396 research outputs found
A Search for Candidate Light Echoes: Photometry of Supernova Environments
Supernova (SN) light echoes could be a powerful tool for determining
distances to galaxies geometrically, Sparks 1994. In this paper we present CCD
photometry of the environments of 64 historical supernovae, the first results
of a program designed to search for light echoes from these SNe. We commonly
find patches of optical emission at, or close to, the sites of the supernovae.
The color distribution of these patches is broad, and generally consistent with
stellar population colors, possibly with some reddening. However there are in
addition patches with both unusually red and unusually blue colors. We expect
light echoes to be blue, and while none of the objects are quite as blue in V-R
as the known light echo of SN1991T, there are features that are unusually blue
and we identify these as candidate light echoes for follow-on observations.Comment: 13 pages, Latex, 5 Postscript Tables, 42 Postscript figures, accepted
for publication in the A&AS. Figures 1 through 36 are available at the web
address: http://www.stsci.edu/~boffi
Nucleon magnetic moments in an extended chiral constituent quark model
We present results for the nucleon magnetic moments in the context of an
extended chiral constituent quark model based on the mechanism of the Goldstone
boson exchange, as suggested by the spontaneous breaking of chiral symmetry in
QCD. The electromagnetic charge-current operator is consistently deduced from
the model Hamiltonian, which includes all force components for the
pseudoscalar, vector and scalar meson exchanges. Thus, the continuity equation
is satisfied for each piece of the interaction, avoiding the introduction of
any further parameter. A good agreement with experimental values is found. The
role of isoscalar two-body operators, not constrained by the continuity
equation, is also investigated.Comment: 7 pages, 1 table, submitted for publication to Phys. Lett. B,
elsart.sty/elsart12.sty include
Electroweak properties of baryons in a covariant chiral quark model
The proton and neutron electromagnetic form factors and the nucleon axial
form factor have been calculated in the Goldstone-boson exchange
constituent-quark model within the point-form approach to relativistic quantum
mechanics. The results, obtained without any adjustable parameter nor quark
form factors, are, due to the dramatic effects of the boost required by the
covariant treatment, in striking agreement with the data.Comment: Proceedings of the Conference N*2001, Mainz; 4 pages, 3 figures
included in eps format; World Scientific style file include
Nucleon electromagnetic and axial form factors in point-form relativistic quantum mechanics
Results for the proton and neutron electric and magnetic form factors as well
as the nucleon axial form factor are presented for constituent quark models,
based on either one-gluon-exchange and Goldstone-boson-exchange dynamics. The
calculations are performed in a covariant framework using the point-form
approach to relativistic quantum mechanics. The only input to the calculations
is the nucleon wave function of the corresponding constituent quark model. A
comparison is given to results of the instanton-induced constituent quark model
treated with the Bethe-Salpeter equation.Comment: 4 pages, 6 figures, contribution to XVIII European Conference on
Few-Body Problems in Physics, September 2002, Bled, Sloveni
Covariant axial form factor of the nucleon in a chiral constituent quark model
The axial form factor G_A of the nucleon is investigated for the
Goldstone-boson-exchange constituent quark model using the point-form approach
to relativistic quantum mechanics. The results, being covariant, show large
contributions from relativistic boost effects. The predictions are obtained
directly from the quark-model wave functions, without any further input such as
vertex or constituent-quark form factors, and fall remarkably close to the
available experimental data.Comment: 10 pages, 1 figure in .eps format, typeset with Elsevier elsart style
files included. Revised version with a newly added section about discussion
of results. To appear in Phys. Lett.
Covariant electroweak nucleon form factors in a chiral constituent quark model
Results for the proton and neutron electric and magnetic form factors as well
asthe nucleon axial and induced pseudoscalar form factors are presented for the
chiral constituent quark model based on Goldstone-boson-exchange dynamics. The
calculations are performed in a covariant framework using the point-form
approach to relativistic quantum mechanics. The direct predictions of the model
yield a remarkably consistent picture of the electroweak nucleon structure.Comment: 4 pages, 3 figures revte
Measuring longitudinal amplitudes for electroproduction of pseudoscalar mesons using recoil polarization in parallel kinematics
We propose a new method for measuring longitudinal amplitudes for
electroproduction of pseudoscalar mesons that exploits a symmetry relation for
polarization observables in parallel kinematics. This polarization technique
does not require variation of electron scattering kinematics and avoids the
major sources of systematic errors in Rosenbluth separation.Comment: intended for Phys. Rev. C as a Brief Repor
Electromagnetic nucleon form factors in instant and point form
We present a study of the electromagnetic structure of the nucleons with
constituent quark models in the framework of relativistic quantum mechanics. In
particular, we address the construction of spectator-model currents in the
instant and point forms. Corresponding results for the elastic nucleon
electromagnetic form factors as well as charge radii and magnetic moments are
presented. We also compare results obtained by different realistic nucleon wave
functions stemming from alternative constituent quark models. Finally, we
discuss the theoretical uncertainties that reside in the construction of
spectator-model transition operators.Comment: 13 pages, 9 figures, updated and extended version for publicatio
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