Gamma-ray burst sources with a high luminosity can produce electron-positron pair cascades in their environment as a result of back-scattering of a seed fraction of their original hard spectrum. The resulting spectral modifications offer the possibility of diagnosing not only the compactness parameter of the γ-ray emitting region but also the baryonic density of the environment external to the ...

We explicitly compute the Compton amplitude for the scattering of a photon and a (massless) “electron/positron” at tree level and one loop, in a four-dimensional fermionic heterotic string model. We comment on the relationship between the amplitudes we compute in string theory and the corresponding ones in field theory. CERN-TH/97-212 August, 1997

This is an introduction to the use of QCD perturbation theory, emphasizing generic features of the theory that enable one to separate short-time and long-time e ects. I also cover some important classes of applications: electron-positron annihilation to hadrons, deeply inelastic scattering, and hard processes in hadron-hadron collisions. Supported by DOE Contract DE-FG03-96ER40969.

This is an introduction to the use of QCD perturbation theory, emphasizing generic features of the theory that enable one to separate short-time and long-time effects. I also cover some important classes of applications: electron-positron annihilation to hadrons, deeply inelastic scattering, and hard processes in hadron-hadron collisions.

The effects of multi-photon-exchange and other higher-order QED corrections on elastic electron-proton scattering have been a subject high experimental theoretical interest since the polarization transfer measurements proton electromagnetic form factor ratio $$G_E^p/G_M^p$$ at large momentum $$Q^2$$ conclusively established strong decrease this with for $$Q^2 \gtrsim 1$$ $$\hbox {GeV}^2$$ . Thi...

Photon–photon scattering in vacuum due to the interaction with virtual electron-positron pairs is a consequence of quantum electrodynamics. A way for detecting this phenomenon has been devised based on interacting modes generated in microwave waveguides or cavities [G. Brodin, M. Marklund and L. Stenflo, Phys. Rev. Lett. 87 171801 (2001)]. Here we materialize these ideas, suggest a concrete cav...

In order to increase the luminosity of electron-positron colliders it is desirable to find a means to reduce the phase-space of the beams. The transverse cooling of positrons imposed by the quantum excitation-free radiation emission in a single crystal is considered as a potential route to achieving ultra-cold beams. An analysis of the problem is presented, including an evaluation of the contri...

We present the positronium formation cross sections for a positron colliding with lithium and sodium for the collision energies from 0.01 eV up to 20 eV by the hyperspherical close-coupling method. For Li, our results agree with the experimental data and with other calculations. Our results for Na remain in agreement with previous close-coupling calculations, but do not support the latest exper...

A small animal positron emission tomography (PET) instrument using a high-resolution solid-state detector insert in a conventional PET system was investigated for its potential to achieve sub-millimeter spatial resolution for mouse imaging. Monte Carlo simulations were used to estimate the effect of detector configurations (thickness, length and radius) on sensitivity. From this initial study, ...

We show that the timing properties of a pulsed low-energy positron lifetime beam can be conveniently tested by an electron beam. We apply this method to study time resolution of the beam and electron scattering in a flat and a ’sawtooth’ shaped choppers. The results show that (i) time resolution of 160 ps is obtained, (ii) the scattering of the electrons and the secondary electron yield of the ...