Cutting edge: Material opposites come in from cold

October 11, 2002

Michael Charlton describes the first controlled production of antimatter, a missing 'half' of the stuff of creation.

In 1931, motivated by little more than an extraordinary faith in the mathematical beauty of an equation, the British physicist Paul Dirac stunned the world by predicting the existence of antiparticles and antimatter.

These entities, Dirac foretold, would have some properties that are opposite to those possessed by their ordinary matter counterparts (for example, charge) and some the same (for example, mass). Thus, there should be positively charged electrons and negatively charged protons - positrons and antiprotons respectively.

Antimatter is a missing "half" of the stuff of creation and by gaining a better understanding of its properties, physicists hope to unlock more of the secrets of the birth of the universe and its fundamental properties.

Positrons and antiprotons - which, when they encounter everyday matter, cause a blast of energy that results in mutual self-destruction - were discovered after Dirac's prediction. But the controlled creation of atomic antimatter has never been achieved before.

How fitting then that 2002, the centenary of Dirac's birth, has witnessed the first controlled production of atomic antimatter, in particular antihydrogen. This, the simplest anti-atom, comprises a positron bound to an antiproton. Underlying this achievement are countless man-years of progress with antiparticles, all the way from understanding their basic properties to learning how to control them and finally to cool and mix them.

To make an atom of hydrogen, be it "anti" or not, one has to arrange things to allow the constituents to move closely together. In essence, one can do this in two ways. First, by having the pair move alongside one another in beams at the same speed, somewhat akin to motorway traffic calming, or by cooling both constituents to a very low temperature and allowing them to mingle, a kind of particle gridlock.

Our experiments this year chose the latter route for a simple reason. Soon we hope to study the antihydrogen by measuring its properties. For this, it is best to have the antihydrogen moving at low speeds or, in the jargon, to be cold. The slower it is, the longer we will have to look at it and the better our view of it is. This common sense translates pretty well into physics; the longer we can make measurements on the antihydrogen, the more precise our results should be.

I lead a team of physicists from the University of Wales Swansea in the international antihydrogen collaboration called Athena. Our experiment is located at Cern, the European Particle Physics Laboratory in Geneva, which is the world's sole supplier of low-energy antiprotons.

Its Antiproton Decelerator spits out about 10 million of them every two minutes or so in a short, sharp burst. Using a special arrangement of electric and magnetic fields, Athena is able to trap a few of these precious charged antiparticles. But to get our antiprotons to mingle effectively with positrons, we have to cool them down.

We do so following a time-honoured tradition - if you want to cool an object, you plunge it into something that is colder. For antiprotons, our cooler is a cloud of electrons held in the same electromagnetic trap. These electrons cool themselves nicely by emitting microwaves as they whiz ferociously round in circles in the strong magnetic field.

The antiprotons fritter away their kinetic energy in collision with the electrons until they flop gently into their midst at a temperature just a few degrees above absolute zero. Once the electrons have done their job, we dump them abruptly and are left with a pure, cold antiproton ensemble.

Meanwhile, on the other side of the apparatus, some 5m away, the Swansea team and their Athena associates have been busy preparing the positrons in a custom-built unit. It was built in the UK then shipped to Cern, where it now butts up against the antiproton trap. We have a prodigious throughput of positrons (more than 100 million every few minutes) that are shot across towards the antiprotons and quickly squeezed into their own electromagnetic cocoon. Unknown to them, the antiprotons are only a few millimetres away.

Then the mingling begins. We release the antiprotons into the positron cloud (energetically) from above and they fall into the trap and sink gently towards the bottom. Before they reach the floor, some have captured a positron to form our antihydrogen bound state. Once formed, the neutral antiatom is no longer confined by the electromagnetic trap, so it wanders to a nearby material object where it annihilates. Athena picks up this event with a uniquely sensitive purpose-built detector.

The reaction is controlled and the antihydrogen is cold. And this is just how it should be for our experiments; but these are for the future. For now, we continue to mix our antiparticles and pick our antihydrogen events before Cern's machines begin their energy-saving winter slumber at the end of October. But rest assured that Athena will be raring to go again when the machines are reawakened in the spring.

Michael Charlton is head of physics at the University of Wales Swansea. He leads the UK involvement in the Athena project at Cern.

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