A mirror at the heart of matter

October 13, 2000

Cern's new antiproton decelerator will allow Michael Charlton to try to explain the abundance of matter over antimatter

For a laboratory famed for its accelerators and large machines, one of which is km in circumference, the latest addition to Cern, the European particle physics laboratory's great instrumentation portfolio might seem modest.

The device formally inaugurated at the European Laboratory for Particle Physics near Geneva on August 10 is small by Cern standards - although at 188m around even this is relative - and what is more, it is a decelerator not an accelerator.

The job of the antiproton decelerator is to capture and slow the antiparticles of protons. Inside the machine, the antiprotons reach a speed of about 10 per cent of that of light - positively relaxed by Cern's usual standards. Every minute or so, a short burst of antiprotons is ejected from the machine in readiness for experimentation.

The new facility will allow researchers to compare, in great detail, the properties of protons and antiprotons. One feature of both these particles is that, given the correct partner, they form stable atomic systems. For the proton we add the familiar electron and obtain hydrogen. The antiproton requires the anti-partner of the electron, the positron, to form antihydrogen.

So far we have had only fleeting man-made glimpses of this object, the simplest atomic antimatter, which was moving too fast to be of any real use to physics investigations. We aim to make precision hydrogen-antihydrogen comparisons by performing laser spectroscopic studies of the energy levels of ultra-slow atoms. To achieve this, techniques have been devised to control positrons and antiprotons and to cool them until they reach a temperature near the boiling point of liquid helium. This somewhat fragile liquid boils in a kettle kept at a temperature of 4 degrees Kelvin (in other words, cooled to 269C). At this temperature, the antiproton speed, 900mph, is bordering on the lethargic and not much greater than the speed of sound in air.

I lead a team of physicists from the University of Wales Swansea that has developed the positron component of an antihydrogen apparatus being built by the Athena collaboration. Our apparatus, now working at Cern, will be joined to Athena's antiproton reservoir.

What is the point of this, you might ask? The answer lies in symmetry - or rather a lack of it. In science, and particularly, physics, we tend to think of symmetry in terms of "mirrors". One of these is the familiar household mirror that interchanges left and right, so-called parity reversal. Another is time reversal, the peculiar notion that the laws of physics apply equally well if the direction of the arrow of time is reversed. A third implies that the universe is unchanged if the sign of the electric charge carried by particles is reversed. What this amounts to is changing a particle into an antiparticle; for instance an electron into a positron or a proton into an antiproton.

In the first half of the 20th century, physics translated our fascination for symmetry into the belief that its laws worked just as well if any one of these mirrors - let us call them C (charge), P (parity) and T (time) - were used individually. It then proceeded slowly to demolish this edifice. C and P fell together in the 1950s, and the peculiar dual mirror, CP, cracked in 1964. T's fall, though much later, was widely expected. We now know that a combination of any two of these mirrors is flawed. This leaves intact one mirror - the combination of all three. We call it CPT.

To some, this might sound like a public-relations nightmare for physics. How could we get it so wrong? In fact, though these mirrors are faulty, the chinks are tiny and apply only to certain particle interactions and reactions. As usual though, the exceptions prove the rule - or point the way towards new ones. For this reason we must test the CPT mirror (and we can do this with our proposed hydrogen-antihydrogen comparisons) because even the smallest blemish will shed light on some of the remaining mysteries of the universe.

The reason for the importance of this is provided by cosmology. As presently understood, the very flaws in these mirrors are responsible for the abundance of matter over antimatter in the universe. The Big Bang is reckoned to have produced exactly equal amounts of matter and antimatter, which should have long since completely cancelled to zero. But they have not, and we want to know why. After all, we are a curious people bound to search for and explore the flaws in nature's symmetry. For therein lie hidden her subtle beauties.

Michael Charlton is head of the experimental physics group at the University of Wales Swansea and the top UK physicist involved in Cern's antiproton decelerator.

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