23 research outputs found
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 that are compatible
with the given parity of the states - the parity-chiral multiplets. We classify
all possible parity-chiral multiplets: (i) that contain
parity doublet for nucleon spectrum;(ii) consists of
the parity doublet for delta spectrum; (iii) 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 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
representation of SU(2)_L\timesSU(2)_R with approximate degeneracy between
positive and negative parity and 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