sions produces two mutually oppo not be reconciled by a simple 'phase ance behaviour is in good agreement site jets, giving typical cigar-shaped space' model which assumes a with a simple quark-antiquark mod events. purely statistical decay pattern. el, the resonance decays show a As the energy is increased, one of Both theoreticians and experi totally different behaviour. these jets is seen to grow wider, but mentalists have shown vivid imagi Collinear back-to-back jets are in a very special way. Rather than nation in inventing procedures and ruled out as the dominant effect, but simply getting fatter with a cylindri names to describe this jet analysis, the data is in good agreement with a cal profile, it becomes planar, and at but the underlying structure is clear three-gluon decay model in which higher energies the jet clearly splits by simple visual inspection. Quanti the gluon fragmentation is assumed into two, giving an overall event with tative analysis is less clear-cut, but to be similar to that of a quark. Thi^ three coplanar jet components. the results of all the PETRA groups three-gluon process is the lowest The observed frequency of these are in line with QCD. order QCD contribution to the decay events agrees with quantum chro- Another scene of QCD predictions of upsilons into hadrons, and is now modynamics (QCD) calculations on is still the decay of the upsilon into clearly distinguishable from a simple the emission of hard gluons. The hadrons, as measured by PLUTO at statistical phase space description. events are incompatible with pure DORIS before its move to PETRA. quark-antiquark formation and can While the neighbouring off-reson
Lepton/Photon highlights
The Lepton/Photon Symposium at CERN (see September issue, been scanty. Also from SPEAR held at Fermilab at the end of August page 249) qualified for inclusion in came news of a candidate pseudo- was dominated by the results of jet the Symposium because the experi scalar (spin zero, negative parity) analysis in PETRA electron-positron ment triggers on lepton pairs, and charmonium state at 2980 MeV. A annihilations (see above). However although no new information was previous pseudoscalar charmonium many other important results available, it was still a feature. candidate at 2820 MeV from DES\ emerged which were highlighted New results on neutrino-electron had not been confirmed at SPEAR by Leon Lederman in his summary scattering came from the fine and a new level is therefore welcome talk. grained calorimeter of a Virginia / (see September issue, page 246). PETRA, as well as giving the new Maryland / Oxford / Pekin collabo An unexpected contribution was evidence for gluon production, has ration. This is the first time that data the new signal seen by the CHARM also shown that quantum electro on this reaction has been available detector in a neutrino beam dump dynamics is as good as ever, even at from a large counter experiment, and experiment at CERN (see page 31 3). high energies. the results underline yet again the While others preferred to adopt a The continued non-appearance of success of the Weinberg-Salam cautious wait and see attitude, the sixth quark flavour at PETRA is model. Preliminary results on anti- Lederman was in more optimistic not yet a problem, but it is unfor neutrino-electron scattering were mood, pointing out that here might tunate that no firm indications of this also presented from the fine be the first signs of the tau quark mass are available from theo grained calorimeter of the CERN / neutrino. ry, unlike the intermediate bosons of Hamburg / Amsterdam / Rome / These Lepton/Photon Symposia weak interactions where confident Moscow ('CHARM') collaboration. started in a modest way at MIT in predictions can be made. Other new Charmed baryons have been re 1 963 to cover a specialized area of data from PETRA comes from two ported before, but now they have particle physics research, but have photon exchange mechanisms, been pinpointed in electron-positron now grown to cover the most excit while at SPEAR, the two photon collisions at SPEAR. These precision ing developments, as was demon process has been used to measure measurements could open up the strated by the Fermilab meeting. the eta prime lifetime. field of charmed baryon spectro The candidate beauty signal seen scopy, where information so far has
308 CERN Courier, October 1979