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Particle Physics: Glueball Sightings Premature NATURE VOL. 323 25 SEPTEMBER 1986 295 -------------NEWSANDVIEWS------------- switching on nuclear events. ger systems in memori'· The photorecep­ electromagnetic coupling, and is so large What these messengers are is still a mys­ tors of Hermissenda provide another ex­ that theorists began to suspect that there is tery, but Ins(1,4,5)P3 and diacylglycerol perimental model for memory where the significant quark content in 1: it is either must be likely candidates because of their closure of potassium channels by protein an excited quark state or a hybrid. Inde­ involvement in neuromodulation. A role kinase C is an essential component of the pendent of the interpretation there is a for calcium has already been implicated in plastic change that occurs during associa­ conundrum: the Q has the same quantum 13 PC12 cells where there is a rapid increase tive learning • The virtuosity of this dual numbers as the photon and so can turn in c-fos transcription following the addi­ signalling system is truly remarkable: not into a photon, this feeding the decay chain tion of agents that open voltage­ only does it control the second-to-second t into two photons. The prominent rate 11 dependent calcium channels • Long-term activity of most cells, but it can apparently into y-Q and this piece of quantum potentiation of synaptic transmission be­ extend its performance to controlling the mechanics implies that the decay of two tween neurones in the hippocampus has long-term plastic changes that underly cell photons should be at least five times larger been used as an experimental model for growth and the acquisition of memory. D than the experimental upper limit! This memory. Because both calcium and pro­ discrepancy must be explained before tein kinase C have been implicated in the conclusions are drawn. Michael Berridge is in the AFRC Unit of Insect acquisition of long-term potentiation Neurophysiology and Pharmacology, Depart­ One possibility is that there are two there already is some experimental evi­ ment of Zoology, University of Cambridge, states in the 1.4 GeV region of mass: the dence for the involvement of such messen- Cambridge CB2 3EJ, UK. t and another. Supporting this is the fact that the mass and width of the signal in the Particle physics y-Q channel seems not to be consistent with the properties of t determined from other experiments. The true t might Glueball sightings premature have small couplings both to y-Q and two photons; a canonical glueball. The from Frank Close y-Q signal might herald the excited quark state expected in this mass region; if HAs a glue ball at last been positively iden­ particle, theorists in the late 1970s realized so then there should be interesting inter­ tified? Aihara et al. (Phys. Rev. Lett. 51, that QCD implied that radiative decays of ference phenomena that will need analysis. 51; 1986) seem to think so; their measure­ the '1/J should be a good source of glue However, there is a a less exciting possi­ ment of the lack of electromagnetic cou­ matter. Prominent in the decay particles bility that has already been discussed in a pling of the iota particle leads them to was a hitherto unknown state - the iota recent Toronto University preprint by T. conclude that the iota is the long-sought (t), a prima facie candidate for resonat­ Barnes. If the t is an excited quark state bundle of gluons. Certainly the iota has ing glue. It has a mass of 1.4 GeV (about (17*) then in addition to its copious y-Q been most people's favourite candidate 50 per cent more than a proton) in line decay it will also have a powerful coupling for some years but a definitive test has with theorists' expectations for glueball to Y-Q* (an excited Q analogous to the been awaited. The present claim comes masses. excited 17*). This latter decay channel is from data which are some 40 per cent Encouraging as this is, there are some closed by energy conservation but it can better than an earlier experiment by the features of its decays which do not fit well contribute destructively to the amplitude 'TASSO' collaboration in Hamburg with the expectations for a glueball. There for t decaying into two photons. The (Althoff et al. Z. Phys. C29, 189; 1985), is also an unfortunate coincience that an apparent anomaly between the small y­ but this group did not risk such a positive excited state of the eta (17) particle, a y and large y-Q is then due to neglect claim, and the evidence is perhaps not as quark state, is expected to occur in this of the (even larger) destructive y-Q* clear-cut as the new data of Aihara et al. vicinity with the same quantum numbers coupling. If this is the explanation in de­ seem to imply. of spin and parity (behaviour of the wave­ cays of the Q*: e* ~ t + y; or in elec­ Atomic nuclei are made of protons and function under mirror reflection) as the tron-positron annihilation: e+e- ~ y ~ neutrons which are in turn clusters of t. So the question is: is the t a gluonic Q* + t. quarks. Quarks possess an unusual type of state or an excited 17 or a hybrid of quarks Until the opening of the large electron­ charge called colour which operates in and glue? positron collider (LEP) in Geneva or the some ways like electrical charge. The re­ To help answer this, difficult studies of Stanford Linear Collider (SLC) and the lativistic quantum field theory of the the electromagnetic interactions of the t hoped-for discovery of bound states of top colour-induced forces is called quantum have been made. Glue is electrically quarks (whose decays should be a copious chromodynamics (QCD) by analogy with neutral and so glueballs should have source of glueballs), it is most promising the mathematically similar theory of rather feeble couplings to photons. In par­ to continue studying '1/J decays, or low­ quantum electrodynamics (QED). The ticular the t should have a much-suppres­ energy annihilation of protons and anti­ photons of QED transmit electromagnetic sed decay into two photons, a mode that is protons (as at LEAR in Geneva) for sight­ forces; similarly gluons transmit the chro­ allowed by quantum mechanics. ings of glue. If there are several spinless modynamic forces in QCD. In 1985 the TASSO collaboration particles with negative parity (pseudo­ These similarities induce theorists to referred to above showed that scalars) in the 1 to 2 Ge V mass range (as seek to build a unified theory of these this mode is somewhat smaller than ex­ data suggest), then quarks will be unable forces. But first, is QCD correct? If it is, pected for a quark state at this mass. The to accommodate them all. Gluonic de­ then in addition to ordinary matter there experiment of Aihara et al. improved this grees of freedom will need to be invoked. should exist glueballs - matter made of by nearly a factor of two, which although Careful examination of pseudoscalars, glue not of quarks. There are no balls of suggestive is hardly conclusive given the and clarification of the electromagnetic light in QED because photons are electri­ various sources of error. There are two coupling of t, are therefore probably the cally neutral and do not mutually attract. problems (Aihara and colleagues com­ best current strategies. D However, the gluons of QCD carry colour ment only on one) which call for more charges and so mutually should attract by work before their claim is established. the same forces that cluster the quarks During the past three years there have Frank Close is at the Rutherford Appleton Laboratory and is visiting Professor of Physics into conventional nuclear particles. been reports that t has a prominent de­ at Queen Mary College, Mile End Road, Lon­ Following the discovery of the psi ('1/l) cay into photon and rho (y-Q). This is an don El, UK. © 1986 Nature Publishing Group.
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