4,951 research outputs found
Quark mass dependence of the pseudoscalar hairpin vertex
In a recent investigation of chiral behavior in quenched lattice QCD, the
flavor-singlet pseudoscalar ``hairpin'' vertex associated with the eta prime
mass was studied for pion masses ranging from approximately 275 to 675 MeV.
Throughout this mass range, the quark-disconnected pseudoscalar correlator is
well-described by a pure double-pion-pole diagram with a p^2 independent mass
insertion. The residue of the double pole was found to exhibit a significant
quark mass dependence, evidenced by a negative slope of the effective mass
insertion (m_0^{eff})^2 as a function of m_{\pi}^2. It has been observed by
Sharpe that, with a consistent NLO calculation in quenched chiral perturbation
theory, this mass dependence is uniquely predicted in terms of the single-pole
coefficient \alpha_{\Phi} and the Leutwyler parameter L_5. Since \alpha_{\Phi}
is found to be approximately zero, the chiral slope of the double-pole residue
determines a value for L_5. This provides a consistency check between the
chiral slope of the hairpin mass insertion and that of the pion decay constant.
We investigate the consistency of these mass dependences in our Monte Carlo
results at two values of lattice spacing. Within statistics, the slopes are
found to be consistent with the Q\chiPT prediction, confirming that the
observed negative slope of m_0^{eff} arises as an effect of the L_5 Leutwyler
term.Comment: 4 pages, 1 figure, LaTe
QED2 as a testbed for interpolations between quenched and full QCD
Lattice QED2 with the Wilson formulation of fermions is used as a convenient
model system to study artifacts of the quenched approximation on a finite
lattice. The quenched functional integral is shown to be ill-defined in this
system as a consequence of the appearance of exactly real modes for physical
values of the fermion mass.
The location and frequency of such modes is studied as a function of lattice
spacing, lattice volume, topological charge and improved action parameters. The
efficacy of the recently proposed modified quenched approximation is examined,
as well as a new approach to the interpolation from the quenched to full
dynamical theory employing a truncated form of the fermion determinant.Comment: Talk presented by A. Duncan at LATTICE97 (theoretical developments
Low Dirac Eigenmodes and the Topological and Chiral Structure of the QCD Vacuum
Several lattice calculations which probe the chiral and topological structure
of QCD are discussed. The results focus attention on the low-lying eigenmodes
of the Dirac operator in typical gauge field configurations.Comment: Talk presented at the DPF2000 Conferenc
Evidence for quenched chiral logs
Using the pole shifting procedure of the modified quenched approximation
(MQA) to cure the exceptional configuration problem, accurate hadron hadron
spectrum calculations can be obtained at very light quark mass. Here we use the
MQA to extend and improve our previous investigation of chiral logs in the pion
mass. At beta=5.7 for Wilson fermion, we see clear evidence for quenched chiral
logarithms in the pion mass as a function of quark mass. The size of the
observed chiral log exponent delta is in good agreement with the value obtained
from a direct calculation of the eta' hairpin diagram.Comment: 3 pages, 4 figures, Lattice 98 tal
The eta-prime propagator in quenched QCD
The calculation of the eta-prime hairpin diagram is carried out in the
modified quenched approximation (MQA) in which the lattice artifact which
causes exceptional configurations is removed by shifting observed poles at
kappa<kappa_c in the quark propagators to the critical value of hop ping
parameter. By this method, the eta-prime propagator can be accurately
calculated even for very light quark mass. A determination of the topological
susceptibility for quenched QCD is also obtained, using the fermionic method of
Smit and Vink to calculate winding numbers.Comment: 3 pages, 3 postscript figure
Chiral Loops and Ghost States in the Quenched Scalar Propagator
The scalar, isovector meson propagator is analyzed in quenched QCD, using the
MQA pole-shifting ansatz to study the chiral limit. In addition to the expected
short-range exponential falloff characteristic of a heavy scalar meson, the
propagator also exhibits a longer-range, negative metric contribution which
becomes pronounced for smaller quark masses. We show that this is a quenched
chiral loop effect associated with the anomalous structure of the
propagator in quenched QCD. Both the time dependence and the quark mass
dependence of this effect are well-described by a chiral loop diagram
corresponding to an intermediate state, which is light and
effectively of negative norm in the quenched approximation. The relevant
parameters of the effective Lagrangian describing the scalar sector of the
quenched theory are determined.Comment: 29 pages, 10 figures, Late
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