrum), Tsukub