On the scent of glue by Frank Close Although this work was not new Soon after the quark model was in­ has been intensifying over the last for the specialists, the underlying vented twenty years ago, people re­ several years. Where have all the message was clear. 'Because of its alized that it was in trouble. The Pauli flowers gone? inherent singularities, classical gen­ exclusion principle ruled out many eral relativity predicts its own down­ well known states, in particular the How colour forces work fall,' stated Hawking, 'just as the configuration of three identical classical picture of the atom was also strange quarks that formed the ome­ Electrical charges are the sources doomed'. ga-minus. The very hadron whose of electromagnetic forces. As every The meeting merited two closing discovery had confirmed the Eight­ schoolchild knows, opposite char­ lectures. For the cosmologists, Mar­ fold Way seemingly killed its off­ ges attract while like charges repel. tin Rees of Cambridge confessed to spring, the quark model. Quarks possess electric charge and finding the symposium an 'unusual In those days, many people were so feel electromagnetic forces. That experience'. For the particle physi­ reluctant to accept the idea of frac­ is why even electrically uncharged cists, John Ellis described particle tionally charged quarks which had particles like neutrons have electro­ physics and cosmology as having 'a never been seen. The Pauli paradox magnetic interactions; they contain brilliant past in front of them' — refer­ suggested that quarks were at best electrically charged constituents. ring to the new common interest in no more than a bookkeeping device, Quarks also have colour and it ap­ the primaeval Big Bang and its imme­ not physical particles. On the other pears that this is a form of charge diate.consequences. hand some aficionados took the atti­ whose behaviour in generating The symposium occasionally re­ tude that as these were funny parti­ forces is analogous to electrical flected communications difficulties cles then perhaps they obeyed funny charge except for the important pro­ between macro- and microphysics. rules, and that application of the Pauli perty that, whereas electrical The meeting may not have bridged principle might involve hidden sub­ charges are either positive or nega­ these gulfs in mutual understanding, tleties. tive, there are three different varie­ but it certainly helped to close them. Today we accept that quarks do ties of colour. Suppose that quarks Most of the participants left feeling obey the Pauli principle, and that the carry positive (colour) charge and an- that they had learned something, (hidden) subtlety is that each quark tiquarks correspondingly carry nega­ however not everyone learned the flavour (up, down, strange, etc.) can tive charge. Then the attraction of same things! While there is still a occur in any of three ways, or 'co­ opposites, such as a red quark and a long way to go, the meeting showed lours'. Thus, for example, if each red antiquark, forms the familiar that some of the barriers on the way strange quark in the omega-minus mesons. to solving the Biggest Problem may has a different colour, they are no The presence of three colours be crumbling. longer identical : Pauli is satisfied ; the makes the possible attractions rath­ omega-minus can exist. er richer than in the simpler electro­ Report by Gordon Fraser The concept of colour has proved magnetic case. Just as like charges seminal in generating a theory of repel, so do like colours repel. For quark forces. Colour appears to be a attractions the rules are generalized form of charge, like electrical charge, slightly; not only can opposites at­ and a relativistic quantum field the­ tract, but unlike colours, such as red ory — quantum chromodynamics and blue quarks, can attract under (QCD) — has been developed. Its certain conditions. We must take successes in describing high energy into account the quantum state of the hadron interactions are well known. two coloured objects. If it is antisym­ But its application to the study of metric under exchange of the colour quark bound states (the hadrons) has labels, then the coloured objects will raised exciting new problems. A mutually attract; if symmetric they whole host of new hadrons is pre­ will mutually repel. dicted; 'glueballs', 'hybrids', 'her­ This subtlety is inherent in quan­ maphrodites', 'meiktons' — none of tum chromodynamics (a 'non-abe- which has yet been seen conclusive­ lian' theory). An analogy is the famil­ ly, even though the search for them iar case of nuclear isospin exchange 6 CERN Courier, January/February 1984 Frank Close — 'where have all the flowers gone ?' (Photo Rutherford) mit electrical forces but do not them­ selves carry charge. Hence, they voyage freely through space. How­ ever the richness of three colours causes the gluons to carry colour. Not only do they directly transmit the forces, but they feel them even while propagating. The spatial behaviour of the colour forces is thus very different from electromagnetism. This leads to a whole new spectro­ scopy of particles. Gluons and gluonic hadrons If gluons carry colour, they can be mutually attracted by the same co­ lour forces that ensnared the quarks in colour-neutral clusters. Thus, one can imagine the existence of bound states of the gluons, originally called gluonic mesons but now widely known as 'glueballs'. There is also the possibility of 'hybrid' hadrons (also called 'hermaphrodites' or 'meiktons') containing both quarks forces. Two protons or neutrons are trio will have the same colour as one and gluons as excited degrees of symmetric in flavour and the forces that is already there and so be re­ freedom. are intrinsically repulsive: a proton pelled by it. But it will be attracted by In the heuristic picture above, a and neutron feel a repulsive force the other two, so does attraction or hybrid meson arises when a red when in the symmetric state but the repulsion win? In this case they ex­ quark is attracted by a blue anti- antisymmetric (isoscalar) state is at­ actly balance: the attraction only quark, for example. These unlike co­ tractive. operates between antisymmetric lours attract but are not mutually The threefold colour forces act in a combinations, there is only a fifty- neutralized; coloured gluons can be manner which is an obvious general­ fifty chance that this configuration is attracted by the pair and so neutral­ ization of this. There are three ways present, and, together with the fact ize the colour forces. Some, at least, that quarks can pair: red and yellow, that there are two quarks attracting of these configurations are more yellow and blue or red and blue. A the newcomer and one repelling it, than mere gauge transformations of third quark will be strongly attracted yields a net cancellation. Although conventional quark-antiquark meso­ by a pair only if its colour differs from this pedagogic example does not ex­ ns. Thus, an uncoloured quark / anti- the initial pair's colours and the quan­ plain why colour ionization (exist­ quark / gluon cluster can form: a tum state is antisymmetric under the ence of free quarks) is forbidden, we hybrid meson. Hybrid baryons have interchange of any pair's colour la­ can at least see how the systematics also been predicted. I shall generical- bels. Thus red-yellow-blue clusters of colour forces have generated the ly refer to glueballs and hybrids as form: protons, neutrons and the observed clusterings of quarks and 'gluonic excitations' to distinguish familiar baryons exist. Notice that the antiquarks. The hadrons have no net them from their established cousins, way the attractions and repulsions colour, but feel the strong interac­ conventional quark excitations. work has necessarily forced three tions because of their coloured con­ Do we have much hope of produc­ quarks, each of a different colour, to stituents. ing and then identifying gluonic exci­ be present in the baryon. Gluons are to colour as photons tations ? A few years ago there was a A fourth quark brought up to this are to electric charge. Photons trans­ lot of optimism that the answers to CERN Courier, January/February 1984 7 The three lightest quarks, as depicted by Frank Close in his book 'The Cosmic Onion ' (published by Heinemann Educational Books, London). The numbers indicate the electric charges carried by each quark. both of these questions was yes. Today there is less certainty about the identification of glueballs, al­ though there still seems to be opti­ mism that they can be readily pro­ duced in the laboratory. The discov­ ery of new hadronic states in the 'right sort of experiments' has, per­ haps paradoxically, led to this change of opinion. Why? For nearly a quarter of a century we have studied millions of interactions of pions, protons and other well known particles, and in the process have discovered scores of reso­ nances that have collectively estab­ lished the quark model of hadronic matter. There is an impressive body of information that indicates that this model is 'correct'. No well estab­ lished resonances exist that cannot be accommodated in the quark mod­ el; the systematics of the hadron masses now have been reasonably well explained by quantum chromo- dynamics; spin-dependent colour forces between the constituent totally irrelevant in the region under J/psi consists of a charmed quark quarks qualitatively and even on oc­ study? Of course, if you were lucky and a charmed antiquark ; the upsilon casion quantitatively describe the enough to discover two or three is similarly built from a bottom quark pattern of observed energy levels. prominent structures which had nev­ and antiquark. These states survive These 'classical' experiments er been seen before elsewhere and only as long as the quark and anti­ have involved beams of quarks and they stood out like the famous J /psi, quark do not mutually annihilate.