Sometimes discoveries in physics come as sudden revelations: the existence of a new kind of particle, or of a new phase of matter. But sometimes profound results take shape gradually, appearing first as suggestive but not unique interpretations of limited data, becoming fully realized only as they are recognized in different contexts and made quantitative. The phenomenon of colour coherence is of the latter type. Its observation1,2 is a very remarkable result, showing in a dramatic way the unity and predictive power of fundamental physical theories; yet it has come upon us so gradually that it has gone almost unremarked.
To set the stage, let me briefly recall the close analogies between quantum electrodynamics (QED) and quantum chromodynamics (QCD), the theory of the ‘strong’ force which binds atomic nuclei and governs most high-energy particle interactions. Both theories are based on the concept that fields respond to the presence and motion of charges. In QED, the electromagnetic field responds to electric charges. In QCD a more complicated system of eight different fields responds to the presence and motion of three different kinds of charge, somewhat inappropriately called colours (‘red’, ‘green’ and ‘blue’). Although the mathematics of QCD is more intricate — and thus, to aficionados, more symmetrical, beautiful and challenging — than that of QED, the basic interactions postulated in the two theories are profoundly similar.
Electrons and positrons (among other particles) carry electric charge, and quarks and antiquarks are their colour counterparts. Photons are the corpuscles of the electromagnetic field of QED; the eight colour gluons are their analogues in QCD. Gluons are now routinely observed as individual particles, signalled by the jets of energetic particles they leave in their wake. But amid all the close analogies, there is one important difference between QED and QCD, which is responsible for their very different dynamics: whereas the photon is electrically neutral, the gluons themselves carry colour charges (Fig. 1).
The soul of QED is the electromagnetic field. When Faraday and Maxwell proposed this field, their contemporaries found it very abstract and mysterious. But as antennas were devised that danced to its tune, the electromagnetic field came to seem undeniably real and almost tangible.
Can one devise antennas for the colour fields of the strong interaction? It is not possible to do this directly, because the wavelengths involved are ridiculously small — 10-14 cm and below, much smaller than X-rays or even ordinary gamma-rays. Fortunately, colour coherence provides an indirect path to the goal.
What follows is an example of colour coherence which makes an especially vivid impression, owing to its use of photons and gluons in contrast.
Usually, when a high-energy electron and its antiparticle, a positron, annihilate, two narrow sprays of particles (jets) emerge, produced by a short-lived quark-antiquark pair. Some slow-moving particles are also left behind. The number of particles produced at a given time and place measures the strength of the colour field there, much as an antenna's response measures the strength of electromagnetic fields. This multiplicity is roughly proportional to the intensity, or energy, of the field, which grows as the square of its magnitude. So, in effect, the slow-moving particles provide a peculiar form of antenna — one that is fairly crude, but unique in its sensitivity to exceedingly short-wavelength colour fields, ready-made and very cheap.
This antenna has been used to distinguish between coherent and incoherent fields, which are produced in two slightly rarer cases of electron-positron collision. In about ten per cent of the events, there is a third jet containing several strongly interacting particles. This third jet signals the radiation of a colour gluon. Still more rarely, in fewer than one per cent of the events, an energetic photon accompanies the quark and antiquark (Fig. 1). Fortunately, as millions of electron-positron annihilation events have been studied, there are many examples of both rare types available for analysis.
Colour charge, like electric charge, is conserved. Because the original electron and positron had zero colour charge, the colour charges of the annihilation products must add up to zero. When the quark and antiquark are accompanied by a photon, their colour charges must be equal and opposite, as the photon is colour neutral. The positive charge (red, say) creates a red colour field that points away from it, whereas the field of the negative red charge points towards it. In the region between, these two contributions reinforce each other — the fields are coherent. But when the quark and antiquark are accompanied by a gluon, their colour charges are generally of completely different types, not equal and opposite, and the fields between them do not reinforce each other.
QCD predicts, therefore, that more particles will be produced between the quark and antiquark jets when they are accompanied by a photon, as compared to when they are accompanied by a gluon jet. Just such an effect is observed. The difference is about a factor of two, which agrees quite well with rigorous theoretical calculations3.
Other, more complex colour coherence effects are seen in other types of event. For example, in collisions of particles other than electrons and positrons — such as quarks and antiquarks4, or quarks and electrons — the colour fields of the initial particles can reinforce, or cancel, the colour fields of the particles produced.
As these patterns are mapped out, colour fields are becoming an ever more tangible aspect of reality. The recent accumulation of definitive results from a variety of experiments makes this an appropriate time to declare a triumph.
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