Positron Interactions

When a positron scatters off an atom, A, the following processes (assuming A contains enough electrons) can take place:

e+ + A ->

e+ + A Elastic Scattering

e+ + A* Excitation

e+ + An+ + ne- Ionization

Ps(nlm) + A+ Positronium (Ps) Formation

Ps- + A++ Positronium Negative Ion Formation

Ps + A(n+1)+ + ne- Transfer Ionization

Ps- + A(n+2)+ + ne- Transfer Ionization with Ps- Formation

A+ + Gamma rays Annihilation

The first three reactions are possible using electrons as a projectile. The remaining reactions are unique to the positron. It is these latter reactions which distinguish the positron as a more subtle projectile than the electron. Positronium formation is the most obvious manifestation of the competition between the positron and the nucleus for electrons while the annihilation process gives "pin-point" information on correlation in the system in that it measures the probability that the positron coincides with an electron.

Considerable success has been achieved using coupled-(pseudo)state methods to treat positron scattering by "one-electron" and "two-electron" atoms, eg, [1-4]. Pseudostates are a very effective way of representing ionization channels. In a coupled-pseudostate approach we have a representation of all the main physical processes, ie, excitation of the atom, positronium formation, ionization, in effect a complete dynamical theory.

Many-body theory has also been applied to positron-atom scattering and has been found to be a very useful and insightful tool. Using many-body theory the role of virtual positronium formation in increasing positron-atom attraction has been identified [5]. This led to the prediction of bound states with neutral atoms [6] that, at the time, were considered to be non-existent. As a result great interest in the problem of positron binding to atoms and molecules was stimulated. Many-body theory has also provided an explanation of the origins of enhanced annihilation rates observed for heavier atoms [7] and has been used for the calculation of the spectra of annihilation gamma quanta [8].

A long-standing problem has been the greatly enhanced annihilation rates in polyatomic molecules. Experiments had observed rates which were orders of magnitude (!) larger than estimates based upon the number of electrons in the molecule. This has now been explained [9] by the existence of positron-molecule bound states which give rise to a dense spectrum of positron vibrational Feshbach resonances. Using a zero-range potential model the main trends of positron binding to alkanes has been explained, including the emergence of a second bound state [10]. It has been found that for small-sized polyatomic molecules with infrared-active modes, such as methyl halides, a complete analytical theory can be formulated [11]. This theory contains one free parameter, namely the positron-molecule binding energy, and yields excellent agreement with experiment.

Positronium Interactions

The development of mono-energetic positronium beams has led to growing interest in positronium-atom collisions. Positronium is the lightest neutral atomic projectile, being like a hydrogen atom but only 1/1000 th of its mass, positronium collisions are therefore of considerable fundamental interest. The fact that positronium has internal degrees of freedom as well as the atom considerably complicates the theoretical description of positronium scattering.

Using the coupled pseudostate formalism a sophisticated set of computer programs has been developed to study positronium-atom scattering. So far, work has been done on positronium scattering by atomic hydrogen, helium, neon, argon, krypton, xenon and lithium [12-16]. This work has shown the importance of virtual excitation of the atom at low energies [12,17] and the important role of resonances associated with negative ions of the atom [15,17]. It has also uncovered problems with measurements of the momentum transfer cross at low energies [13,14,17] and given the first reliable results for H- production in Ps + H collisions [15] as well as for e+ + H- scattering [18]. Most recently, differential fragmentation of positronium in collision with inert gases has been studied for comparison with experimental measurements from University College London [19].

Antihydrogen

Antihydrogen is a bound state of an antiproton and a positron. As far as we understand at present it is exactly like a hydrogen atom but its antiparticle. The end of 2002 saw the announcement of the first production of cold (< 15K) antihydrogen at CERN by two experimental groups, ATHENA and ATRAP. A primary motivation for the production of antihydrogen is that it offers the opportunity of making very high precision tests of the Weak Equivalence Principle of General Relativity for antimatter and of the CPT invariance of relativistic quantum mechanics. To make these tests, the antihydrogen is required to be in a low lying quantum state, preferably the 1s ground state.

Relevant to these experiments are questions of collisional cooling and survivability of the antihydrogen. Collisional cooling takes place through elastic scattering with ordinary atoms, but any sort of scattering also involves the possibility of destruction either through a rearrangement collision or by direct annihilation of the antiproton or positron. Work is in progress to adapt the powerful coupled-pseudostate technique, which has been successfully used for positron and positronium scattering, to give definitive quantitative information on these processes.

Antiprotons

In parallel with the antihydrogen work, it is informative to look at what should be the simpler problem of antiproton collisions. So far our studies of antiproton interactions have been at somewhat higher energies [20,21], but even here serious discrepancies between theory and experiment are found. Work to resolve these problems is in progress as well as extensions to much lower energies.