Excited Nucleon and Delta Spectra From Lattice QCD

We calculate the nucleon and delta excited state spectra from lattice QCD. Operators which transform as irreducible representations of the lattice symmetry group are used as bases for variational calculations. We compute matrices of corre- lation functions between all the operators in the variational bases. From the time dependence of the eigenvalues of these matrices, we extract energy eigenvalues. By subducing the continuum SU(3) rotation group to the octahedral group, we can identify the spins of the continuum states which correspond to the lattice states. In the nucleon spectrum calculation, we use 24^3 × 64 anisotropic lattices with pion masses of 416 MeV and 576 MeV. The lattices have a spacing of about 0.1 fm and an anisotropy of 3. We use the Wilson gauge and the Wilson fermion actions with two flavors of dynamical light quarks. The low-lying spectrum has many of the qualitative features of the physical spectrum and we are able to identify the continuum states which correspond to several of the lattice states. This includes one of the first observations of a spin- 5 state on the lattice. For the delta spectrum calculation, we use 16^3 × 128 anisotropic lattices. The gauge action is the tree-level tadpole improved Wilson gauge action, while in the fermion sector we use the clover action. The pion mass is about 390 MeV and the anisotropy is 3.5. We have two flavors of dynamical light quarks as well as dynamical strange quarks. To compute the correlation functions, we use the distillation method in which operators are projected on the the low lying eigenmodes of the Laplacian operator, allowing for an exact computation of all-to-all propagators between the distilled source and sink operators. We are able to identify four low-lying states with continuum delta states

First results from 2+1 dynamical quark flavors on an anisotropic lattice: light-hadron spectroscopy and setting the strange-quark mass

We present the first light-hadron spectroscopy on a set of $N_f=2+1$ dynamical, anisotropic lattices. A convenient set of coordinates that parameterize the two-dimensional plane of light and strange-quark masses is introduced. These coordinates are used to extrapolate data obtained at the simulated values of the quark masses to the physical light and strange-quark point. A measurement of the Sommer scale on these ensembles is made, and the performance of the hybrid Monte Carlo algorithm used for generating the ensembles is estimated.Comment: 24 pages. Hadron Spectrum Collaboratio

Excited State Nucleon Spectrum with Two Flavors of Dynamical Fermions

Highly excited states for isospin 1/2 baryons are calculated for the first time using lattice QCD with two flavors of dynamical quarks. Anisotropic lattices are used with two pion masses: 416(36) MeV and 578(29) MeV. The lowest four energies are reported in each of the six irreducible representations of the octahedral group at each pion mass. The lattices used have dimensions 24^3x64, spatial lattice spacing a_s = 0.11 fm and temporal lattice spacing a_t = 1/3 a_s. Clear evidence is found for a 5/2^-state in the pattern of negative-parity excited states. This agrees with the pattern of physical states and spin 5/2 has been realized for the first time on the lattice.Comment: 21 pages, 12 figure

Nucleon, Delta and Omega excited state spectra at three pion mass values

The energies of the excited states of the Nucleon, Delta and Omega are computed in lattice QCD, using two light quarks and one strange quark on anisotropic lattices. The calculations are performed at three values of the pion mass: 392(4), 438(3) and 521(3) MeV. We employ the variational method with a basis of about ten interpolating operators enabling six energies to be distinguished clearly in each irreducible representation of the octahedral group. We compare our calculations of nucleon excited states with the low-lying experimental spectrum. There is reasonable agreement for the pattern of states