119 research outputs found
One-loop matching coefficients for improved staggered bilinears
We calculate one-loop matching factors for bilinear operators composed of
improved staggered fermions. We compare the results for different improvement
schemes used in the recent literature, all of which involve the use of smeared
links. These schemes aim to reduce, though not completely eliminate, O(a^2)
discretization errors. We find that all these improvement schemes substantially
reduce the size of matching factors compared to unimproved staggered fermions.
The resulting corrections are comparable to, or smaller than, those found with
Wilson and domain-wall fermions. In the best case (``Fat-7'' and mean-field
improved HYP links) the corrections are 10 % or smaller at 1/a = 2 GeV.Comment: 13 pages, 1 figure (misleading sentence in sec. II removed; version
to appear in Physical Review D
Performance of the Electromagnetic Calorimeter of the HERMES Experiment
The performance of the electromagnetic calorimeter of the HERMES experiment
is described. The calorimeter consists of 840 radiation resistant F101
lead-glass counters. The response to positrons up to 27.5 GeV, the comparison
between the measured energy and the momentum reconstructed from tracking,
long-term stability, hadron rejection and neutral meson invariant mass
reconstruction are shown.Comment: 22 pages, 13 figures, LaTeX, accepted by NI
Renormalisation of Composite Operators in Lattice Perturbation Theory with Clover Fermions: Non-forward Matrix Elements
We consider the renormalisation of lattice QCD operators with one and two
covariant derivatives related to the first and second moments of generalised
parton distributions and meson distribution amplitudes. Employing the clover
fermion action we calculate their non-forward quark matrix elements in one-loop
lattice perturbation theory. For some representations of the hypercubic group
commonly used in simulations we determine the sets of all possible mixing
operators and compute the matrices of renormalisation factors in one-loop
approximation. We describe how tadpole improvement is applied to the results.Comment: 16 pages; v2: eqs.(27), (28), (74)-(79) corrected, several signs
changed in section III.
Kaon B Parameter in Quenched QCD
I calculate the kaon B-parameter with a lattice simulation in quenched
approximation. The lattice simulation uses an action possessing exact lattice
chiral symmetry, an overlap action. Computations are performed at two lattice
spacings, about 0.13 and 0.09 fm (parameterized by Wilson gauge action
couplings beta=5.9 and 6.1) with nearly the same physical volumes and quark
masses. I describe particular potential difficulties which arise due to the use
of such a lattice action in finite volume. My results are consistent with other
recent lattice determinations using domain-wall fermions.Comment: 23 pages, Revtex, 16 postscript figure
A lattice calculation of vector meson couplings to the vector and tensor currents using chirally improved fermions
We present a quenched lattice calculation of , the coupling of
vector mesons to the tensor current normalized by the vector meson decay
constant. The chirally improved lattice Dirac operator, which allows us to
reach small quark masses, is used. We put emphasis on analyzing the quark mass
dependence of and find only a rather weak dependence. Our
results at the and masses agree well with QCD sum rule
calculations and those from previous lattice studies.Comment: 6 pages, 3 figures, one sentence remove
Application of heavy-quark effective theory to lattice QCD: II. Radiative corrections to heavy-light currents
We apply heavy-quark effective theory to separate long- and short-distance
effects of heavy quarks in lattice gauge theory. In this approach, the inverse
heavy-quark mass and the lattice spacing are treated as short distances, and
their effects are lumped into short-distance coefficients. We show how to use
this formalism to match lattice gauge theory to continuum QCD, order by order
in the heavy-quark expansion. In this paper, we focus on heavy-light currents.
In particular, we obtain one-loop results for the matching factors of lattice
currents, needed for heavy-quark phenomenology, such as the calculation of
heavy-light decay constants, and heavy-to-light transition form factors.
Results for the Brodsky-Lepage-Mackenzie scale are also given.Comment: 32 pages, 8 figures. v2 corrects Eqs. (4.9) and (4.10) and adds a
reference. Program LatHQ2QCD to compute matching one-loop coefficients
available at http://theory.fnal.gov/people/kronfeld/LatHQ2QCD
Transverse lattice calculation of the pion light-cone wavefunctions
We calculate the light-cone wavefunctions of the pion by solving the meson
boundstate problem in a coarse transverse lattice gauge theory using DLCQ. A
large-N_c approximation is made and the light-cone Hamiltonian expanded in
massive dynamical fields at fixed lattice spacing. In contrast to earlier
calculations, we include contributions from states containing many gluonic
link-fields between the quarks.The Hamiltonian is renormalised by a combination
of covariance conditions on boundstates and fitting the physical masses M_rho
and M_pi, decay constant f_pi, and the string tension sigma. Good covariance is
obtained for the lightest 0^{-+} state, which we identify with the pion. Many
observables can be deduced from its light-cone wavefunctions.After perturbative
evolution,the quark valence structure function is found to be consistent with
the experimental structure function deduced from Drell-Yan pi-nucleon data in
the valence region x > 0.5. In addition, the pion distribution amplitude is
consistent with the experimental distribution deduced from the pi gamma^* gamma
transition form factor and diffractive dissociation. A new observable we
calculate is the probability for quark helicity correlation. We find a 45%
probability that the valence-quark helicities are aligned in the pion.Comment: 27 pages, 9 figure
Flavor Singlet Axial Vector Coupling of the Proton with Dynamical Wilson Fermions
We present the results of a full QCD lattice calculation of the flavor
singlet axial vector coupling of the proton. The simulation has been
carried out on a lattice at with dynamical
Wilson fermions. It turns out that the statistical quality of the connected
contribution to is excellent, whereas the disconnected part is
accessible but suffers from large statistical fluctuations. Using a 1st order
tadpole improved renormalization constant , we estimate .Comment: 13 pages, 5 eps figures, minor changes to text and citation
The HERMES Dual-Radiator Ring Imaging Cerenkov Detector
The construction and use of a dual radiator Ring Imaging Cerenkov(RICH)
detector is described. This instrument was developed for the HERMES experiment
at DESY which emphasizes measurements of semi-inclusive deep-inelastic
scattering. It provides particle identification for pions, kaons, and protons
in the momentum range from 2 to 15 GeV, which is essential to these studies.
The instrument uses two radiators, C4F10, a heavy fluorocarbon gas, and a wall
of silica aerogel tiles. The use of aerogel in a RICH detector has only
recently become possible with the development of clear, large homogeneous and
hydrophobic aerogel. A lightweight mirror was constructed using a newly
perfected technique to make resin-coated carbon-fiber surfaces of optical
quality. The photon detector consists of 1934 photomultiplier tubes for each
detector half, held in a soft steel matrix to provide shielding against the
residual field of the main spectrometer magnet.Comment: 25 pages, 23 figure
Standard Model Matrix Elements for Neutral B-Meson Mixing and Associated Decay Constants
We present results of quenched lattice calculations of the matrix elements
relevant for B_d-\bar B_d and B_s-\bar B_s mixing in the Standard Model.
Results for the corresponding SU(3)-breaking ratios, which can be used to
constrain or determine |V_{td}|, are also given. The calculations are performed
at two values of the lattice spacing, corresponding to \beta = 6.0 and \beta =
6.2, with quarks described by a mean-field-improved Sheikholeslami-Wohlert
action. As a by-product, we obtain the leptonic decay constants of B and D
mesons. We also present matrix elements relevant for D^0-\bar D^0 mixing. Our
results are summarized in the Introduction.Comment: 27 pages (RevTeX), 26 figures, version published in Phys. Rev. D:
improved estimate of the systematic error associated with the uncertainty on
the strange quark mass and other small improvements to analysis (results
change only slightly); correction of typos and minor changes to text; RevTeX
formattin
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