Speaker
Description
At the fundamental level, the annihilation of a positron with an electron is described by QED. As such, it allows one to tests QED by comparing high-precision calculations with experiment, e.g., for the lifetime of positronium (Ps), the simplest matter-antimatter "atom". Positron annihilation and the associated characteristic gamma-ray signal is also a unique probe which provides information on antimatter in the Universe and in our Galaxy. It also lies at the heart of diagnostics, such as PALS (positron annihilation lifetime spectroscopy), used to probe condensed matter systems, and PET (positron emission tomography), a functional medical imagining technique. Hence, in most circumstances positron annihilation takes place in complex atomic or molecular environments, making positron interaction with atoms and molecules an important problem. As it turns out, this interaction is characterised by very strong correlation effects, so it is a challenging theoretical problem. In addition to the usual target polarisation, low-energy positrons also experience attraction due to virtual Ps formation. The combined attraction enables positrons to form bound states with many atoms, a prediction that has not yet been verified in experiment. (Experiments with positrons are also very challenging, though much progress has been made since the creation of positron traps and trap-based positron beams by the group of the late Professor Cliff Surko at the University of San Diego.) For molecules, the existence of bound states leads to a spectacular phenomenon of resonant annihilation. This process involves positron capture accompanied by excitation of molecular vibrations, leading to orders-of-magnitude enhancement of positron annihilation rates in molecular gases. When studied with a high-resolution positron beam, experiment reveals a complex spectrum of vibrational Feshbach resonances (VFR) characteristic of the molecular species. In addition to simple single-quantum vibrations, these spectra contain signatures of multimode vibrations, and the observed level of enhanced annihilation points to the importance of intramolecular vibrational energy redistribution. When interacting with matter, positrons can ionise it by forming Ps. This gives rise to a range of questions concerning Ps scattering from atoms and molecules, and the accompanying process of pick-off annihilaton. Theoretical progress in this area is impeded by both the projectile and target being composite particles, so much of the experimental data remains poorly understood. In my talk I will review the current state of understanding of these phenomena, including recent advances in the application of many-body-theory methods to positron-molecule interactions, calculations of positron binding and VFR spectra, and collisions involving positronium.
| Theoretical or experimental | Theoretical |
|---|