146 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

### 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

### 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.

### 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

### 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 $O_{VA}=(\bar s \gamma_\mu
d)(\bar s \gamma_\mu \gamma_5 d)$.Comment: Lattice2002(improve

### Non-perturbative running of the average momentum of non-singlet parton densities

We determine non-perturbatively the anomalous dimensions of the second moment
of non-singlet parton densities from a continuum extrapolation of results
computed in quenched lattice simulations at different lattice spacings. We use
a Schr\"odinger functional scheme for the definition of the renormalization
constant of the relevant twist-2 operator. In the region of renormalized
couplings explored, we obtain a good description of our data in terms of a
three-loop expression for the anomalous dimensions. The calculation can be used
for exploring values of the coupling where a perturbative expansion of the
anomalous dimensions is not valid a priori. Moreover, our results provide the
non-perturbative renormalization constant that connects hadron matrix elements
on the lattice, renormalized at a low scale, with the experimental results,
renormalized at much higher energy scales.Comment: Latex2e file, 6 figures, 25 pages, Corrected errors on linear fit in
table 2 and discussion on anomalous dimension of f_

### Non-perturbative renormalization of moments of parton distribution functions

We compute non-perturbatively the evolution of the twist-2 operators
corresponding to the average momentum of non-singlet quark densities. The
calculation is based on a finite-size technique, using the Schr\"odinger
Functional, in quenched QCD. We find that a careful choice of the boundary
conditions, is essential, for such operators, to render possible the
computation. As a by-product we apply the non-perturbatively computed
renormalization constants to available data of bare matrix elements between
nucleon states.Comment: Lattice2003(Matrix); 3 pages, 3 figures. Talk by A.

### Exact Nonperturbative Renormalization

We propose an exact renormalization group equation for Lattice Gauge
Theories, that has no dependence on the lattice spacing. We instead relate the
lattice spacing properties directly to the continuum convergence of the support
of each local plaquette. Equivalently, this is formulated as a convergence
prescription for a characteristic polynomial in the gauge coupling that allows
the exact meromorphic continuation of a nonperturbative system arbitrarily
close to the continuum limit.Comment: 12 page

### Quenched Spectroscopy for the N=1 Super-Yang-Mills Theory

We present results for the Quenched SU(2) N=1 Super-Yang-Mills spectrum at
$\beta=2.6$, on a $V=16^3 \times 32$ lattice, in the OZI approximation. This is
a first step towards the understanding of the chiral limit of lattice N=1 SUSY.Comment: 3 pages, Latex, 2 ps figures, contribution to Lattice 97, Edinburgh
22-26 July 1997; to appear on Nucl. Phys. B. (Proc. Suppl.

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