Markus Limmer

Hadron spectroscopy with dynamical chirally improved fermions

We simulate two dynamical, mass-degenerate light quarks on

Meson and baryon spectrum for QCD with two light dynamical quarks

{16}^{3}\ifmmode\times\else\texttimes\fi{}32

Lattice study of light scalar tetraquarks with I=0,2,1/2,3/2: Are \sigma and \kappa tetraquarks?

lattices with a spatial extent of 2.4 fm using the chirally improved Dirac operator. The simulation method, the implementation of the action, and signals of equilibration are discussed in detail. Based on the eigenvalues of the Dirac operator we discuss some qualitative features of our approach. Results for ground-state masses of pseudoscalar and vector mesons as well as for the nucleon and delta baryons are presented.

Fermion-loop simulation of the lattice Gross-Neveu model

We present results of meson and baryon spectroscopy using the Chirally Improved Dirac operator on lattices of size 16{sup 3}x32 with two mass-degenerate light sea quarks. Three ensembles with pion masses of 322(5), 470(4), and 525(7) MeV and lattice spacings close to 0.15 fm are investigated. Results for ground and excited states for several channels are given, including spin two mesons and hadrons with strange valence quarks. The analysis of the states is done with the variational method, including two kinds of Gaussian sources and derivative sources. We obtain several ground states fairly precisely and find radial excitations in various channels. Excited baryon results seem to suffer from finite size effects, in particular, at small pion masses. We discuss the possible appearance of scattering states, considering masses and eigenvectors. Partially quenched results in the scalar channel suggest the presence of a 2-particle state, however, in most channels we cannot identify them. Where available, we compare our results to results of quenched simulations using the same action.

Dynamical Chirally Improved Quarks: First Results for Hadron Masses

We investigate whether the lightest scalar mesons {sigma} and {kappa} have a large tetraquark component qqqq, as is strongly supported by many phenomenological studies. A search for possible light tetraquark states with J{sup PC}=0{sup ++} and I=0,2,1/2,3/2 on the lattice is presented. We perform the two-flavor dynamical simulation with chirally improved quarks and the quenched simulation with overlap quarks, finding qualitative agreement between both results. The spectrum is determined using the generalized eigenvalue method with a number of tetraquark interpolators at the source and the sink, and we omit the disconnected contractions. The time dependence of the eigenvalues at the finite temporal extent of the lattice is explored also analytically. In all the channels, we unavoidably find the lowest scattering states {pi}(k){pi}(-k) or K(k){pi}(-k) with back-to-back momentum k=0,2{pi}/L,.... However, we find an additional light state in the I=0 and I=1/2 channels, which may be interpreted as the observed resonances {sigma} and {kappa} with a sizable tetraquark component. In the exotic repulsive channels I=2 and I=3/2, where no resonance is observed, we find no light state in addition to the scattering states.

Angular momentum content of the rho meson in lattice QCD.

We present a numerical simulation of the Gross-Neveu model on the lattice using a new representation in terms of fermion loops. In the loop representation all signs due to Pauli statistics are eliminated completely and the partition function is a sum over closed loops with only positive weights. In the loop representation a different type of boundary condition appears: sectors of constant winding number. We demonstrate that the new formulation allows one to simulate volumes which are 2 orders of magnitude larger than those accessible with standard methods.

The Chiral and Angular Momentum Content of the ρ-Meson

We present first results for a study with two mass-degenerate dynamical Chirally Improved (CI) fermions on lattices of spatial extent 2.4 fm. The CI Dirac operator obeys the Ginsparg-Wilson condition in good approximation. The pion mass we use is still large O(470 MeV) for the 16**3*32 lattices with lattice spacing of 0.15 fm. The hadron masses are obtained with the variational technique and the results compared with earlier quenched calculations with similar lattice parameters. We find indications for the isovector, scalar meson a0(980) near the experimental value, in contradistinction to quenched results which always produced a mass value compatible with the first excitation a0(1450).

Excited hadrons in n f = 2 QCD

The variational method allows one to study the mixing of interpolators with different chiral transformation properties in the nonperturbatively determined physical state. It is then possible to define and calculate in a gauge-invariant manner the chiral as well as the partial wave content of the quark-antiquark component of a meson in the infrared, where mass is generated. Using a unitary transformation from the chiral basis to the ;{2S+1}L_{J} basis one may extract a partial wave content of a meson. We present results for the ground state of the rho meson using quenched simulations as well as simulations with n_{f} = 2 dynamical quarks, all for lattice spacings close to 0.15 fm. We point out that these results indicate a simple ;{3}S_{1}-wave composition of the rho meson in the infrared, like in the SU(6) flavor-spin quark model.

Chiral symmetry breaking and the spin content of the ρ and ρ′ mesons

It is possible to define and calculate in a gauge-invariant manner the chiral as well as the partial wave content of the quark–antiquark Fock component of a meson in the infrared, where mass is generated. Using the variational method and a set of interpolators that span a complete chiral basis we extract in a lattice QCD Monte Carlo simulation with nf = 2 dynamical light quarks the orbital angular momentum and spin content of the ρ-meson. We obtain in the infrared a simple 3S1 component as a leading component of the ρ-meson with a small admixture of the 3D1 partial wave, in agreement with the SU(6) flavor–spin symmetry.

Baryon axial charges from Chirally Improved fermions - first results

Inst. f. Theoret. Physik, Univ. Regensburg, Regensburg, GernmayE-mail:georg.engel@uni-graz.at, christof.gattringer@uni-graz.at,christian.lang@uni-graz.at, markus.limmer@uni-graz.at,daniel.mohler@uni-graz.at, andreas.schaefer@physik.uni-regensburg.deThe chirally improved (CI) fermion action allows us to obtain results for pion masses down to320 MeV on (in lattice units) comparatively small lattices with physical extent of 2.4 fm. We usedifferently smeared quarks sources to build sets of several interpolators. The variational methodthenleads to excellentgroundstate masses formost mesons andbaryons. Theexcitedstate signalsweaken in quality towards smaller quark masses. In particular the excited baryons come out toohigh.The XXVII International Symposium on Lattice Field Theory -LAT2009July 26-31 2009Peking University, Beijing, China