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Chulwoo Jung

Publications and source records attributed to Chulwoo Jung.

At least 73 records · Page 4Linked to original sources
arXiv2010

Systematic Uncertainties in $B_K$ with Improved Staggered Fermions

We study three sources of error in our calculation of $B_K$ using HYP-smeared staggered fermions on the MILC asqtad lattices. These are (1) dependence on the light sea quark mass; (2) finite volume effects; and (3) the impact of an order of magnitude increase in the number of measurements. Our main results are (1) the dependence on the light sea-quark mass is weaker than expected by naive dimensional analysis, (2) including finite volume effects in SU(2) staggered chiral perturbation theory fits leads to a very small change in $B_K$, of size $\approx 0.1%$, and (3) increasing the statistics on one of the coarse MILC lattices resolves a potential discrepancy with other coarse results.

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arXiv2010

First results for $B_K$ on the ultrafine ($a=0.045$ fm) ensemble

We present preliminary results for $B_K$ from the MILC ultrafine lattices, based on a partial ensemble of 305 configurations. We use HYP-smeared improved staggered valence quarks. The analysis is done using fitting forms based on both SU(2) and SU(3) staggered chiral perturbation thery. For the SU(2) analysis, we find that the result using the NLO fit function is consistent with that from a partial NNLO fit. For the SU(3) analysis, where we have to use partially constrained fits due to the number of fit parameters, we find that our two preferred fits ("N-BB1" and "N-BB2") are also consistent, both with each other and with the results of the SU(2) fits. These results are used in companion proceedings to improve the control over the continuum extrapolation.

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arXiv2010

Status of dynamical ensemble generation

I give an overview of current and future plans of dynamical QCD ensemble generation activities. A comparison of simulation cost between different discretizations is made. Recent developments in techniques and algorithms used in QCD dynamical simulations, especially mass reweighting, are also discussed.

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arXiv2009

Determination of $B_K$ using improved staggered fermions (II) SU(2) chiral perturbation theory fit

We present results for $B_K$ calculated using HYP-smeared improved staggered fermions on the MILC asqtad lattices. In this report, the data is analyzed using the results of SU(2) staggered chiral perturbation theory (SChPT). We outline the derivation of the NLO SU(2) SChPT result, explain our fitting procedure, and outline how we estimate systematic errors. We also show the light sea-quark mass and lattice spacing dependence for both SU(2) and SU(3)-based analyses. Our preliminary result from the SU(2) analysis is $B_K(\text{NDR}, μ= 2 \text{GeV}) = 0.512 \pm 0.014 \pm 0.034$ and $\hat{B}_K = B_K(\text{RGI})= 0.701 \pm 0.019 \pm 0.047$. This is somewhat more accurate than our result from the SU(3) analysis. It is consistent with results obtained using valence domain-wall fermions

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arXiv2009

Determination of $B_K$ using improved staggered fermions (I): SU(3) chiral perturbation theory fit

We present the results for $B_K$ calculated using HYP-smeared staggered fermions using lattices generated by the MILC collaboration using the asqtad staggered action. We have done the calculation on 8 ensembles of these lattices, including three different lattice spacings ($a=0.12, 0.09, 0.06$ fm). We fit the data to forms based on those predicted by SU(3) mixed-action partially-quenched staggered chiral perturbation theory. Our preliminary result is $ B_K(\text{NDR},μ=2\text{GeV}) = 0.528 \pm 0.011 \pm 0.048 $, where the first error is statistical and the second systematic. The error turns out to be larger than that from an analysis using SU(2) chiral perturbation theory.

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arXiv2009

Determination of $B_K$ using improved staggered fermions (III) Finite volume effects

We study the finite-volume effects in our calculation of $B_K$ using HYP-smeared improved staggered valence fermions. We calculate the predictions of both SU(3) and SU(2) staggered chiral perturbation theory at one-loop order. We compare these to the results of a direct calculation, using MILC coarse lattices with two different volumes: $20^3$ and $28^3$. From the direct calculation, we find that the finite volume effect is $\approx 2%$ for the SU(3) analysis and $\approx 0.9%$ for the SU(2) analysis. We also show how the statistical error depends on the number of measurements made per configuration, and make a first study of autocorrelations.

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arXiv2008

Scaling behavior and sea quark dependence of pion spectrum with HYP-smeared staggered fermions

We study the pion spectrum (and in particular taste-symmetry breaking within it) using HYP-smeared valence staggered fermions on the coarse and fine MILC lattices (which have asqtad staggered sea quarks). We focus on the dependence on lattice spacing and sea-quark mass. We also update our results on source dependence. Our main conclusion is that on the MILC fine lattices the appropriate power-counting for SU(3) staggered chiral perturbation theory may have discretization errors entering at next-to-leading order rather than at leading-order.

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arXiv2008

Calculating $B_K$ using HYP staggered fermions

We give an update on our calculation of $B_K$ using HYP-smeared valence staggered quarks. We have results for $B_K$ at tree-level on several coarse MILC lattices ($a\approx 0.12 $fm) and one of the fine lattices ($a\approx 0.09 $fm), using 10 light valence quarks ranging down to $m_s^{\rm phys}/10$. We have generalized staggered chiral perturbation theory to our mixed action setup, and outline the results. We explain our present fitting strategy, and give some preliminary results.

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arXiv2008

Taste symmetry breaking with HYP-smeared staggered fermions

We study the impact of hypercubic (HYP) smearing on the size of taste breaking for staggered fermions, comparing to unimproved and to asqtad-improved staggered fermions. As in previous studies, we find a substantial reduction in taste-breaking compared to unimproved staggered fermions (by a factor of 4-7 on lattices with spacing $a\approx 0.1 $fm). In addition, we observe that discretization effects of next-to-leading order in the chiral expansion (${\cal O}(a^2 p^2)$) are markedly reduced by HYP smearing. Compared to asqtad valence fermions, we find that taste-breaking in the pion spectrum is reduced by a factor of 2.5-3, down to a level comparable to the expected size of generic ${\cal O}(a^2)$ effects. Our results suggest that, once one reaches a lattice spacing of $a\approx 0.09 $fm, taste-breaking will be small enough after HYP smearing that one can use a modified power counting in which ${\cal O}(a^2) \ll {\cal O}(p^2)$, simplify fitting to phenomenologically interesting quantities.

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arXiv2007

Localization and chiral symmetry in 2+1 flavor domain wall QCD

We present results for the dependence of the residual mass of domain wall fermions (DWF) on the size of the fifth dimension and its relation to the density and localization properties of low-lying eigenvectors of the corresponding hermitian Wilson Dirac operator relevant to simulations of 2+1 flavor domain wall QCD. Using the DBW2 and Iwasaki gauge actions, we generate ensembles of configurations with a $16^3\times 32$ space-time volume and an extent of 8 in the fifth dimension for the sea quarks. We demonstrate the existence of a regime where the degree of locality, the size of chiral symmetry breaking and the rate of topology change can be acceptable for inverse lattice spacings $a^{-1} \ge 1.6$ GeV.

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arXiv2007

Status of 2+1 flavor, 32^3x64 domain wall fermion simulations

We describe details of 1/a ~ 2.2Gev, L ~ 3 fm dynamical domain wall fermion simulations which will allow us to do a more systematic continuum extrapolation in combination with existing simu- lations. Details of the simulations such as algorithm choices and machine performance, as well as results of basic measurements are presented. These configurations are presently being generated on the QCDOC machine at Edinburgh and the DOE QCDOC machine at Brookhaven as part of a joint project with LHPC.

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arXiv2007

Improvement of the pion spectrum with HYP-smeared staggered fermions

We extend our previous study of taste-symmetry breaking using HYP-smeared staggered fermions in two ways. First, we improve the statistics of a comparison of unimproved and HYP-smeared staggered fermions on quenched lattices with $a\approx 0.1 $fm. This allows us to obtain a signal for all pion tastes, rather than just a subset, and thus to make a complete comparison. In addition, it allows us to differentiate between wall and local sources. Second, we compare HYP-smeared valence quarks to asqtad valence quarks on 2+1 flavor unquenched MILC lattices. We find that taste breaking is substantially reduced by HYP-smearing, bringing the size of this discretization effect (which is the dominant such effect with staggered fermions) down to the size expected generically for any fermion type.

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arXiv2005

Thermodynamics using p4-improved staggered fermion action on QCDOC

We present an exploratory study of the thermodynamics of $N_f=3$ QCD with an improved staggered fermions using the QCDOC supercomputer. We use a p4 action with MILC-style smeared links (Fat 7). Some details of the implementation of the p4 action on QCDOC are discussed and performance benchmarks are given. We show preliminary results for the quark mass dependence of the pseudo-critical temperature $T_c$ from several lattice volumes . We also make a comparison between p4fat7 and the old p4 action.

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arXiv2001

A Lattice Study of the Magnetic Moment and the Spin Structure of the Nucleon

Using an approach free from momentum extrapolation, we calculate the nucleon magnetic moment and the fraction of the nucleon spin carried by the quark angular momentum in the quenched lattice QCD approximation. Quarks with three values of lattice masses, 210, 124 and 80 MeV, are formulated on the lattice using the standard Wilson approach. At every mass, 100 gluon configurations on 16^3 x 32 lattice with β=6.0 are used for statistical averaging. The results are compared with the previous calculations with momentum extrapolation. The contribution of the disconnected diagrams is studied at the largest quark mass using noise theory technique.

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arXiv2001

Photon Structure Functions from Quenched Lattice QCD

We calculate the first moment of the photon structure function, $ ^γ=\int^1_0 dx F^γ_2(x,Q^2)$, on the quenched lattices with $β=6.0$ using the formalism developed by the authors recently. In this exploratory study, we take into account only the connected contractions. The result is compared with the experimental data as well as model predictions.

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arXiv2001

Studying Hadronic Structure of the Photon in Lattice QCD

We show that the matrix element of a local quark-gluon operator in the photon state, $<γ(kλ)|\hat O| γ(kλ)>$, can be calculated in lattice QCD. The result is generalized to other quantities involving space-like photons, including the transition form factor $γγ^*\to π^0$ and the virtual-photon-nucleon Compton amplitude $<γ^*N |γ^*N>$ which can be used to define the generalized Drell-Hearn-Gerasimov and Bjorken sum rules.

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arXiv2000

Residual Chiral Symmetry Breaking in Domain-Wall Fermions

We study the effective quark mass induced by the finite separation of the domain walls in the domain-wall formulation of chiral fermion as the function of the size of the fifth dimension ($L_s$), the gauge coupling ($β$) and the physical volume ($V$). We measure the mass by calculating the small eigenvalues of the hermitian domain-wall Dirac operator ($H_{\rm DWF}(m_0=1.8))$ in the topologically-nontrivial quenched SU(3) gauge configurations. We find that the induced quark mass is nearly independent of the physical volume, decays exponentially as a function of $L_s$, and has a strong dependence on the size of quantum fluctuations controlled by $β$. The effect of the choice of the lattice gluon action is also studied.

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