English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

... Fig. 1: The variation of the BEC transition temperature as a I function of cavity sizes and for different, possible Ps densities in J the cavity (adapted from ref. [14]). could therefore be able to tell us what happens in that region of interaction where scattering states become close to the weakly bound states of the interacting species, if they exist at all. Both the annihilation process and the Ps formation cross section are expected, from general theoretical estimates, to increase proportionally to 1/v, v being the relative velocity of the partners, as the energy tends to zero. One of the puzzling questions pertaining to such very low-energy scenario is provided by the possibility to find experimental evidence and theoretical confirmation on the existence ofbound states or of metastable resonant states associated to cold positrons and Ps that are made to interact with fairly cold atomic and molecular gases. This is an aspect of the physics of antiparticles that has made very impressive progress, in the last 4-5 years, on the theoretical and computational predictions of such states while still rather absent at the experimental level. The definition of a stable antimatter compound could be had, in its simplest form, by changing a bound proton of a known stable compound with a positron. Thus PsH (positronium hydride) and PS2 become real chemical species that are related to the stable molecule of H2• Naturally, all such systems invariably annihilate and their lifetimes are of the order of 100 picoseconds. Hence, an operational definition of a stable antimatter species could be had whenever the latter has at least one bound state that does not dissociate or autoionize before the inevitable annihilation occurs. Several of the properties of such apparently strange compounds are of direct interest at the fundamental level, besides being useful in many areas of applied, engineering studies [4]. Their binding energies, their annihilation rates and the momentum distributions of the emitted annihilation photons are amenable to calculations which have made tremendous progress in the last few years, thereby hopefull spurring future experimental attempts at producing comparable measurements: it may not The subject of positron and positronium (Ps) physics with atoms and molecules is indeed one with a long history, but also a subject where a surprisingly large number of fundamental questions raised by a variety ofrecent observations still remain to be answered. Many of the problems in this area come up when discussing low-energy interaction of positrons and Ps with atomic and molecular aggregates and therefore provide the fundamental understanding for setting up a more quantitative approach to the chemistry of matter-antimatter processes. To illustrate the status of modern research in positron physics and in the chemistry of positron/Ps processes, I have chosen to discuss only a few of the current issues which, hopefully,will convey to the reader the excitement coming from the resurgence of interest in this field and the current wave of experiments with their new findings. For a more extensive discussion that endeavours to better document the details ofthe topics which I shall only lightly touch on in this overview, I refer the interested reader to a recently publishedvolume containing a broad range ofarticles on antimatter processes [1]. The increase in the interest of the scientific community on positrons as a research tool follows the development oflow-energy positron beams, particularly over the last 15 years or so. It was realised early on, in fact, that to have such sources available might offer additional tests on the polarisation forces and scattering states probed by the counterpart studies with low energy electron beams of atomic and molecular gases. Furthermore, such studies indicated that correlation forces between the impinging positron and the bound electrons, either atomic or molecular, are much more important than the same effects in electron collisions, a consideration with profound implications in the sense that the virtual positronium (Ps) formation channels (that can be viewed as transient bound state complexes moving in the field of the atomic or the molecular ion) now make the treatment of positron-atom/molecule collision events a much more complicated situation to handle with theoretical modellings. Thus, to have removed the non-locality of the exchange interaction that affects electron scattering processes turned out to be a simplification outweighted by the more complicated features of the electron-positron correlation effects which play a more explicit role, together with the one coming from the Ps formation channel. The latter is certainly one of the most fundamental examples of a charge exchange reaction that definitely requires for its description a quantum mechanical formulation of target-projectile interactions and dynamics. To this end, the new experimental techniques which exploit positron accumulators [2,3] have markedly increased our capabilities for investigating positron and Ps interactions with matter at very low energies: they work already in the meV range and with possible developments which could bring us down to the fleV region. The elastic channels and the annihilation channels are always open as the kinetic energy of the probe approaches zero. Ps formation could also be energetically allowed whenever the target ionization energy is below 6.8 eV, the binding energy of the ground state Ps. Experiments

Positron and positronium physics in atomic and molecular gases: Challenges for the 21stcentury

Positron and positronium physics in atomic and molecular gases: Challenges for the 21^stcentury