134 research outputs found
Bottomonium spectrum at order v^6 from domain-wall lattice QCD: precise results for hyperfine splittings
The bottomonium spectrum is computed in dynamical 2+1 flavor lattice QCD,
using NRQCD for the b quarks. The main calculations in this work are based on
gauge field ensembles generated by the RBC and UKQCD collaborations with the
Iwasaki action for the gluons and a domain-wall action for the sea quarks.
Lattice spacing values of approximately 0.08 fm and 0.11 fm are used, and
simultaneous chiral extrapolations to the physical pion mass are performed. As
a test for gluon discretization errors, the calculations are repeated on two
ensembles generated by the MILC collaboration with the Luscher-Weisz gauge
action. Gluon discretization errors are also studied in a lattice potential
model using perturbation theory for four different gauge actions. The
nonperturbative lattice QCD results for the radial and orbital bottomonium
energy splittings obtained from the RBC/UKQCD ensembles are found to be in
excellent agreement with experiment. To get accurate results for spin
splittings, the spin-dependent order-v^6 terms are included in the NRQCD
action, and suitable ratios are calculated such that most of the unknown
radiative corrections cancel. The cancellation of radiative corrections is
verified explicitly by repeating the calculations with different values of the
couplings in the NRQCD action. Using the lattice ratios of the S-wave hyperfine
and the 1P tensor splitting, and the experimental result for the 1P tensor
splitting, the 1S hyperfine splitting is found to be
60.3+-5.5(stat)+-5.0(syst)+-2.1(exp) MeV, and the 2S hyperfine splitting is
predicted to be 23.5+-4.1(stat)+-2.1(syst)+-0.8(exp) MeV.Comment: 36 pages, 14 figures. v2: added Appendix D containing detailed
analysis of gluon discretization errors using a lattice potential model and
comparison to results from MILC ensembles. Estimates of systematic errors in
hyperfine splittings now include gluon discretization errors and b-bbar
annihilation contribution. Accepted for publication in PR
Localized eigenmodes of the covariant lattice Laplacian
We study numerically the eigenmode spectrum of the covariant lattice
Laplacian, in the fundamental SU(2) color group representation. It is found
that eigenmodes at the lower and upper ends of the spectrum are localized, and
that the localization volume scales. In contrast, the eigenmodes of the lattice
Faddeev-Popov operator are all extended rather than localized (as required for
confinement) despite the similarity of the kinetic and Faddeev-Popov operators.Comment: Talk presented by J. Greensite at Lattice2005 (Topology and
Confinement), Dublin, July 25-30, 2005; 6 pages, 4 figures, uses PoS.cls; to
appear in Proceedings of Scienc
Nucleon electromagnetic form factors from lattice QCD using a nearly physical pion mass
We present lattice QCD calculations of nucleon electromagnetic form factors
using pion masses = 149, 202, and 254 MeV and an action with
clover-improved Wilson quarks coupled to smeared gauge fields, as used by the
Budapest-Marseille-Wuppertal collaboration. Particular attention is given to
removal of the effects of excited state contamination by calculation at three
source-sink separations and use of the summation and generalized
pencil-of-function methods. The combination of calculation at the nearly
physical mass = 149 MeV in a large spatial volume ( = 4.2)
and removal of excited state effects yields agreement with experiment for the
electric and magnetic form factors and up to = 0.5
GeV.Comment: v2: published version; 30 pages, 25 figures, 6 table
Initial nucleon structure results with chiral quarks at the physical point
We report initial nucleon structure results computed on lattices with 2+1
dynamical M\"obius domain wall fermions at the physical point generated by the
RBC and UKQCD collaborations. At this stage, we evaluate only connected quark
contributions. In particular, we discuss the nucleon vector and axial-vector
form factors, nucleon axial charge and the isovector quark momentum fraction.
From currently available statistics, we estimate the stochastic accuracy of the
determination of and to be around 10%, and we expect to
reduce that to 5% within the next year. To reduce the computational cost of our
calculations, we extensively use acceleration techniques such as low-eigenmode
deflation and all-mode-averaging (AMA). We present a method for choosing
optimal AMA parameters.Comment: 7 pages, 11 figures; talk presented at the 32nd International
Symposium on Lattice Field Theory, 23-28 June, 2014, Columbia University, New
York, US
Quark Contributions to Nucleon Momentum and Spin from Domain Wall fermion calculations
We report contributions to the nucleon spin and momentum from light quarks
calculated using dynamical domain wall fermions with pion masses down to 300
MeV and fine lattice spacing a=0.084 fm. Albeit without disconnected diagrams,
we observe that spin and orbital angular momenta of both u and d quarks are
opposite, almost canceling in the case of the d quark, which agrees with
previous calculations using a mixed quark action. We also present the full
momentum dependence of n=2 generalized form factors showing little variation
with the pion mass.Comment: 7 pages, 5 figures, NT-LBNL-11-020, MIT-CTP-4323. Presented at the
29th International Symposium on Lattice Field Theory (Lattice 2011), Squaw
Valley, California, 10-16 Jul 201
Peculiarities in the Spectrum of the Adjoint Scalar Kinetic Operator in Yang-Mills Theory
We study the spectrum of low-lying eigenmodes of the kinetic operator for
scalar particles, in the color adjoint representation of Yang-Mills theory. The
kinetic operator is the covariant Laplacian, plus a constant which serves to
renormalize mass. In the pure gauge theory, our data indicates that the
interval between the lowest eigenvalue and the mobility edge tends to infinity
in the continuum limit. On these grounds, it is suggested that the perturbative
expression for the scalar propagator may be misleading even at distance scales
that are small compared to the confinement scale. We also measure the density
of low-lying eigenmodes, and find a possible connection to multi-critical
matrix models of order m=1.Comment: 9 pages, 14 figure
High Energy Physics from High Performance Computing
We discuss Quantum Chromodynamics calculations using the lattice regulator.
The theory of the strong force is a cornerstone of the Standard Model of
particle physics. We present USQCD collaboration results obtained on Argonne
National Lab's Intrepid supercomputer that deepen our understanding of these
fundamental theories of Nature and provide critical support to frontier
particle physics experiments and phenomenology.Comment: Proceedings of invited plenary talk given at SciDAC 2009, San Diego,
June 14-18, 2009, on behalf of the USQCD collaboratio
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