12,465 research outputs found
The Low-Lying Dirac Spectrum of Staggered Quarks
We investigate and clarify the role of topology and the issues surrounding
the epsilon regime for staggered quarks. We study unimproved and improved
staggered quark Dirac operators on quenched lattice QCD gluon backgrounds
generated using a Symanzik-improved gluon action. For the improved Dirac
operators we find a clear separation of the spectrum into would-be zero modes
and others. The number of would-be zero modes depends on the topological charge
as predicted by the continuum Index Theorem, and the expectation values of
their chirality are large for the most improved actions (approx 0.7). The
remaining modes have low chirality and show clear signs of clustering into
quartets that become degenerate in the continuum limit. We demonstrate that the
lattice spacing and volume dependence of the eigenvalues follow expectations.
Furthermore, the non-zero modes follow the random matrix theory predictions for
all topological charge sectors. The values of the chiral condensate extracted
from fits to the theoretical distributions are consistent with each other, and
with the results obtained from the total density of eigenvalues using the
Banks-Casher relation. We conclude that staggered quarks respond correctly to
QCD topology when both fermion and gauge actions are improved.Comment: 17 pages, a few typos corrected, part of one figure change
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
The B Meson Decay Constant from Unquenched Lattice QCD
We present determinations of the B meson decay constant f_B and of the ratio
f_{B_s}/f_B using the MILC collaboration unquenched gauge configurations which
include three flavors of light sea quarks. The mass of one of the sea quarks is
kept around the strange quark mass, and we explore a range in masses for the
two lighter sea quarks down to m_s/8.
The heavy b quark is simulated using Nonrelativistic QCD, and both the
valence and sea light quarks are represented by the highly improved (AsqTad)
staggered quark action.
The good chiral properties of the latter action allow for a much smoother
chiral extrapolation to physical up and down quarks than has been possible in
the past. We find f_B = 216(9)(19)(4) (6) MeV and f_{B_s} /f_B = 1.20(3)(1).Comment: 4 pages, 2 figure
Improvement and Taste Symmetry Breaking for Staggered Quarks
We compare several improved actions for staggered quarks. We study the effect
of improvement on the taste changing interactions by calculating the splitting
in the pion spectrum. We investigate the effect of the improvement on some
topological properties.Comment: 3 pages, 3 figures, Lattice 2003 proceeding
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