44 research outputs found
Insight into nucleon structure from generalized parton distributions
The lowest three moments of generalized parton distributions are calculated
in full QCD and provide new insight into the behavior of nucleon
electromagnetic form factors, the origin of the nucleon spin, and the
transverse structure of the nucleon.Comment: 3 pages, Lattice2003(Theoretical developments
Moments of nucleon spin-dependent generalized parton distributions
We present a lattice measurement of the first two moments of the
spin-dependent GPD H-tilde(x,xi,t). From these we obtain the axial coupling
constant and the second moment of the spin-dependent forward parton
distribution. The measurements are done in full QCD using Wilson fermions. In
addition, we also present results from a first exploratory study of full QCD
using Asqtad sea and domain-wall valence fermions.Comment: Lattice2003(Theory), 3 pages, 3 figures, to appear in the Proceedings
of Lattice 200
Moments of Nucleon Light Cone Quark Distributions Calculated in Full Lattice QCD
Moments of the quark density, helicity, and transversity distributions are
calculated in unquenched lattice QCD. Calculations of proton matrix elements of
operators corresponding to these moments through the operator product expansion
have been performed on lattices for Wilson fermions at using configurations from the SESAM collaboration and at
using configurations from SCRI. One-loop perturbative renormalization
corrections are included. At quark masses accessible in present calculations,
there is no statistically significant difference between quenched and full QCD
results, indicating that the contributions of quark-antiquark excitations from
the Dirac Sea are small. Close agreement between calculations with cooled
configurations containing essentially only instantons and the full gluon
configurations indicates that quark zero modes associated with instantons play
a dominant role. Naive linear extrapolation of the full QCD calculation to the
physical pion mass yields results inconsistent with experiment. Extrapolation
to the chiral limit including the physics of the pion cloud can resolve this
discrepancy and the requirements for a definitive chiral extrapolation are
described.Comment: 53 Pages Revtex, 26 Figures, 9 Tables. Added additional reference and
updated referenced data in Table I
Exploring morphological correlations among H2CO, 12CO, MSX and continuum mappings
There are relatively few H2CO mappings of large-area giant molecular cloud
(GMCs). H2CO absorption lines are good tracers for low-temperature molecular
clouds towards star formation regions. Thus, the aim of the study was to
identify H2CO distributions in ambient molecular clouds. We investigated
morphologic relations among 6-cm continuum brightness temperature (CBT) data
and H2CO (111-110; Nanshan 25-m radio telescope), 12CO (1--0; 1.2-m CfA
telescope) and midcourse space experiment (MSX) data, and considered the impact
of background components on foreground clouds. We report simultaneous 6-cm H2CO
absorption lines and H110\alpha radio recombination line observations and give
several large-area mappings at 4.8 GHz toward W49 (50'\times50'), W3
(70'\times90'), DR21/W75 (60'\times90') and NGC2024/NGC2023 (50'\times100')
GMCs. By superimposing H2CO and 12CO contours onto the MSX color map, we can
compare correlations. The resolution for H2CO, 12CO and MSX data was about 10',
8' and 18.3", respectively. Comparison of H2CO and 12CO contours, 8.28-\mu m
MSX colorscale and CBT data revealed great morphological correlation in the
large area, although there are some discrepancies between 12CO and H2CO peaks
in small areas. The NGC2024/NGC2023 GMC is a large area of HII regions with a
high CBT, but a H2CO cloud to the north is possible against the cosmic
microwave background. A statistical diagram shows that 85.21% of H2CO
absorption lines are distributed in the intensity range from -1.0 to 0 Jy and
the \Delta V range from 1.206 to 5 km/s.Comment: 18 pages, 22 figures, 5 tables. Accepted to be published in
Astrophysics and Space Scienc
Nonthermal phenomena in clusters of galaxies
Recent observations of high energy (> 20 keV) X-ray emission in a few
clusters of galaxies broaden our knowledge of physical phenomena in the
intracluster space. This emission is likely to be nonthermal, probably
resulting from Compton scattering of relativistic electrons by the cosmic
microwave background (CMB) radiation. Direct evidence for the presence of
relativistic electrons in some 50 clusters comes from measurements of extended
radio emission in their central regions. We briefly review the main results
from observations of extended regions of radio emission, and Faraday rotation
measurements of background and cluster radio sources. The main focus of the
review are searches for nonthermal X-ray emission conducted with past and
currently operating satellites, which yielded appreciable evidence for
nonthermal emission components in the spectra of a few clusters. This evidence
is clearly not unequivocal, due to substantial observational and systematic
uncertainties, in addition to virtually complete lack of spatial information.
If indeed the emission has its origin in Compton scattering of relativistic
electrons by the CMB, then the mean magnetic field strength and density of
relativistic electrons in the cluster can be directly determined. Knowledge of
these basic nonthermal quantities is valuable for the detailed description of
processes in intracluster gas and for the origin of magnetic fields.Comment: 23 pages, 7 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 5; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke