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We present early results from a lattice QCD study seeking a bound $H$-dibaryon using $N_f=2$ flavors of $O(a)$ improved Wilson fermions and a quenched strange quark. We compute a matrix of two-point functions using operators consisting of the two independent local products of six positive-parity-projected quarks with the appropriate quantum numbers, which belong to the singlet and 27-plet irreducible representations of flavor SU(3). To expand this basis, we also independently vary the quark-field smearing, and apply a new scheme to reduce the noise caused by smearing. We then find the ground-state mass by solving the generalized eigenvalue problem. We show results from ensembles with pion masses 451 MeV and 1 GeV, and compare with other lattice calculations.
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We present evidence for the existence of a bound H-dibaryon, an I=0, J=0, s=-2 state with valence quark structure uuddss, at a pion mass of m_pi ~ 389 MeV. Using the results of Lattice QCD calculations performed on four ensembles of anisotropic clover gauge-field configurations, with spatial extents of L ~ 2.0, 2.5, 3.0 and 3.9 fm at a spatial lattice spacing of b ~ 0.123 fm, we find an H-dibaryon bound by B = 16.6 +- 2.1 +- 4.6 MeV at a pion mass of m_pi ~ 389 MeV.
We present preliminary results from a lattice QCD calculation of the H-dibaryon using two flavors of $mathcal{O}(a)$ improved Wilson fermions. We employ local six-quark interpolating operators at the source with a combination of local six-quark and two-baryon operators at the sink with the appropriate quantum numbers of the H-dibaryon and its coupling to the two-baryon channels. We find that the two-baryon operators provide an improved overlap onto the ground state in comparison to the local six-quark operators. We also apply Luschers finite volume formalism to obtain information on the nature of the infinite-volume interaction of two particles. Further, the momentum projection to three moving frames enables the isolation of the pole in the infinite-volume scattering amplitude. Preliminary results at pion masses of 450 MeV and 1 GeV clearly indicate the presence of states below the $Lambda Lambda$ threshold while a finite-volume analysis fails to conclusively show the existence of an infinite-volume bound state.
Energies for excited light baryons are computed in quenched QCD with a pion mass of 490 MeV. Operators used in the simulations include local operators and the simplest nonlocal operators that have nontrivial orbital structures. All operators are designed with the use of Clebsch-Gordan coefficients of the octahedral group so that they transform irreducibly under the group rotations. Matrices of correlation functions are computed for each irreducible representation, and then the variational method is applied to separate mass eigenstates. We obtained 17 states for isospin 1/2 and 11 states for isospin 3/2 in various spin-parity channels including $J^P=5/2^pm$. The pattern of the lowest-lying energies from each irrep is discussed. We use anisotropic lattices of volume $24^3times 64$ with temporal lattice spacing $a_t^{-1}=6.05$ GeV with renormalized anisotropy $xi=3.0$.
We present the first determination of the binding energy of the $H$ dibaryon in the continuum limit of lattice QCD. The calculation is performed at five values of the lattice spacing $a$, using O($a$)-improved Wilson fermions at the SU(3)-symmetric point with $m_pi=m_Kapprox 420$ MeV. Energy levels are extracted by applying a variational method to correlation matrices of bilocal two-baryon interpolating operators computed using the distillation technique. Our analysis employs Luschers finite-volume quantization condition to determine the scattering phase shifts from the spectrum and vice versa, both above and below the two-baryon threshold. We perform global fits to the lattice spectra using parametrizations of the phase shift, supplemented by terms describing discretization effects, then extrapolate the lattice spacing to zero. The phase shift and the binding energy determined from it are found to be strongly affected by lattice artifacts. Our estimate of the binding energy in the continuum limit of three-flavor QCD is $B_H=3.97pm1.16_{rm stat}pm0.86_{rm syst}$ MeV.
New extended interpolating operators made of quenched three dimensional fermions are introduced in the context of lattice QCD. The mass of the 3D fermions can be tuned in a controlled way to find a better overlap of the extended operators with the states of interest. The extended operators have good renormalisation properties and are easy to control when taking the continuum limit. Moreover the short distance behaviour of the two point functions built from these operators is greatly improved. The operators have been numerically implemented and a comparison to point sources and Jacobi smeared sources has been performed on the new CLS configurations.
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