1,765 research outputs found
B_s Mesons using Staggered Light Quarks
Last year we proposed using staggered fermions as the light quarks, combined
with nonrelativistic heavy quarks, in simulations of heavy-light mesons. A
first round of tests which focuses on the B_s meson has been completed using
quenched lattices, and results are presented here for the kinetic B_s mass, the
B_s^* - B_s splitting, and f_{B_s}. The next project, already underway, is to
compute the B and B_s decay constants and spectra on the n_f = 2+1 and 3 MILC
lattices. We report on progress with one set of these configurations.Comment: Talk presented by M.W. at Lattice2002(heavyquark
Perturbative Wilson loops from unquenched Monte Carlo simulations at weak couplings
Perturbative expansions of several small Wilson loops are computed through
next-to-next-to-leading order in unquenched lattice QCD, from Monte Carlo
simulations at weak couplings. This approach provides a much simpler
alternative to conventional diagrammatic perturbation theory, and is applied
here for the first time to full QCD. Two different sets of lattice actions are
considered: one set uses the unimproved plaquette gluon action together with
the unimproved staggered-quark action; the other set uses the one-loop-improved
Symanzik gauge-field action together with the so-called ``asqtad''
improved-staggered quark action. Simulations are also done with different
numbers of dynamical fermions. An extensive study of the systematic
uncertainties is presented, which demonstrates that the small third-order
perturbative component of the observables can be reliably extracted from
simulation data. We also investigate the use of the rational hybrid Monte Carlo
algorithm for unquenched simulations with unimproved-staggered fermions. Our
results are in excellent agreement with diagrammatic perturbation theory, and
provide an important cross-check of the perturbation theory input to a recent
determination of the strong coupling by the HPQCD
collaboration.Comment: 14 pages, 8 figure
Highly Improved Staggered Quarks on the Lattice, with Applications to Charm Physics
We use perturbative Symanzik improvement to create a new staggered-quark
action (HISQ) that has greatly reduced one-loop taste-exchange errors, no
tree-level order a^2 errors, and no tree-level order (am)^4 errors to leading
order in the quark's velocity v/c. We demonstrate with simulations that the
resulting action has taste-exchange interactions that are at least 3--4 times
smaller than the widely used ASQTAD action. We show how to estimate errors due
to taste exchange by comparing ASQTAD and HISQ simulations, and demonstrate
with simulations that such errors are no more than 1% when HISQ is used for
light quarks at lattice spacings of 1/10 fm or less. The suppression of (am)^4
errors also makes HISQ the most accurate discretization currently available for
simulating c quarks. We demonstrate this in a new analysis of the psi-eta_c
mass splitting using the HISQ action on lattices where a m_c=0.43 and 0.66,
with full-QCD gluon configurations (from MILC). We obtain a result of~111(5)
MeV which compares well with experiment. We discuss applications of this
formalism to D physics and present our first high-precision results for D_s
mesons.Comment: 21 pages, 8 figures, 5 table
Unquenching effects on the coefficients of the L\"uscher-Weisz action
The effects of unquenching on the perturbative improvement coefficients in
the Symanzik action are computed within the framework of L\"uscher-Weisz
on-shell improvement. We find that the effects of quark loops are surprisingly
large, and their omission may well explain the scaling violations observed in
some unquenched studies.Comment: 7 pages, 5 figures, uses revtex4; version to appear in Phys.Rev.
Perturbative expansions from Monte Carlo simulations at weak coupling: Wilson loops and the static-quark self-energy
Perturbative coefficients for Wilson loops and the static-quark self-energy
are extracted from Monte Carlo simulations at weak coupling. The lattice
volumes and couplings are chosen to ensure that the lattice momenta are all
perturbative. Twisted boundary conditions are used to eliminate the effects of
lattice zero modes and to suppress nonperturbative finite-volume effects due to
Z(3) phases. Simulations of the Wilson gluon action are done with both periodic
and twisted boundary conditions, and over a wide range of lattice volumes (from
to ) and couplings (from to ).
A high precision comparison is made between the simulation data and results
from finite-volume lattice perturbation theory. The Monte Carlo results are
shown to be in excellent agreement with perturbation theory through second
order. New results for third-order coefficients for a number of Wilson loops
and the static-quark self-energy are reported.Comment: 36 pages, 15 figures, REVTEX documen
Update: Accurate Determinations of alpha_s from Realistic Lattice QCD
We use lattice QCD simulations, with MILC configurations (including vacuum
polarization from u, d, and s quarks), to update our previous determinations of
the QCD coupling constant. Our new analysis uses results from 6 different
lattice spacings and 12 different combinations of sea-quark masses to
significantly reduce our previous errors. We also correct for
finite-lattice-spacing errors in the scale setting, and for nonperturbative
chiral corrections to the 22 short-distance quantities from which we extract
the coupling. Our final result is alpha_V(7.5GeV,nf=3) = 0.2120(28), which is
equivalent to alpha_msbar(M_Z,n_f=5)= 0.1183(8). We compare this with our
previous result, which differs by one standard deviation.Comment: 12 pages, 2 figures, 4 table
B Physics on the Lattice: Present and Future
Recent experimental measurements and lattice QCD calculations are now
reaching the precision (and accuracy) needed to over-constrain the CKM
parameters and . In this brief review, I discuss the
current status of lattice QCD calculations needed to connect the experimental
measurements of meson properties to quark flavor-changing parameters.
Special attention is given to , which is becoming a competitive
way to determine , and to mixings, which now include
reliable extrapolation to the physical light quark mass. The combination of the
recent measurement of the mass difference and current lattice
calculations dramatically reduces the uncertainty in . I present an
outlook for reducing dominant lattice QCD uncertainties entering CKM fits, and
I remark on lattice calculations for other decay channels.Comment: Invited brief review for Mod. Phys. Lett. A. 15 pages. v2: typos
corrected, references adde
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