14,131 research outputs found
On Multipolar Analytical Potentials for Galaxies
We present analytical potential-density pairs in three dimensions for the
gravitational field of galaxies, obtained by thickening the multipolar
expansion up to the quadrupole term. These may be interpreted as
generalizations of the Miyamoto and Nagai potential-density pairs. With a
suitable restriction on the possible values of the multipole moments, the
density distributions are positive and monotone decreasing functions of the
radial and axial coordinates
A General Relativistic Model for the Gravitational Field of Active Galactic Nuclei Surrounded by a Disk
An exact but simple general relativistic model for the gravitational field of
active galactic nuclei is constructed, based on the superposition in Weyl
coordinates of a black hole, a Chazy-Curzon disk and two rods, which represent
matter jets. The influence of the rods on the matter properties of the disk and
on its stability is examined. We find that in general they contribute to
destabilize the disk. Also the oscillation frequencies for perturbed circular
geodesics on the disk are computed, and some geodesic orbits for the superposed
metric are numerically calculated.Comment: 24 pages, LATEX. Phys. Rev. D (in press
The annihilation of virtual photons into pseudoscalar mesons
We investigate the possibility to constrain the pion distribution amplitude
from the gamma* gamma* -> pi transition. For a surprisingly large range in the
two photon virtualities we find that the transition form factor is essentially
independent of the distribution amplitude. This in turn entails a
parameter-free prediction of QCD. The gamma* gamma* -> eta, eta' form factors
are also briefly discussed. We estimate that experimental studies might be
feasible at the existing e+ e- experiments BaBar, Belle, and CLEO.Comment: 22 pages latex, 9 figure
An Online Tutor for Astronomy: The GEAS Self-Review Library
We introduce an interactive online resource for use by students and college
instructors in introductory astronomy courses. The General Education Astronomy
Source (GEAS) online tutor guides students developing mastery of core
astronomical concepts and mathematical applications of general astronomy
material. It contains over 12,000 questions, with linked hints and solutions.
Students who master the material quickly can advance through the topics, while
under-prepared or hesitant students can focus on questions on a certain topic
for as long as needed, with minimal repetition. Students receive individual
accounts for study and course instructors are provided with overview tracking
information, by time and by topic, for entire cohorts of students. Diagnostic
tools support self-evaluation and close collaboration between instructor and
student, even for distance learners. An initial usage study shows clear trends
in performance which increase with study time, and indicates that distance
learners using these materials perform as well as or better than a comparison
cohort of on-campus astronomy students. We are actively seeking new
collaborators to use this resource in astronomy courses and other educational
venues.Comment: 15 pages, 9 figures; Vogt, N. P., and A. S. Muise. 2015. An online
tutor for general astronomy: The GEAS self-review library. Cogent Education,
2 (1
Two-Photon Annihilation into Baryon-Antibaryon Pairs
We study the handbag contribution to two-photon annihilation into
baryon-antibaryon pairs at large energy and momentum transfer. We derive
factorization of the process amplitude into a hard gamma gamma -> q qbar
subprocess and form factors describing the soft q qbar -> B Bbar transition,
assuming that the process is dominated by configurations where the (anti)quark
approximately carries the full momentum of the (anti)baryon. The form factors
represent moments of time-like generalized parton distributions, so-called B
Bbar distribution amplitudes. A characteristic feature of the handbag mechanism
is the absence of isospin-two components in the final state, which in
combination with flavor symmetry provides relations among the form factors for
the members of the lowest-lying baryon octet. Assuming dominance of the handbag
contribution, we can describe current experimental data with form factors of
plausible size, and predict the cross sections of presently unmeasured B Bbar
channels.Comment: 20 pages latex, 4 figures. v2: minor clarifications, references
update
- …