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We study the effects of two dynamical sea quarks on the spectrum of heavy quarkonia. Within the non-relativistic approach to Lattice QCD we found sizeable changes to the hyperfine splitting, but we could not observe any changes for the fine structure. We also investigated the scaling behaviour of our results for several different lattice spacings.
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We present results showing that the strong coupling constant measured in two-flavor full QCD with dynamical Kogut-Susskind quarks at $beta=5.7$ exhibit a 15% increase due to sea quarks over that for quenched QCD at the scale $muapprox 7$GeV . (talk at lattice93)
We present a dynamical lattice calculation with 2 flavours for bottomonium states with an additional gluonic excitation. Using improved actions for the quarks and gauge fields at a lattice spacing of $a approx 0.1$ fm, we find 10.977(61)(62) GeV for the energy of the lowest lying $bbar bg$-hybrid, where the first error is statistical and the second denotes the systematic uncertainty due to the determination of scale. In a parallel quenched simulation we demonstrate explicitly that vacuum polarisation effects are less than 10% of the splitting with the ground state.
We explore sea quark effects in the light hadron mass spectrum in a simulation of two-flavor QCD using the nonperturbatively O(a)-improved Wilson fermion action. In order to identify finite-size effects, light meson masses are measured on 12^3x48, 16^3x48 and 20^3x48 lattices with a~0.1 fm. On the largest lattice, where the finite-size effect is negligible, we find a significant increase of the strange vector meson mass compared to the quenched approximation. We also investigate the quark mass dependence of pseudoscalar meson masses and decay constants and test the consistency with (partially quenched) chiral perturbation theory.
We estimate the effects on the decay constants of charmonium and on heavy meson masses due to the charm quark in the sea. Our goal is to understand whether for these quantities $N_f=2+1$ lattice QCD simulations provide results that can be compared with experiments or whether $N_f=2+1+1$ QCD including the charm quark in the sea needs to be simulated. We consider two theories, $N_f=0$ QCD and QCD with $N_f=2$ charm quarks in the sea. The charm sea effects (due to two charm quarks) are estimated comparing the results obtained in these two theories, after matching them and taking the continuum limit. The absence of light quarks allows us to simulate the $N_f=2$ theory at lattice spacings down to $0.023$ fm that are crucial for reliable continuum extrapolations. We find that sea charm quark effects are below $1%$ for the decay constants of charmonium. Our results show that decoupling of charm works well up to energies of about $500$ MeV. We also compute the derivatives of the decay constants and meson masses with respect to the charm mass. For these quantities we again do not see a significant dynamical charm quark effect, albeit with a lower precision. For mesons made of a charm quark and a heavy antiquark, whose mass is twice that of the charm quark, sea effects are only about $0.1%$ in the ratio of vector to pseudoscalar masses.
We report on a study of heavy hybrid states using the NRQCD approach on coarse and asymmetric lattices, where we discard vacuum polarisation effects and neglect all spin-correction terms. We find a clear hybrid signal on all our lattices ($a_s= 0.15 ... 0.47$ fm). We have studied in detail the lattice spacing artefacts, finite volume effects and mass dependence. Within the above approximations we predict the hybrid excitation in Charmonium to be 1.323(13) GeV above its ground state. The bottomonium hybrid was found to be 1.542(8) GeV above its ground state.
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