What's in matter

Studies confirm the presence of gluons as constituents of matter

Published: Wednesday 31 December 1997

the theory of Quantum Chromodynamics ( qcd ) explains the interactions between particles such as protons and neutrons. Quarks -- the elementary constituents of the nuclear particles -- are postulated by qcd that have already been observed in accelerators around the world. But another important elementary particle required by the theory, the gluon has proved to be more elusive. Now, researchers at the Brookhaven National Laboratory, New York, usa , and cern , the European particle physics laboratory, Geneva, Switzerland, have reported the existence of gluons as constituents of matter ( Physical Review Letters , Vol 79, September 1, 1997).

Both the laboratories, in different processes have found evidence of a short lived hybrid particle made-up of quarks and gluons. Several experiments have established that quarks are the elementary constituents from which strongly interacting particles (the proton, neutron and others collectively called hadrons) are made. But qcd demands that these quarks be held together by the exchange of other particles called gluons.

The existence of gluons has been indirectly verified at high energies in many experiments. But at lower energies, there has been a problem. Owing to its peculiar structure, the equations of qcd break down at low energies that are typical of ordinary matter.

In 1980s, several theorists had modelled the behaviour of matter at ordinary energies and predicted the existence of a hybrid quark-gluon particle. This hybrid particle was predicted to have peculiar properties that would make it stand out from other more mundane particles. The gluon component would give it a spin and charge, which would be different from particles made up of just quarks. And because of these peculiar properties, the hybrid particles would decay in a characteristic fashion which would make them identifiable.

The lightest of such particles was predicted to have a mass of about 1.8 g e v (billion electron volts or about twice as massive as the proton) and a lifetime of about 10 -23 seconds.

In 1994, an old detector at Brookhaven was remodelled and used together with the accelerator called the Alternating Gradient Synchrotron. A beam of pions, an ordinary particle made-up of a quark and an antiquark was collided with a target of liquid hydrogen and the collision products were analysed. Looking for the exotic meson was a very complex task because in about 200 million collisions, only 40,000 were selected with the correct properties.

After analysing the data, the group of 51 researchers have reported that a new particle with a life of about 10 -23 seconds and a mass of about 1.4 g e v is present. The other properties of this particle also rule out a more mundane quark-antiquark pair.

In another study, the Crystal Barrel collaboration at cern reported a similar result using a completely different process. The group at cern studied the collisions of a beam of antiprotons and heavy hydrogen. The data from this experiment also reveals the existence of a subsystem with strange properties. The properties seem to agree with those reported by Brookhaven.

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