373 research outputs found
Lattice hadron matrix elements with the Schroedinger functional: the case of the first moment of non-singlet quark density
We present the results of a non-perturbative determination of the pion matrix
element of the twist-2 operator corresponding to the average momentum of
non-singlet quark densities. The calculation is made within the Schroedinger
functional scheme. We report the results of simulations done with the standard
Wilson action and with the non-perturbatively improved clover action and we
show that their ratio correctly extrapolates, in the continuum limit, to a
value compatible with the residual correction factor expected from perturbation
theory.Comment: LaTeX, 10 pages, 5 figure
Non-perturbative Pion Matrix Element of a twist-2 operator from the Lattice
We give a continuum limit value of the lowest moment of a twist-2 operator in
pion states from non-perturbative lattice calculations. We find that the
non-perturbatively obtained renormalization group invariant matrix element is
_{RGI} = 0.179(11), which corresponds to ^{MSbar}(2 GeV) = 0.246(15). In
obtaining the renormalization group invariant matrix element, we have
controlled important systematic errors that appear in typical lattice
simulations, such as non-perturbative renormalization, finite size effects and
effects of a non-vanishing lattice spacing. The crucial limitation of our
calculation is the use of the quenched approximation. Another question that
remains not fully clarified is the chiral extrapolation of the numerical data.Comment: 26 pages, 10 figures, v2: final version, accepted for publication in
EPJ
Heavy quark masses in the continuum limit of quenched Lattice QCD
We compute charm and bottom quark masses in the quenched approximation and in
the continuum limit of lattice QCD. We make use of a step scaling method,
previously introduced to deal with two scale problems, that allows to take the
continuum limit of the lattice data. We determine the RGI quark masses and make
the connection to the MSbar scheme. The continuum extrapolation gives us a
value m_b^{RGI} = 6.73(16) GeV for the b-quark and m_c^{RGI} = 1.681(36) GeV
for the c-quark, corresponding respectively to m_b^{MSbar}(m_b^{MSbar}) =
4.33(10) GeV and m_c^{MSbar}(m_c^{MSbar}) = 1.319(28) GeV. The latter result,
in agreement with current estimates, is for us a check of the method. Using our
results on the heavy quark masses we compute the mass of the Bc meson, M_{Bc} =
6.46(15) GeV.Comment: 29 pages, 9 figures, version accepted for publication in Nucl. Phys.
Pion parton distribution functions from lattice QCD
We report on recent results for the pion matrix element of the twist-2
operator corresponding to the average momentum of non-singlet quark densities.
For the first time finite volume effects of this matrix element are
investigated and come out to be surprisingly large. We use standard Wilson and
non-perturbatively improved clover actions in order to control better the
extrapolation to the continuum limit. Moreover, we compute, fully
non-perturbatively, the renormalization group invariant matrix element, which
allows a comparison with experimental results in a broad range of energy
scales. Finally, we discuss the remaining uncertainties, the extrapolation to
the chiral limit and the quenched approximation.Comment: Lattice2003(matrix), 3 pages, 4 figure
Continuous external momenta in non-perturbative lattice simulations: a computation of renormalization factors
We discuss the usage of continuous external momenta for computing
renormalization factors as needed to renormalize operator matrix elements.
These kind of external momenta are encoded in special boundary conditions for
the fermion fields. The method allows to compute certain renormalization
factors on the lattice that would have been very difficult, if not impossible,
to compute with standard methods. As a result we give the renormalization group
invariant step scaling function for a twist-2 operator corresponding to the
average momentum of non-singlet quark densities.Comment: 28 pages, 10 figure
SSOR preconditioning in simulations of the QCD Schr\"odinger functional
We report on a parallelized implementation of SSOR preconditioning for O(a)
improved lattice QCD with Schr\"odinger functional boundary conditions.
Numerical simulations in the quenched approximation at parameters in the light
quark mass region demonstrate that a performance gain of a factor 1.5
over even-odd preconditioning can be achieved.Comment: 15 pages, latex2e, 4 Postscript figures, uses packages elsart and
epsfi
Non-perturbative scale evolution of four-fermion operators
We apply the Schroedinger Functional (SF) formalism to determine the
renormalisation group running of four-fermion operators which appear in the
effective weak Hamiltonian of the Standard Model. Our calculations are done
using Wilson fermions and the parity-odd components of the operators.
Preliminary results are presented for the operator .Comment: Lattice2002(improve
The charm quark's mass in quenched QCD
We present our preliminary result for the charmed quark mass, which follows
from taking the D_s and K meson masses from experiment and r0=0.5 fm (or,
equivalently F_K=160 MeV) to set the scale. For the renormalization group
invariant quark mass we obtain M_c = 1684(64) MeV, which translates to
m_c(m_c)= 1314 (40)(20)(7) MeV for the running mass in the MSbar scheme.
Renormalization is treated non-perturbatively, and the continuum limit has been
taken, so that the only uncontrolled systematic error consists in the use of
the quenched approximation.Comment: Lattice2001(spectrum), 3 page
Renormalization group invariant average momentum of non-singlet parton densities
We compute, within the Schr\"odinger functional scheme, a renormalization
group invariant renormalization constant for the first moment of the
non-singlet parton distribution function. The matching of the results of our
non-perturbative calculation with the ones from hadronic matrix elements allows
us to obtain eventually a renormalization group invariant average momentum of
non-singlet parton densities, which can be translated into a preferred scheme
at a specific scale.Comment: Latex2e file, 4 figures, 12 page
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