1,195 research outputs found
Lattice calculation of the strangeness and electromagnetic nucleon form factors
We report on recent lattice QCD calculations of the strangeness magnetic
moment of the nucleon and the nucleon electromagnetic form factors, when we
allow the electromagnetic current to connect to quark loops as well as to the
valence quarks. Our result for the strangeness magnetic moment is
G_M^s(0)=-0.36+/-0.20. The sea contributions from the u and d quarks are about
80% larger. However, they cancel to a large extent due to their electric
charges, resulting in a smaller net sea contribution of -0.097+/-0.037 mu_N to
the nucleon magnetic moment. As far as the neutron to proton magnetic moment
ratio is concerned, this sea contribution tends to cancel out the cloud-quark
effect from the Z-graphs and results in a ratio of -0.68+/-0.04 which is close
to the SU(6) relation and the experiment. The strangeness Sachs electric
mean-square radius _E is found to be small and negative and the total
sea contributes substantially to the neutron electric form factor.Comment: LATTICE98(matrixelement); 3 pages, no figures, to appear in Lattice
'98 proceeding
Lattice Calculation of the Strangeness Magnetic Moment of the Nucleon
We report on a lattice QCD calculation of the strangeness magnetic moment of
the nucleon. Our result is . The sea contributions
from the u and d quarks are about 80% larger. However, they cancel to a large
extent due to their electric charges, resulting in a smaller net sea
contribution of to the nucleon magnetic moment. As
far as the neutron to proton magnetic moment ratio is concerned, this sea
contribution tends to cancel out the cloud-quark effect from the Z-graphs and
result in a ratio of which is close to the SU(6) relation and
the experiment. The strangeness Sachs electric mean-square radius
is found to be small and negative and the total sea contributes substantially
to the neutron electric form factor.Comment: 10 pages, 5 figures, LaTex, UK/97-23, ADP-97-55/T28
Age Estimations of M31 Globular Clusters from Their Spectral Energy Distributions
This paper presents accurate spectral energy distributions (SEDs) of 16 M31
globular clusters (GCs) confirmed by spectroscopy and/or high
spatial-resolution imaging, as well as 30 M31 globular cluster candidates
detected by Mochejska et al. Most of these candidates have m_V > 18, deeper
than previous searches, and these candidates have not yet been confirmed to be
globular clusters. The SEDs of these clusters and candidates are obtained as
part of the BATC Multicolor Survey of the Sky, in which the
spectrophotometrically-calibrated CCD images of M31 in 13 intermediate-band
filters from 4000 to 10000 A were observed. These filters are specifically
designed to exclude most of the bright and variable night-sky emission lines
including the OH forest. In comparison to the SEDs of true GCs, we find that
some of the candidate objects are not GCs in M31. SED fits show that
theoretical simple stellar population (SSP) models can fit the true GCs very
well. We estimate the ages of these GCs by comparing with SSP models. We find
that, the M31 clusters range in age from a few ten Myr to a few Gyr old, as
well as old GCs, confirming the conclusion that has been found by Barmby et a,
Williams & Hodge, Beasley et al., Burstein et al. and Puzia et al. in their
investigations of the SEDs of M31 globular clusters.Comment: Accepted for Publication in A&Ap, 13 pages, 6 figure
A Lattice Study of the Magnetic Moment and the Spin Structure of the Nucleon
Using an approach free from momentum extrapolation, we calculate the nucleon
magnetic moment and the fraction of the nucleon spin carried by the quark
angular momentum in the quenched lattice QCD approximation. Quarks with three
values of lattice masses, 210, 124 and 80 MeV, are formulated on the lattice
using the standard Wilson approach. At every mass, 100 gluon configurations on
16^3 x 32 lattice with \beta=6.0 are used for statistical averaging. The
results are compared with the previous calculations with momentum
extrapolation. The contribution of the disconnected diagrams is studied at the
largest quark mass using noise theory technique.Comment: 14 pages, 3 figures, Talk given at Lattice2001, Berlin, German
Inelastic production in polarized photon-hadron collisions
Presented here is a calculation of inelastic production in polarized
photon-hadron collisions under the framework of NRQCD factorization formalism.
We consider the photoproduction of \jpsi in the energy range relevant to
HERA. The Weizs\"acker-Williams approximation is adopted in the evaluation of
the cross sections for collisions. We found that this process can give
another independent test for the color-octet mechanism, and the different
features for the two color-octet processes may provide further informations on
the mechanism for inelastic \jpsi photoproduction. And the discrepancy on the
production asymmetry between various sets of polarized gluon distribution
functions is also found to be distinctive.Comment: 14pages, 6 PS figure
ACS imaging of star clusters in M51. I. Identification and radius distribution
We use HST/ACS observations of the spiral galaxy M51 in F435W, F555W and
F814W to select a large sample of star clusters with accurate effective radius
measurements in an area covering the complete disc of M51. We present the
dataset and study the radius distribution and relations between radius, colour,
arm/interarm region, galactocentric distance, mass and age. We select a sample
of 7698 (F435W), 6846 (F555W) and 5024 (F814W) slightly resolved clusters and
derive their effective radii by fitting the spatial profiles with analytical
models convolved with the point spread function. The radii of 1284 clusters are
studied in detail. We find cluster radii between 0.5 and ~10 pc, and one
exceptionally large cluster candidate with a radius of 21.6 pc. The median
radius is 2.1 pc. We find 70 clusters in our sample which have colours
consistent with being old GC candidates and we find 6 new "faint fuzzy"
clusters in, or projected onto, the disc of M51. The radius distribution can
not be fitted with a power law, but a log-normal distribution provides a
reasonable fit to the data. This indicates that shortly after the formation of
the clusters from a fractal gas, their radii have changed in a non-uniform way.
We find an increase in radius with colour as well as a higher fraction of
redder clusters in the interarm regions, suggesting that clusters in spiral
arms are more compact. We find a correlation between radius and galactocentric
distance which is considerably weaker than the observed correlation for old
Milky Way GCs. We find weak relations between cluster luminosity and radius,
but we do not observe a correlation between cluster mass and radius.Comment: 17 pages, 23 figures. Accepted for publication in A&
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