According to theory, the missing particle is made from gluons, the
force-carrying particles that hold quarks together inside protons and neutrons.
Because of their intrinsic “stickiness”, gluons should clump together to form
“glueballs” of pure subnuclear energy.

Ever since gluons were discovered in the 1970s, there has been little doubt
that glueballs exist. However, despite a host of experiments at CERN, the
European centre for particle physics near Geneva, no glueball has been
identified.

But at the International Conference on High Energy Physics in Warsaw last
week, experts announced that a particle discovered in 1994 matched the
theoretical description of a glueball. The particle, known as the fo(1500)
resonance, had been passed off then as a relatively unexciting vibrating quark
pair. The experts decided it could be a glueball after examining thousands of
data tapes held in computer archives across the world.

“The search has been rather like a criminal investigation, with lots of
detective work being required to eliminate suspects from our enquiries,” says
Rolf Landua of CERN.

The standard model predicts, with astonishing accuracy, many aspects of the behaviour of the known elementary particles and the forces which influence them. But physicists know that the standard model gives an incomplete picture: it cannot, for instance, be used to predict the mass of a particle. So theorists argue that a more fundamental theory must be lurking somewhere in the background, and have been confident that departures from the standard model’s predictions would show up sooner or later.

One such flaw may have at last been discovered at CERN. Last month in Brussels, at a meeting of the European Physical Society, David Charlton of the University of Birmingham described preliminary results obtained by hundreds of physicists working with CERN’s 27-kilometre circumference Large Electron Positron collider.

At LEP, electrons and positrons are smashed into one another at huge energies, producing various short-lived particles. These include the Z₀ boson, one of the force-carrying particles which unify two of the four forces of nature, the weak and electromagnetic interactions. The Z₀ bosons decay rapidly into a range of other particles, and experiments at LEP have measured the fraction of bottom quarks produced by this decay.

This fraction, called R_b, turned out to be significantly larger than the figure predicted by the standard model. One possible explanation is supersymmetry. Using the standard model, R_b can be worked out only after a series of corrections are applied to the initial number crunching. The existence of supersymmetric particles would affect these corrections, skewing R_b towards the new figure.

Many theorists hope that this explanation is correct because supersymmetry gives a more complete and internally consistent description of the Universe which could eventually lead to an all-encompassing “theory of everything”. But there are other explanations, including a statistical fluke or some undiscovered bias in the results. “One has to be very careful with the present measurements,” says Howard Haber of the University of California, Santa Cruz.

Clinching evidence could soon be at hand, however, as LEP is now being modified to work at even higher energies. “If supersymmetry is responsible, then we might expect to see the first superparticles being produced for real when LEP’s collision energy is upgraded,” says Haber.