497,131 research outputs found
On cutoff effects in lattice QCD from short to long distances
We discuss kinematical enhancements of cutoff effects at short and
intermediate distances. Starting from a pedagogical example with periodic
boundary conditions, we switch to the case of the Schroedinger Functional,
where the theoretical analysis is checked by precise numerical data with Nf=2
dynamical O(a)-improved Wilson quarks. Finally we present an improved
determination of the renormalization of the axial current in that theory.Comment: 16 pages, 4 figures and 2 tables. References modifie
Cutoff-effects in the spectrum of dynamical Wilson fermions
We investigate the low-lying eigenvalues of the improved Wilson-Dirac
operator in the Schroedinger functional with two dynamical quark flavors. At a
lattice spacing of approximately 0.1 fm we find more very small eigenvalues
than in the quenched case. These cause problems with HMC-type algorithms and in
the evaluation of fermionic correlation functions. Through a simulation at a
finer lattice spacing we are able to establish their nature as cutoff-effectsComment: Lattice2004(machines), 3 pages, 3 figures, talk by R.
Towards precision heavy flavour physics from lattice QCD
I convey an idea of the significant recent progress, which opens up good
perspectives for high-precision ab-initio computations in heavy flavour physics
based on lattice QCD. This report focuses on the strategy and the challenges of
fully non-perturbative investigations in the B-meson sector, where the b-quark
is treated within an effective theory, as followed by the ALPHA Collaboration.
As an application, I outline its use to determine the b-quark mass and
summarize the status of our ongoing project in the two dynamical flavour
theory.Comment: Invited talk at the Third Workshop on Theory, Phenomenology and
Experiments in Heavy Flavour Physics, 5-7 July 2010, Capri, Italy; 8 pages
including figures, latex2e, uses espcrc2.st
Quark disconnected diagrams in chiral perturbation theory
We show how quark-disconnected and quark-connected contributions to hadronic
n-point functions can be written as independent correlators for which one can
derive expressions in partially quenched chiral effective theory. As an example
we apply the idea to the case of the hadronic vacuum polarisation. In
particular, we consider the cases of the Nf = 2 theory without and with a
partially quenched strange quark and also the Nf = 2 + 1 theory. In the latter
two cases a parameter-free prediction for the disconnected contribution at NLO
in the effective theory is given. Finally we show how twisted boundary
conditions can then be used in lattice QCD to improve the q^2 resolution in the
connected contributions even when flavour singlet operators are considered.Comment: 19 pages, 4 figures; results unchanged; typos corrected, references
added and discussion extended; accepted for publication in JHE
Glueball masses from ratios of path integrals
By generalizing our previous work on the parity symmetry, the partition
function of a Yang-Mills theory is decomposed into a sum of path integrals each
giving the contribution from multiplets of states with fixed quantum numbers
associated to parity, charge conjugation, translations, rotations and central
conjugations. Ratios of path integrals and correlation functions can then be
computed with a multi-level Monte Carlo integration scheme whose numerical
cost, at a fixed statistical precision and at asymptotically large times,
increases power-like with the time extent of the lattice. The strategy is
implemented for the SU(3) Yang-Mills theory, and a full-fledged computation of
the mass and multiplicity of the lightest glueball with vacuum quantum numbers
is carried out at two values of the lattice spacing (0.17 and 0.12 fm).Comment: Poster contribution to Lattice 2011. 7 pages, 2 tables, 2 figure
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