Two-photon annihilation into pion pairs

We discuss pion pair production in two-photon collisions in two different kinematical regimes. When both photons are real and at moderately large center-of-mass energy s we elaborate on partonic transverse momentum and Sudakov corrections within the hard scattering approach. We also point out the difference between our approach and that of other authors. When one of the photons is highly virtual the produced pion pair can be described in terms of a two-pion distribution amplitude, for which we derive the perturbative limit at large s.Comment: Latex, 7 pages, 6 Postscript figures, Talk given at "PHOTON2000", Ambleside, UK, August 2000; to appear in the proceedings. Eq. (2) and typo correcte

The Handbag Contribution to Two-Photon Annihilation Into Meson Pairs

We report on the handbag contribution to two-photon annihilation into pion and kaon pairs at large energy and momentum transfer. The underlying physics of the mechanism is outlined and characteristic features and predictions are presented.Comment: 6 pages, 4 figures, uses ws-procs9x6.cls. Talk presented at workshops QCD-N'02, Ferrara, Italy, April 3-6, and Exclusive Processes at High Momentum Transfer, Jefferson Lab, Newport News, VA, USA, May 15-18, 200

Graptemys pseudogeographica

Number of Pages: 6Integrative BiologyGeological Science

Graptemys oculifera

Number of Pages: 2Integrative BiologyGeological Science

Graptemys

Number of Pages: 3Integrative BiologyGeological Science

Pseudemys alabamensis

Number of Pages: 2Integrative BiologyGeological Science

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

Magnetic Field Seeding by Galactic Winds

The origin of intergalactic magnetic fields is still a mystery and several scenarios have been proposed so far: among them, primordial phase transitions, structure formation shocks and galactic outflows. In this work we investigate how efficiently galactic winds can provide an intense and widespread "seed" magnetisation. This may be used to explain the magnetic fields observed today in clusters of galaxies and in the intergalactic medium (IGM). We use semi-analytic simulations of magnetised galactic winds coupled to high resolution N-body simulations of structure formation to estimate lower and upper limits for the fraction of the IGM which can be magnetised up to a specified level. We find that galactic winds are able to seed a substantial fraction of the cosmic volume with magnetic fields. Most regions affected by winds have magnetic fields in the range -12 < Log B < -8 G, while higher seed fields can be obtained only rarely and in close proximity to wind-blowing galaxies. These seed fields are sufficiently intense for a moderately efficient turbulent dynamo to amplify them to the observed values. The volume filling factor of the magnetised regions strongly depends on the efficiency of winds to load mass from the ambient medium. However, winds never completely fill the whole Universe and pristine gas can be found in cosmic voids and regions unaffected by feedback even at z=0. This means that, in principle, there might be the possibility to probe the existence of primordial magnetic fields in such regions.Comment: 14 pages, 5 figures. Accepted for publications by MNRAS. A high resolution version of the paper is available at http://astronomy.sussex.ac.uk/~sb207/Papers/bb.ps.g