412 research outputs found
The Schr\"odinger Functional for Improved Gluon and Quark Actions
The Schr\"odinger Functional (quantum/lattice field theory with Dirichlet
boundary conditions) is a powerful tool in the non-perturbative improvement and
for the study of other aspects of lattice QCD. Here we adapt it to improved
gluon and quark actions, on isotropic as well as anisotropic lattices.
Specifically, we describe the structure of the boundary layers, obtain the
exact form of the classically improved gauge action, and outline the
modifications necessary on the quantum level. The projector structure of
Wilson-type quark actions determines which field components can be specified at
the boundaries. We derive the form of O(a) improved quark actions and describe
how the coefficients can be tuned non-perturbatively. There is one coefficient
to be tuned for an isotropic lattice, three in the anisotropic case.
Our ultimate aim is the construction of actions that allow accurate
simulations of all aspects of QCD on coarse lattices.Comment: 39 pages, LaTeX, 11 embedded eps file
One-Loop Self Energy and Renormalization of the Speed of Light for some Anisotropic Improved Quark Actions
One-loop corrections to the fermion rest mass M_1, wave function
renormalization Z_2 and speed of light renormalization C_0 are presented for
lattice actions that combine improved glue with clover or D234 quark actions
and keep the temporal and spatial lattice spacings, a_t and a_s, distinct. We
explore a range of values for the anisotropy parameter \chi = a_s/a_t and treat
both massive and massless fermions.Comment: 45 LaTeX pages with 4 postscript figure
Lattice QCD on Small Computers
We demonstrate that lattice QCD calculations can be made -- times
faster by using very coarse lattices. To obtain accurate results, we replace
the standard lattice actions by perturbatively-improved actions with
tadpole-improved correction terms that remove the leading errors due to the
lattice. To illustrate the power of this approach, we calculate the
static-quark potential, and the charmonium spectrum and wavefunctions using a
desktop computer. We obtain accurate results that are independent of the
lattice spacing and agree well with experiment.Comment: 15 pages, 3 figs incl as LaTex pictures Minor additions to tables and
tex
The Anisotropic Wilson Gauge Action
Anisotropic lattices, with a temporal lattice spacing smaller than the
spatial one, allow precision Monte Carlo calculations of problems that are
difficult to study otherwise: heavy quarks, glueballs, hybrids, and high
temperature thermodynamics, for example. We here perform the first step
required for such studies with the (quenched) Wilson gauge action, namely, the
determination of the renormalized anisotropy as a function of the bare
anisotropy and the coupling. By, essentially, comparing the
finite-volume heavy quark potential where the quarks are separated along a
spatial direction with that where they are separated along the time direction,
we determine the relation between and to a fraction of 1% for
weak and to 1% for strong coupling. We present a simple parameterization of
this relation for and , which
incorporates the known one-loop result and reproduces our non-perturbative
determinations within errors. Besides solving the problem of how to choose the
bare anisotropies if one wants to take the continuum limit at fixed
renormalized anisotropy, this parameterization also yields accurate estimates
of the derivative needed in thermodynamic studies.Comment: 24 pages, LaTeX, 15 ps figures (added high statistics simulations
confirming our results; to appear in Nucl. Phys. B
Shell model calculations of stellar weak interaction rates: I. Gamow-Teller distributions and spectra of nuclei in the mass range A=45-65
Electron capture and beta-decay rates on nuclei in the mass range A=45-65
play an important role in many astrophysical environments. The determination of
these rates by large-scale shell model calculations is desirable, but it
requires to reproduce the Gamow-Teller strength distributions and spectra of
the pf shell nuclei. We show in this paper that large-scale shell model
calculations, employing a slightly monopole-corrected version of the wellknown
KB3 interaction, fulfill these necessary requirements. In particular, our
calculations reproduce the experimentally available GT+ and GT- strength
distributions and the nuclear halflives, and describe the nuclear spectra
appropriately.Comment: 11 pages, 5 figure
Scalar meson properties from D-meson decays
Decay amplitudes of D(D_s)->f0(980)X, X=pi, K, are compared to experimental
branching ratios with the aim of singling out the poorly known D->f0(980)
transition form factor in these amplitudes. Since the other elements of the
amplitudes are either calculable in an effective QCD theory using operator
product expansion or are known from experiment (e.g. the pion and kaon decay
constants), we can take advantage of these reactions to constrain the
transition form factors obtained in relativistic quark models. In these models,
the f0(980) wavefunction requires an unknown size parameter for both its
non-strange ubar u(dbar d) and strange sbar s components, which we fit to the
D(D_s) decay data.Comment: Talk given at the Few Body 18 conference in Santos, Brazil. 4 pages,
to be published in Nuclear Physics
Quarkonium spin structure in lattice NRQCD
Numerical simulations of the quarkonium spin splittings are done in the
framework of lattice nonrelativistic quantum chromodynamics (NRQCD). At leading
order in the velocity expansion the spin splittings are of , where
is the renormalized quark mass and is the mean squared quark
velocity. A systematic analysis is done of all next-to-leading order
corrections. This includes the addition of relativistic
interactions, and the removal of discretization errors in the
leading-order interactions. Simulations are done for both S- and P-wave mesons,
with a variety of heavy quark actions and over a wide range of lattice
spacings. Two prescriptions for the tadpole improvement of the action are also
studied in detail: one using the measured value of the average plaquette, the
other using the mean link measured in Landau gauge. Next-to-leading order
interactions result in a very large reduction in the charmonium splittings,
down by about 60% from their values at leading order. There are further
indications that the velocity expansion may be poorly convergent for
charmonium. Prelimary results show a small correction to the hyperfine
splitting in the Upsilon system.Comment: 16 pages, REVTEX v3.1, 5 postscript figures include
Gamow-Teller strength distributions for nuclei in pre-supernova stellar cores
Electron-capture and -decay of nuclei in the core of massive stars
play an important role in the stages leading to a type II supernova explosion.
Nuclei in the f-p shell are particularly important for these reactions in the
post Silicon-burning stage of a presupernova star. In this paper, we
characterise the energy distribution of the Gamow-Teller Giant Resonance (GTGR)
for mid-fp-shell nuclei in terms of a few shape parameters, using data obtained
from high energy, forward scattering (p,n) and (n,p) reactions. The energy of
the GTGR centroid is further generalised as function of nuclear
properties like mass number, isospin and other shell model properties of the
nucleus. Since a large fraction of the GT strength lies in the GTGR region, and
the GTGR is accessible for weak transitions taking place at energies relevant
to the cores of presupernova and collapsing stars, our results are relevant to
the study of important -capture and -decay rates of arbitrary,
neutron-rich, f-p shell nuclei in stellar cores. Using the observed GTGR and
Isobaric Analog States (IAS) energy systematics we compare the coupling
coefficients in the Bohr-Mottelson two particle interaction Hamiltonian for
different regions of the Isotope Table.Comment: Revtex, 28 pages +7 figures (PostScript Figures, uuencoded, filename:
Sutfigs.uu). If you have difficulty printing the figures, please contact
[email protected]. Accepted for publication in Phys. Rev. C, Nov 01,
199
Heavy-light mesons with staggered light quarks
We demonstrate the viability of improved staggered light quarks in studies of
heavy-light systems. Our method for constructing heavy-light operators exploits
the close relation between naive and staggered fermions. The new approach is
tested on quenched configurations using several staggered actionsn combined
with nonrelativistic heavy quarks. The B_s meson kinetic mass, the hyperfine
and 1P-1S splittings in B_s, and the decay constant f_{B_s} are calculated and
compared to previous quenched lattice studies. An important technical detail,
Bayesian curve-fitting, is discussed at length.Comment: 38 pages, figures included. v2: Entry in Table IX corrected and other
minor changes, version appearing in Phys. Rev.
Comparison Studies of Finite Momentum Correlators on Anisotropic and Isotropic Lattices
We study hadronic two- and three-point correlators relevant for heavy to
light pseudoscalar meson semi-leptonic decays, using Symanzik improved glue,
D234 light quark and NRQCD heavy quark actions. Detailed comparisons are made
between simulations on anisotropic and isotropic lattices involving finite
momentum hadrons. We find evidence that having an anisotropy helps in
extracting better signals at higher momenta. Initial results for the form
factors f_+(q^2) and f_0(q^2) are presented with tree-level matching of the
lattice heavy-light currents.Comment: 43 pages with 50 postscript figure
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