Heart of the antimatter

If the Big Bang created matter and its opposite in equal quantities, why is there so little antimatter around? Brian Foster reports on an experiment that could solve one of the most fundamental and puzzling mysteries of science

In the Bay Area of California, not far from San Francisco, a three-kilometre concrete tunnel runs straight as an arrow under the freeway, pointing towards the Pacific. This is the Stanford Linear Accelerator, whose remarkable history stretches back to the Sixties and the discovery that inside the "fundamental" particles that make up the atomic nucleus lie much smaller objects - quarks.

Now this venerable accelerator is to be deployed in an investigation into beauty - at least, beauty as understood by particle physicists. The goal is to discover why the universe is lopsided and is composed almost entirely of matter - the familiar stuff in the world around us - and contains very little antimatter, even though the laws of physics make almost no distinction between matter and antimatter.

This may seem esoteric, but it is fortunate for us all that this asymmetry exists. Otherwise the universe would be a totally featureless cavity filled with nothing but countless billions of photons - pure radiation energy - at a temperature of just 3 degrees above absolute zero. Antimatter is fictionally familiar to devotees of Star Trek - the engines of the Starship Enterprise derive their power by combining matter and antimatter to yield pure energy. In real life, antimatter consists of particles many of whose properties, such as electric charge, are the opposite of those found in ordinary matter. Should a particle come into contact with its antiparticle counterpart, they annihilate, producing a burst of radiation as their mass is converted into energy according to Einstein's famous equation E= mc2. So if matter and antimatter were created equally during the Big Bang that started the universe, their mutual collisions since should have resulted in the annihilation of everything that makes life possible and makes the universe in which we live remotely interesting.

Recently, experimenters at the European centre for particle physics, Cern, in Geneva, succeeded for the first time in producing not just the nucleus of anti-hydrogen, the antiproton, but also an orbiting anti-electron (positron), and produced three atoms of anti-hydrogen. It took an enormous degree of scientific expertise to achieve this, so the question is, why are there so few anti-atoms naturally present?

The key is in the phrase that the laws of physics make "almost no distinction" between the two. The laws of physics were indeed found, in 1964, to show a small asymmetry between particle and anti-particle, a totally unexpected discovery which gained a Nobel prize. The origin of this asymmetry was a total mystery when it was discovered and it remains so to this day.

Until we discover the reason for this tiny asymmetry we will never understand how the universe has evolved to its present state. This month, after more than two years of prototyping and preparation, the UK agreed to participate in BaBar, an experiment that should solve this matter-antimatter mystery.

Despite the elephantine connotations of its name, BaBar is not a behemoth the size of a cathedral, as are the detectors for the Large Hadron Collider under construction at Cern. Even though it is a much more homely device, it is still the size of a small house, and crammed with sophisticated particle detectors and electronics. BaBar will look at collisions between particles and antiparticles that contain heavy quarks and heavy antiquarks respectively. This is where the beauty comes in: for beauty in the eye of the particle physicist is actually a specific type of heavy quark. (Sometimes more prosaically known as the bottom quark.) The physicists expect that, extremely rarely, those beauty-containing heavy particles that do not carry any electrical charge will suddenly flip into antiparticles. They hope that the effect will be much bigger in these heavy quark systems than was the case for the light quarks which were all that it was possible to produce in 1994, thereby enabling a much more accurate measurement of the asymmetry. The problem is that although the asymmetry is much bigger, it only occurs in a tiny fraction of the final states to which the heavy bottom quarks and antiquarks decay. Thus in order to measure the effect, many millions of particles containing bottom quarks must be produced. This necessity means that the BaBar detector will sit around an accelerator known as a beauty factory.

In many ways the most challenging part of the project is to construct the accelerator capable of delivering this unprecedented number of b-particles to the BaBar experiment. Beams collide in the centre of BaBar approximately 200 million times per second. The extremely difficult experimental conditions to which the accelerator gives rise mean that although relatively small, BaBar needs to be extremely smart.

Since only a tiny fraction of the possible states that can be produced provide evidence of matter-antimatter asymmetry, it is essential that the BaBar apparatus be able to completely identify and measure all the particles produced by the initial electron-positron annihilation. The detector has to be accurate as well as complex, and must measure both electrically charged and neutral particles well. The UK groups, coming from 10 universities, are building a large part of the neutral particle detector, which consists of several thousand crystals of caesium iodide doped with small amounts of thallium. The UK groups are also responsible for the sophisticated electronics required to digitise the information and record it on computer. Very few manufacturers are capable of producing high quality CsI crystals, and the UK universities have worked closely with one, Hilger, in Kent. As a result the company has been awarded a contract from the US worth $4m. Indeed, other UK firms such as Micron and EEV have been awarded contracts worth $3m and $1m respectively, so that already the orders received by UK industry have exceeded by a factor of 2 the pounds 2.5m capital cost of UK participation in the experiment.

The design and construction of the detector as well as the accelerator is well under way. The schedule is tight, calling for the experiment to be taking data in 1999. The project is truly international, with teams from France, Germany, Italy and Norway as well as the US, Canada, Russia, China and Taiwan. The precision results that BaBar will provide on the matter-antimatter asymmetry in the fundamental forces will arrive in time for the new millennium.