Positrons and antiatoms

By Thomas Farrell on February 26, 2016

The existence of antimatter is pretty well known. Whether it’s from an episode of Star Trek, the plot of a Dan Brown novel or learning about nuclear decay at school, most people have at least heard of it. Fewer people may know what a widely studied and utilised area of physics antimatter makes up, and even fewer that there are all kinds of exotic antimatter-containing substances in existence and under study today.

First of all, thanks to Physics World and Dr Helen Heath for this 100 second(!) rundown of what antimatter actually is:

So the positron, mentioned in the video, is an antielectron. When the two meet in space they annihilate, converting their mass to energy. However, before this happens, they can form a bound state called positronium. Positronium is an exotic atom-like state consisting of an electron and a positron – to picture it classicly, imagine they orbit each other around a common point. It even has its own chemical symbol, Ps.

Positronium can form in a number of ways – for example, through electron capture in collisions between atoms and positrons, or at the surfaces of metals. One area of interest in positronium studies is that of positronium’s behaviour under gravity and how various challenges such as positronium’s lifetime and its production in an ultracold environment can be overcome to perform free-fall experiments. It is also useful in spectroscopy – when positronium annihilates it produces gamma rays, which can be detected from, say, stellar gas.

You can also find positrons in antihydrogen – the antimatter mirror of hydrogen. The most basic form of hydrogen consists of a proton and an electron; antihydrogen is made up of an antiproton and a positron. Similarly important to positronium, there are intense research efforts currently underway to produce, trap and experiment on antihydrogen such as the ALPHA collaboration at CERN. Looking at the way antihydrogen behaves compared to its matter cousin promises insight into the behaviour of various forces, and is relevant to discovering why there is a distinct lack of antimatter in the universe. In a recent review (which gives an excellent overview of the state of antihydrogen physics), W A Bertsche et al explain:

Performing measurements of the properties of antihydrogen, the bound state of an antiproton and a positron, and comparing the results with those for ordinary hydrogen, has long been seen as a route to test some of the fundamental principles of physics. There has been much experimental progress in this direction in recent yearsand antihydrogen is now routinely created and trapped and a range of exciting measurements probing the foundations of modern physics are planned or underway

Sketch of experimental device for shaping velocity distribution of antiatoms. Antiatom trap is shown as a red circle in the center of the shaping disks. The bottom disk is a mirror and, upper disk is an absorber. The disk radius is R, the trap radius , the size of the slit between disks is h, and the height of a bottom mirror above detector plate is H. Classical trajectories of antiatoms are shown by the curved lines with arrows. From A Yu Voronin et al 2016 J. Phys. B: At. Mol. Opt. Phys. 49 054001

At JPhysB, we are currently publishing a special issue on Antihydrgoen and Positronium, which is looking to be a fascinating collection of papers and reviews covering a range of topics including antihydrogen-hydrogen interaction, positronium forming in plasmas and experiments studying the effects of gravity on antihydrogen. What we’ve published so far is available here, and there is plenty more to come soon. Our Guest Editors, Mike Charlton, Allen P Mills and Yasunori Yamazaki, summarise:

For over forty years it has been possible to investigate the interactions of low energy positrons using well-defined beams colliding with atoms, molecules and surfaces. The ability to control positrons in collision has led directly to recent advances in, for example, the formation of dense clouds of positronium atoms, the observation of the positronium molecule and the development of novel sources of low energy positronium in vacuum. This special atom continues to be of fundamental interest as the exemplar QED-only bound state, and its properties, including gravitational, are the subject of continued and vigorous study. Furthermore, its interactions with atoms, molecules and within condensed media, and its production and annihilation in astrophysical and terrestrial milieux, are topics of substantial current interest.

Slow antiproton physics began around thirty years ago and involved studies of their interaction with atoms and molecules, and the first trapping experiments. After about ten years, the Antiproton Decelerator, a machine devoted purely to atomic physics investigations, was constructed at CERN, thereby opening up a host of new possibilities with ultra-low energy antiprotons. This facilitated experimentation with antihydrogen, which has recently included its capture in magnetic minimum neutral atom traps and the first explorations of its properties.

Many crucial advances have been made in the last decade or so, and we are on the cusp of a new era in antimatter physics involving its study, control, interrogation and application.

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In case you missed it, you can catch up on more about positronium in our interview with Professor David B. Cassidy of University College London.

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