Mid Term Review

Positron bound states and scattering resonances

Zoe Cheong
Laser Physics Centre, ANU

Positrons are the anti-matter counterpart to electrons, with the primary difference between the two particles being that positrons have a positive charge. Positrons can bind to matter to form various exotic atoms and molecules, such as in positronium (Ps), where a positron forms a bound state with an electron. Other examples of this phenomenon include positronium anions (Ps-), positronium molecules (Ps2) and positronium hydride (PsH). These bound state systems are of interest to study as they are a novel testing ground for fundamental physics calculations. It has also been predicted that certain atoms are able to bind positrons to form positron-atomic bound states, such as Cd or a pseudo-bound state such as the prediction that Mg can support a shape resonance with a positron. However, neither e+-Cd bound state nor positronic shape resonances of Mg have been observed experimentally.

Notably, the Surko group in UC San Diego have routinely measured resonance structures in the positron annihilation cross section for various hydrocarbons. These annihilation cross sections are much larger than expected at incident energies close to vibrational state energy thresholds of the target molecules. These resonances have been identified as Feshbach resonances and are analogous to negative ion resonances that are routinely measured in electron scattering experiments with most target molecules. Due to their association with vibrational states of molecules in positron scattering, they are referred to as vibrational Feshbach resonances (VFR). Positron VFRs are another type of pseudo-bound state of positrons with target molecules and as such are closely associated with positron-molecule binding. Using the annihilation cross section and the information inferred from them, theoretical predictions have been made on the formation and the decay of the positron VFRs. Notably, positron VFRs can decay in multiple ways, and another such method is where the positron escapes the system and leaves the target molecule in an exited state. By investigating its decay through vibrational excitation channels, insight into the mechanics of their formation can be gained. However, the decay of positron VFRs through vibrational excitation channels have not been experimentally measured.

My PhD project explores positron resonances and binding through experimental measurements of low energy positron scattering. In this talk, I will discuss the theoretical background and the experimental techniques we plan to apply to perform these measurements. I will also present the first measurements of the vibrational excitation cross section of positron scattering from ethane and talk about the future plans for the project, such as the target atoms and molecules of interest.


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