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