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Looking for Gluons Looking for gluons Whole body image of a patient with thyroid cancer taken using iodine-123 produced at the TRIUMF cyclotron. 500 MeV contains an irreducible 0.5 In his concluding talk at the Geneva per cent of iodine-125 as an impuri­ high energy physics conference earl­ ty. This contributes excessively to ier this year, Abdus Salam predicted the patient radiation dose beyond that the gluon is likely to be discov­ 28 hours after production. As noted ered sooner than the long-awaited in the April issue of the COURIER, intermediate particles of weak inter­ the ideal energy for producing pure actions. In the light of the new high iodine-123 is near 70 MeV, and energy results^ from PETRA (see TRIUMF has implemented a third September issue, page 307), this extracted beam covering the 60 to prediction now looks a pretty safe 100 MeV range, with the production bet. of iodine-123 (and other potential The existence of gluon is far from radiopharmaceuticals) as its main being physics history, but evidence is use. A 5 |iA extracted beam of good steadily mounting. Even if high quality was obtained and a molten energy proton-antiproton collider sodium iodide target is under devel­ projects live up to their promise and opment for early installation in this quickly reveal the intermediate weak beam for iodine-123 production. particles, the gluon seems to be Higher intensities are planned for a winning the race at the moment. later stage, which will require several Spin one gluons are postulated to shielded target stations. be the carriers of the 'colour' force In addition to the above research acting between quarks, and are thus programme, TRIUMF radiopharma­ ultimately responsible for all strong ceuticals will be distributed com­ interaction phenomena. Theorists mercially via an agreement nego­ are gradually piecing together a tiated with the Commercial Products quantitative theory of quantum by this gland is not observed; activity Division of Atomic Energy of Canada chromodynamics (QCD) to describe in the salivary glands, the gastroin­ Ltd. Financing from the British these forces at work deep inside testinal tract and the bladder is, Columbia Development Corpora­ hadrons. however, present and diffuse uptake tion, through the University of British Delicate effects (so-called 'scaling is visible in the lung. In addition, a Columbia, is paying for the construc­ violations') seen in neutrino experi­ small region of intense iodine uptake tion of high-level radioisotope labo­ ments had previously given useful is seen in the vicinity of the left hip; ratory facilities at TRIUMF for encouragement to the QCD theo­ this proved to be a metastatised processing irradiated targets into rists. But the contributions from thyroid tumour, exhibiting thyroid radiopharmaceuticals. Also a com­ QCD mechanisms can be masked by function in its new location. mercial variable energy 42 MeV other effects and so be difficult to The spallation reaction is also negative hydrogen ion cyclotron will measure. being exploited by this team for the be installed to provide additional However high energy annihilation research production of xenon-127, production facilities for radioiso­ of electrons and positrons at DESY, iron-52 and other materials, and by a topes produced with proton energies first at the souped-up DORIS ring second TRIUMF/University of Bri­ between 11 and 42 MeV. Grants and then at the new PETRA ring, tish Columbia team for the produc­ from the British Columbia Provincial have provided a new and effective tion of short-lived positron emitters, Government have allowed the sec­ way of testing QCD. carbon-11, nitrogen-1 3, oxygen-1 5 ond cyclotron to be upgraded to There are a number of big and fluorine-18. These are destined become a potential neutron source problems which beset QCD. Firstly, it for application in positron emission for cancer therapy and the equipping is limited, like all relativistic field tomography studies after incorpora­ of the radioisotope laboratories for theories, to a perturbation-style ap­ tion into appropriate radiopharma­ the research programme. proach where progressively smaller ceuticals by chemical synthetic contributions are added together to techniques. converge (hopefully) on a final result. The iodine-123 produced at This convergence should improve at 358 CERN Courier, November 1979 Momentum distributions in three projections for 1.- a two-jet event and 2.- a three-jet event as measured by the TASSO detector in high energy electron-positron collisions at PETRA. higher energies. In addition, no recipe has been found to explain the apparent con­ finement of quarks and gluons within hadrons — at least at the energies we know, quarks and gluons do not appear as free parti­ cles. The perturbative QCD formalism therefore has to be embedded in an empirical 'hadronization' formula which describes how quarks and gluons produce jets of hadronic matter in high energy collisions. This limits the predictive power of the theory to situations where quark/gluon behaviour is not com­ pletely masked by hadronization. This is what makes the DESY elec­ tron-positron data on the formation and subsequent decay of heavy quark-antiquark bound states and on the emission of 'hard' (energetic) gluons so interesting. The first evidence came from DORIS data on the decay of the upsilon resonance. As a vector (spin one, negative parity) particle, this •should decay into an odd number of vector gluons, analogous to the well- known decay of positronium (an electromagnetic bound state of an electron and a positron) into three photons. Hopefully these gluons would pro­ duce three clearly-defined jets of hadrons, but in the upsilon mass range, this behaviour might not show up as clearly as might be hoped. It is difficult at these energies to differentiate between true jets and the amorphous behaviour given by a statistical decay model. To distinguish clearly between three-jet events and the two jets coming from quark-antiquark pairs, the experimentalists need some means to measure the 'jettiness' of their data. This jet analysis should also be amenable to QCD calcula­ tions. CERN Courier, November 1979 359 Momenta of the particles produced at 31.6 GeV total energy in the PLUTO detector at PETRA. Solid and dotted lines correspond to charged and neutral particles, respectively, and the thick bars show the directions of the jet axes. One such parameter which has emerged takes the name of 'thrust'. A decay producing two back-to- back particles would have a thrust value of exactly one, and the devia­ tion from this value indicates the likelihood of additional energetic particles. Analysis of upsilon decays rev­ ealed more events with low thrust than would be expected from the production of quark-antiquark pairs, and found decay products grouped in a plane. This gave the first hint that a new kind of behaviour was being seen, and the hunt for the gluon began in earnest. The next step came with the avai­ lability of higher electron-positron annihilation energies in PETRA to search for signs of three distinct jets produced by a quark, an antiquark and a hard gluon. The annihilation energies at DORIS are too low for the perturbative QCD mechanisms to pierce through the accompanying hadronization and produce observa­ ble effects. Following preliminary evidence from the TASSO collaboration, all the PETRA groups now have some evidence which suggests that hard gluon emission (called 'Gluestrah- lung' by some) is being seen. While the data up to collision energies of 1 7 GeV can be explained by quark- antiquark production, energies round the 30 GeV mark show clear signs of three-body behaviour. The next step is to measure the spin of the additional particle which is emitted along with the quark and the antiquark. Indications from upsi- The Mark J detector at PETRA. As well as finding evidence for three-jet events, the Aachen / DESY / MIT / NIKHEF (Amsterdam) / Peking collaboration has also made important studies of the production of electron, muon and tau particle-antiparticle pairs. (Photo DESY) 360 CERN Courier, November 1979 Exploiting muon spin rotation Ion decays and from lepton scatter­ Last year, some twenty per cent of characteristic precession of the ing experiments suggest spin one, the available beam time at the CERN muons. In this way the stopped but a final conclusion cannot yet be 600 MeV synchro-cyclotron (SC) muons can be used to probe the made. Confirmation should come was taken up by studies using the inner structure of a wide range of from analysis of the jet angular distri­ technique of muon spin rotation materials. butions. (HSR). Early experiments had shown that While it is important to confirm the The idea of muon spin rotation at sufficiently low temperatures, existence of the gluon and to pin dates back some twenty years to the muons corné to rest in metals and 'pwn its quantum numbers, another pioneer experiments on parity viola­ that the muon precession rates in fital test is to search for evidence of tion in weak decays, but it has only ferromagnets can be used to meas­ a single gluon decaying into a gluon come into its own as an experimen­ ure internal magnetic fields at the pair. This three-gluon coupling has tal technique in the 1 970s. muon sites. Early JISR applications no analogue in more familiar field Polarized positive muons are at CERN were aimed at ferromag­ theories, such as quantum electro­ brought to a stop in a target and netic materials. dynamics, but is a necessary conse­ precess in the local magnetic fields. However it was soon discovered quence of QCD. (Negative muons are quickly cap­ that, first, much more had to be This three-gluon coupling, which tured by nuclei and are much less learnt about the way muons interact would be seen as a softening and useful.) Because of parity violation, in metals, and in particular, what broadening of gluon jets at high positrons from the decay of these types of sites in the crystals the energy compared to the showers positive muons are preferentially muon energies prefer.
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