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H. Hoeber

Publications and source records attributed to H. Hoeber.

At least 19 recordsLinked to original sources
arXiv1999

Alpha_S from Upsilon Spectroscopy with Dynamical Wilson Fermions

We estimate the QCD coupling constant from a lattice calculation of the bottomonium spectrum. The second order perturbative expansion of the plaquette expectation value is employed to determine alpha_S at a scale set by the 2S-1S and 1P-1S level splittings. The latter are computed in NRQCD in a dynamical gauge field background with two degenerate flavours of Wilson quarks at intermediate masses and extrapolated to the chiral limit. Combining the N_f=2 result with the quenched result at equal lattice spacing we extrapolate to the physical number of light flavours to find a value of alpha_{\bar MS}^{(5)}(m_Z) = 0.1118(17). The error quoted covers both statistical and systematic uncertainties in the scale determination. An additional 5% uncertainty comes from the choice of the underlying sea quark formulation and from truncation errors in perturbative expansions.

hep-lat↗
arXiv1998

Light and Strange Hadron Spectroscopy with Dynamical Wilson Fermions

We present the final analysis of the light and strange hadron spectra from a full QCD lattice simulation with two degenerate dynamical sea quark flavours at $β= 5.6$ on a $16^3 \times 32$ lattice. Four sets of sea quark masses corresponding to the range $.69 \leq m_π/m_ρ\leq .83$ are investigated. For reference we also ran a quenched simulation at $β_{\sf eff} = 6.0$, which is the point of equal lattice spacing, $a_ρ^{-1}$. In the light sector, we find the chiral extrapolation to physical u- and d- masses to present a major source of uncertainty, comparable to the expected size of unquenching effects. From linear and quadratic fits we can estimate the errors on the hadron masses made from light quarks to be on a 15 % level prior to the continuum extrapolation. For the hadrons with strange valence quark content, the $N_F = 2$ approximation to QCD appears not to cure the well-known failure of quenched QCD to reproduce the physical $K-K^*$ splitting.

hep-lat↗
arXiv1998

An Estimate of alpha_S from Bottomonium in Unquenched QCD

We estimate the strong coupling constant from the perturbative expansion of the plaquette. The scale is set by the 2S-1S and 1P-1S splittings in bottomonium which are computed in NRQCD on dynamical gauge configurations with nf=2 degenerate Wilson quarks at intermediate masses. We have increased the statistics of our spectrum calculation in order to reliably extrapolate in the sea-quark mass. We find a value of alpha_MS(m_Z) = 0.1118(26) which is somewhat lower than previous estimates within NRQCD.

hep-lat↗
arXiv1998

Scanning the Topological Sectors of the QCD Vacuum with Hybrid Monte Carlo

We address a long standing issue and determine the decorrelation efficiency of the Hybrid Monte Carlo algorithm (HMC), for full QCD with Wilson fermions, with respect to vacuum topology. On the basis of five state-of-the art QCD vacuum field ensembles (with 3000 to 5000 trajectories each and m_pi/m_rho-ratios in the regime >0.56, for two sea quark flavours) we are able to establish, for the first time, that HMC provides sufficient tunneling between the different topological sectors of QCD. This will have an important bearing on the prospect to determine, by lattice techniques, the topological susceptibility of the vacuum, and topology sensitive quantities like the spin content of the proton, or the eta' mass.

hep-lat↗
arXiv1997

Critical Dynamics of the Hybrid Monte Carlo Algorithm

We investigate the critical dynamics of the Hybrid Monte Carlo algorithm approaching the chiral limit of standard Wilson fermions. Our observations are based on time series of lengths O(5000) for a variety of observables. The lattice sizes are 16^3 x 32 and 24^3 x 40. We work at beta=5.6, and kappa=0.156, 0.157, 0.1575, 0.158, with 0.83 > m_pi/m_rho > 0.55. We find surprisingly small integrated autocorrelation times for local and extended observables. The dynamical critical exponent $z$ of the exponential autocorrelation time is compatible with 2. We estimate the total computational effort to scale between V^2 and V^2.25 towards the chiral limit.

hep-lat↗
arXiv1997

Improving Stochastic Estimator Techniques for Disconnected Diagrams

Disconnected diagrams are expected to be sensitive to the inclusion of dynamical fermions. We present a feasibility study for the observation of such effects on the nucleonic matrix elements of the axial vector current, using SESAM full QCD vacuum configurations with Wilson fermions on $16^3\times 32$ lattices, at $β=5.6$. Starting from the standard methods developed by the Kentucky and Tsukuba groups, we investigate the improvement from various refinements thereof.

hep-lat↗
arXiv1997

Glueballs and string breaking from full QCD

We present results on the static potential, and torelon and glueball masses from simulations of QCD with two flavours of dynamical Wilson fermions on $16^3\times 32$ and $24^3\times 40$ lattices at $β=5.6$.

hep-lat↗
arXiv1997

Bottomonium from NRQCD with Dynamical Wilson Fermions

We present results for the b \bar b spectrum obtained using an O(M_bv^6)-correct non-relativistic lattice QCD action. Propagators are evaluated on SESAM's three sets of dynamical gauge configurations generated with two flavours of Wilson fermions at beta = 5.6. Compared to a quenched simulation at equivalent lattice spacing we find better agreement of our dynamical data with experimental results in the spin-independent sector but observe no unquenching effects in hyperfine-splittings. To pin down the systematic errors we have also compared quenched results in different ``tadpole'' schemes and used a lower order action.

hep-lat↗
arXiv1997

SSOR Preconditioning of Improved Actions

We generalize local lexicographic SSOR preconditioning for the Sheikholeslami-Wohlert improved Wilson fermion action and the truncated perfect free fermion action. In our test implementation we achieve performance gains as known from SSOR preconditioning of the standard Wilson fermion action.

hep-lat↗
arXiv1997

Light and Strange Quark Masses with Dynamical Wilson Fermions

We determine the masses of the light and the strange quarks in the MS-bar-scheme using our high-statistics lattice simulation of QCD with dynamical Wilson fermions. For each of our three sea quarks we have analyzed our data at five different values of the valence quark mass, enabling us to parameterize our fit results in the (sea-quark mass, valence quark mass) plane. For the light quark mass we find m_light(2 GeV) = 2.7(2) MeV, which is lower than in quenched simulations. Applying a new method, which we propose to extract the strange quark mass in a sea of two dynamical light quarks, we obtain m_strange(2 GeV) = 140(20) MeV.

hep-ph↗
arXiv1997

Improved Upsilon Spectrum with Dynamical Wilson Fermions

We present results for the b \bar b spectrum obtained using an O(M_bv^6)-correct non-relativistic lattice QCD action, where M_b denotes the bare b-quark mass and v^2 is the mean squared quark velocity. Propagators are evaluated on SESAM's three sets of dynamical gauge configurations generated with two flavours of Wilson fermions at beta = 5.6. These results, the first of their kind obtained with dynamical Wilson fermions, are compared to a quenched analysis at equivalent lattice spacing, beta = 6.0. Using our three sea-quark values we perform the ``chiral'' extrapolation to m_eff = m_s/3, where m_s denotes the strange quark mass. The light quark mass dependence is found to be small in relation to the statistical errors. Comparing the full QCD result to our quenched simulation we find better agreement of our dynamical data with experimental results in the spin-independent sector but observe no unquenching effects in hyperfine-splittings. To pin down the systematic errors we have also compared quenched results in different ``tadpole'' schemes as well as using a lower order action. We find that spin-splittings with an O(M_bv^4) action are O(10%) higher compared to O(M_bv^6) results. Relative to the results obtained with the plaquette method the Landau gauge mean link tadpole scheme raises the spin splittings by about the same margin so that our two improvements are opposite in effect.

hep-lat↗
arXiv1997

SESAM and TXL Results for Wilson Action--A Status Report

Results from two studies of full QCD with two flavours of dynamical Wilson fermions are presented. At beta=5.6, the region 0.83 > m_pi/m_rho > 0.56 at m_pia > 0.23 L^{-1} is explored. The SESAM collaboration has generated ensembles of about 200 statistically independent configurations on a 16^3 x 32-lattice at three different kappa-values and is entering the final phase of data analysis. The TXL simulation on a 24^3 x 40-lattice at two kappa-values has reached half statistics and data analysis has started recently, hence most results presented here are preliminary. The focus of this report is fourfold: we demonstrate that algorithmic improvements like fast Krylov solvers and parallel preconditioning recently introduced can be put into practise in full QCD simulations, we present encouraging observations as to the critical dynamics of the Hybrid Monte Carlo algorithm in the approach to the chiral limit, we mention signal improvements of noisy estimator techniques for disconnected diagrams to the pi-N sigma term, and we report on SESAM's results for light hadron spectrum, light quark masses, and heavy quarkonia.

hep-lat↗
arXiv1997

Light Quark Masses with Dynamical Wilson Fermions

We determine the masses of the light and the strange quarks in the $\bar{MS}$-scheme using our high-statistics lattice simulation of QCD with dynamical Wilson fermions. For the light quark mass we find $m^{light}_{\bar{MS}}(2 GeV) = 2.7(2) MeV$, which is lower than in quenched simulations. For the strange quark, in a sea of two dynamical light quarks, we obtain $m^{strange}_{\bar{MS}}(2 GeV) = 140(20) MeV$.

hep-lat↗
arXiv1996

Full QCD with dynamical Wilson fermions on a 24^3 x 40-lattice -- a feasibility study

The investigation of light sea-quark effects in lattice QCD with dynamical Wilson fermions requires both larger physical volumes and finer lattice resolutions than achieved previously. As high-end supercomputers like the 512-node APE Tower provide the compute power to perform a major step towards the chiral limit (T-chi-L), we have launched a feasibility study on a 24^3 x 40 lattice. We approach the chiral limit--while refining the resolution--, using the standard Wilson fermion action. Following previous work, our Hybrid Monte Carlo simulation runs at beta=5.6 and two kappa-values, 0.1575 and 0.158. From our study, we are confident that, for the APE Tower, a realistic working point has been found corresponding to a volume of 2 fm^3, with chirality characterized by 1/(a m_pi) = 5.6.

hep-lat↗
arXiv1996

Towards the glueball spectrum of full QCD

We present first results on masses of the scalar and tensor glueballs as well as of the torelon from simulations of QCD with two light flavours of Wilson fermions. The gauge configurations of extent 16^3*32 at beta = 5.6 and kappa = 0.156, 0.157 and 0.1575 have been generated as part of the SESAM collaboration programme. The present lattice resolutions correspond to 1/a = 2.0-2.3 GeV and ratios m(pi)/m(rho) = 0.83, 0.76 and 0.71, respectively. Studies on larger lattice volumes and closer to the chiral limit are in progress.

hep-lat↗