On the scent of glue

by Frank Close

Although this work was not new Soon after the 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 was also strange that formed the ome­ Electrical charges are the sources doomed'. ga-minus. The very 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 . Quarks possess and finding the symposium an 'unusual In those days, many people were so feel electromagnetic forces. That experience'. For the physi­ reluctant to accept the idea of frac­ is why even electrically uncharged cists, John Ellis described particle tionally charged quarks which had like 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 in the omega-minus . 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 ) 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; '', '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 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 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 . 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 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 pelled by it. But it will be attracted by In the heuristic picture above, a and feel a repulsive force the other two, so does attraction or hybrid 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 / cluster can form: a tum state is antisymmetric under the ence of free quarks) is forbidden, we hybrid meson. Hybrid 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 . Gluons are to colour as 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 , 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 . 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; -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 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. targets of quarks (buried in their par­ then the world would beat a path to When they do, their energy is con­ ent hadrons), so quark clusters were your door. Unfortunately, nature verted into radiation, photons dominantly produced, especially if does not yield up such gifts very oft­ and/or gluons. The gluons can then one studied the debris in the near for­ en. Even when she does, it is not form gluonic clusters. This at least is ward direction. easy to know what you have what theory predicts. Decays into a To produce glueballs you must found. plus mesons are particularly either first get rid of the quarks or This is illustrated by what has hap­ interesting. Theory says that one in look in a place where the quarks do pened in experiments that destroy seven psi decays should be of this not tend to go. The latter approach quarks. New states have been seen form. has recently been employed at clearly there and theorists have About 15 per cent of the psi de­ CERN, where several groups have found a new way of generating pa­ cays are indeed radiative, so the been looking at the hadrons pro­ pers, debating whether these are re­ qualitative suspicion that glue is duced in the central collision region, ally the long-sought glueballs or around appears to be quantitatively in particular at the ISR. The gluons merely excited configurations of validated. If a new state appears in which are generated as the two conventional quark matter. the debris accompanying the pho­ quark beams brush by could reson­ These experiments involve elec­ ton, then it has good credentials for ate and form glueballs. tron- annihilation at an ener­ being gluonic. And new states have The difficulties here are obvious. gy tuned to create heavy mesons been discovered, iota, theta, and last Can you be certain that quarks are such as the J/psi or the upsilon. The summer the 2.2 GeV two- reso-

8 CERN Courier, January/February 1984 nance seen by the Mark III group at quarks permits exotic states whose tion of protons into hybrids has been the SPEAR ring (see October issue, quantum numbers are otherwise for­ calculated and seems likely to be page 311). This good news is some­ bidden. Theory gives some hope for rare. Baryon spectroscopy appears what clouded by the fact that several certain such states, which although safe. well known mesons are also promi­ suppressed in radiative decays of So what are the prospects for nent in these data, notably the J/psis, might be visible in the forth­ establishing the existence of hybrid f( 1270), eta and eta prime. Optimists coming low energy proton-antipro­ or gluonic mesons? Theoretical have taken this as proof that the etas ton experiments at the new CERN ideas on the expected spectroscopy do indeed have an intimate relation­ LEAR ring. are converging. For gluonic mesons, ship with glue and point out that for These new states cannot all be all approaches seem to agree that several years interesting structure theoretically avoided by gauge trans­ the scalar (vacuum-like) state should has been seen and ignored in the formation tricks. The spin-depend­ be the lightest, somewhere between vicinity of the f. This suggests to ent energy shifts that arise from co­ 0.5 and 1.5 GeV. Other states some that the f may be more than the lour forces between the quarks and should be somewhat heavier. The innocent ideally-mixed quark state gluons have been calculated now by has the widest theore­ we had thought. various groups and their results tical support, but is not seen in the The case for gluonic states might agree. The results are reminiscent of radiative decays of J/psis, sup­ have been more clear cut if the states what is seen with the well known posedly the best glue factory had been better behaved in their de­ meson nonet including the pions and around. Other gluonic meson candi­ cays. The starting guess was that, . dates are seen instead. since gluons have no flavour, their This has been exploited by some Could the scalar be hiding under decay products should be uniformly theorists who have shown that the the f(1270)? Could it be narrow and distributed among nonstrange and mass pattern of these mesons can down at 600 MeV, and consequent­ strange particles. However the iota be understood at the price of their ly have been overlooked in analyses decays copiously into final states being rather impure mixtures of of two- spectra ? Are there theo­ containing kaons (), but quark-antiquark and hybrid states. retical problems related to the va­ not into other non-strange states. This possibility is rather harder to cuum? Do the interactions of quarks Quite the opposite of 'folklore' ex­ eliminate than one might expect. In­ and gluons radically change the the­ pectations for a . Similarly, deed, in attempts to discredit the oretical picture for gluons alone? the theta and the new 2.2 GeV state whole idea of hybrids, the implica­ This last point in particular is worth are both reluctant to decay into lots tions for baryons have been widely facing. Just as some mesons are of pions. Perhaps these states are investigated during recent months. mixtures of various flavours of conventional quark states or just While in the case of mesons, hy­ quarks, so gluonic mesons might be unusual multiquark combinations. brids could be hidden in the exper­ strongly mixed superpositions of Prompted in part by difficulties imental uncertainties remaining in quark and glue components. If me­ with glueballs, theorists have recent­ the field, surely baryon spectrosco­ sons are generally very impure mix­ ly been paying increasing attention py is so well understood that there is tures of quarks and gluons, then the to the possibility that other manifes­ no room for a plethora of new search for individual, identifiable, tations of gluons in spectroscopy states? The results have been rather pure gluonic states is in trouble. In­ might be more amenable to experi­ surprising. First electromagnetic stead we may have to look for evi­ ment. Particular interest has been fo- properties : if the proton has a com­ dence of an 'overpopulation' of cussed on the 'hybrid' hadrons, in ponent which consists of quarks in a states in certain mass regions, in­ which both quarks and gluons occur colour octet with a gluon neutralizing compatible with the quark model as excited degrees of freedom. the colour, one would expect that the alone. New degrees of freedom such Their properties have been studied requirements of the Pauli principle as gluonic excitations would need to in various ways. In some ('bag') would so upset the quarks' flavour be invoked. models, the lowest energy states and spin balance that baryon mag­ In the absence of such a gift of consist of quarks and a single gluon. netic moment predictions, in particu­ nature there is another strategy that In others they correspond to excita­ lar the famous 3/2 ratio between may help identification of states with tions of 'strings' between the proton and neutron, would be ruined. significant gluonic components. The quarks. A gluon accompanying the In fact it is preserved. Photoexcita­ folklore that gluonic hadrons decay

CERN Courier, January/February 1984 9 'Close-up' of the charmed quark.

but decays preferring four kaons at Everyone seems to believe that the expense of pions would be a rath­ gluonic states should exist, but to er unexpected phenomenon. Tensor identify them with confidence may (spin two) glueballs above 2 GeV are require an improved understanding expected to contain two gluons, of conventional meson spectrosco­ each of which will prefer to generate py. Given the fundamental role that strange quarks. Thus, phi meson gluons play in the nature of things, it pairs and four kaons could be distinc­ would be troubling if gluonic states tive signatures. The glueball interpre­ could only be seen by experts in tation of the iota in the bag model hadron spectroscopy. requires that it contains one gluon with this preference for strange (As well as being an expert in ha­ quarks. Their colour doesn't neutral­ dron spectroscopy, Frank Close of equally to all flavours is based on the ize, so they cannot emerge as a phi the Rutherford Appleton Laboratory, flavour independence of gluon-quark meson, but must separate and end UK, is a prominent writer and broad­ couplings. However this is too naïve. up in different strange hadrons. This caster on . His fasci­ The forces acting on strange quarks is consistent with the apparent fond­ nating book The Cosmic Onion — are in general different from those on ness for decays of Quarks and the Nature of the Uni­ nonstrange quarks. One naturally the various new states seen in J/psi verse' has recently been published might expect this to suppress decays. by Heinemann Educational Books of strange particle production relative Three tensor mesonsi have been London.) to the lighter nonstrange; such a claimed in the 'glue-preferred' pro­ phenomenon is well known in ha- duction of phi meson pairs in nega- dron physics, and here the folklore tive-pion/proton collisions (see De­ says it is easier to produce light cember 1982 issue, page 416), and quarks out of the vacuum than to have not been seen in other chan­ produce the heavier strange nels. In the region just above 2 GeV, quarks. a tensor glueball and a hybrid tensor It may be dangerous to assume are expected with this 'strange ten­ that this folklore is necessarily ap­ dency'; excited tensor mesons con­ plicable to the decays of gluonic sisting of a strange quark-antiquark states. Indeed, when couplings of pair are also expected at this mass, gluons and quarks are calculated (in and so significant mixing should oc­ bag models), one finds that the effec­ cur. This could help explain the large tive coupling strength of a gluon to a decay widths of these candidates. quark and antiquark pair can be This preference for producing greater for a strange pair than a non­ strange particles in gluonic decays strange one. So even if the exchange may be no more than an artefact of of a gluon between heavy quarks is the model. However, if nature does suppressed relative to light quarks, indeed behave this way, then the stu­ the production of a heavy pair can be dy of multikaon channels could be enhanced in certain cases. profitable. This suggests that gluonic states If structures are seen in these may preferentially produce strange channels and are being produced in quarks and antiquarks in their de­ processes that are 'gluon-friendly' cays, some glueballs and hybrid then there could be a real prospect of mesons having important decay establishing gluonic excitation spec­ channels containing up to four troscopy. Proliferation of low-lying kaons. Conventional bound states of pseudoscalars or a prominent me- a strange quark and strange anti­ tastable state that fits perfectly with quark will decay into a pair of kaons. prediction are other hopes.

10 CERN Courier, January/February 1984