1995 EPS HEPP PRIZE - SPECIAL EPS PRIZE

Gluon Discovery Honoured Fig. 1 - Illustrations showing the narrow bundles (Jets) of particles with unusual The 1995 EPS High Energy and (HEPP) Prize has been awarded to topologies produced in the fragmentation Paul Söding, DESY-Institute of High-Energy Physics, , Björn Wiik and of a -antiquark pair and a . Günther Wolf, Deutsches Elektronen- (DESY), , and Sau Lan Wu, Left: jet broadening; right: three-jet event. University of Wisconsin, Madison, Wi, USA, for the first evidence for three-jet events in e+e' collisions at DESY’s PETRA collider while working with the TASSO Collaboration. DESY’s DORIS collider taken around the A Special EPS Prize has been awarded by the EPS Executive Committee to the JADE, ypsilon resonances, where the predicted MARK-J, PLUTO, and TASSO Collaborations, Deutsches Elektronen-Synchrotron three-gluon decay of the spin-1 bound (DESY), Hamburg, each responsible for one of the four detectors initially installed at states of b-bbar should be different from the DESY's PETRA collider whose results independently confirmed the gluon’s existence. quark-antiquark final state of the continuum. However, the final state here had a collision The , the most powerful became more pronounced [Phys. Rev. Lett. energy of only 10 GeV to share among the force we know, binds together the 35 (1975) 1609], where sphericity, defined final-state particles, so the jets of hadrons from which , neutrons and all other as S = (3/2) min ZpT2/Zp2, is essentially were not easily visible. fundamental particles are made. It is so the sum of the transverse momenta pT of strong that it is impossible to produce a particles measured relative to an event axis Theory Gave Some Guidance single, unbounded quark or antiquark (the which is varied in order to minimize S. quarks and antiquarks are “glued”). Field The PLUTO Collaboration at DESY’s are required in any field theory for theory tells us that all forces are transmitted DORIS accelerator subsequently extended the strong interaction, and quantum chro­ by carrier particles, called gauge bosons, this analysis to 9.5 GeV. Fragmentation modynamics (QCD) predicts gluon emission such as the for the electromagnetic models describing how the primary partons by one of the quarks (bremsstrahlung) force and the massless gluon for the strong (quarks) transformed into jets of hadrons analogous to photon emission by force. Whereas can travel through­ were now available, and the Collaboration in the field theory of quantum electro­ out the whole of space, gluons are only able showed that jet axes defined by either dynamics. It was therefore well-known to move within a region 10-15 m in radius charged or neutral particles were the same. before PETRA experiments that QCD pre­ — the typical size of protons and neutrons, As a function of the scattering angle 9, dicted an increase of gluon bremsstrahlung the building blocks of Nature. the jets followed a (1 + cos20) distribution at high energy [see, e.g., J. Ellis et al., Nucl. The 1995 HEPP Prize was awarded by associated with the spin-1/2 of the quarks. Phys. B 111 (1976) 253; A. De Rujula et al., the EPS HEPP Division to four individuals ibid 138 (1978) 387]. Ellis et al. in fact working with TASSO Collaboration at DESY, Search Underway proposed that gluons should manifest them­ Hamburg for “the first evidence of three-jet selves as jets, and that planar events and events at PETRA”. This contribution stimu­ The deep-inelastic scattering experiments even three-jet topologies (illustrated in Fig. lated an enormous analytical effort by all at SLAC in the late-1960s (recognized by 1) should be observable at the higher four PETRA Collaborations that led to the the 1990 Nobel Prize in Physics) and the energies which would be available. How­ confirmation of the gluon’s existence in identification of partons with quarks together ever, the formation of three-jet events was several independent ways. The Special with subsequent neutrino scattering expe­ regarded as very speculative at the time. It Prize awarded by the Executive Committee riments led one to expect that the structure- was not understood how gluons would of EPS to the four Collaborations acknow­ function integral describing the momentum fragment, a first assumption being that ledges this major contribution. distribution of the quarks should equal unity, gluons would reveal themselves by i.e., generating jets of particles not very different Jets Identified from those for quarks. It was a US group from the Stanford Linear Accelerator Center and the Lawrence since the total fractional momentum Berkeley Laboratory which first showed that summed over all constituents is unity (Fe2N for e+e_ annihilation in the 3-7.4 GeV energy and Fv2N are the structure factors for elec­ range, the final-state hadrons (particles with tron-nucleon and neutrino-nucleon scatter­ the strong interaction) emitted in collisions ing, respectively). But it turned out that the organize themselves into two preferred contribution from scattering via the directions, i.e., into two bundles of particles ’ electric and weak charges only called jets (Fig. 1). This phenomena could accounted for one-half of the observed be seen as a decrease of sphericity S with momentum. So it was known that quarks did increasing energy as the jet structure not carry all of the momentum of the in which they were imprisoned, and the PETRA existence of another type of constituent PETRA is the name of the 2.3 km storage besides the quarks, namely gluons, was ring at DESY which, under the leadership postulated. J.G.H. de Groot et al. [Phys. G. A. Voss, started to operate at 13 GeV in Lett. B 82 (1979) 292 and 456] then de­ the centre-of-mass in November 1978, in­ monstrated excellent agreement between creasing in energy to 27 GeV in the spring data on high-energy neutrino interactions Fig. 2 - The total fractional momentum of of 1979. Four experiments were installed in and QCD predictions for the structure all nucleon constituents as a function of the the initial phase: JADE, PLUTO, MARK-J, functions F2 and F3 based on the gluon momentum transfer squared for neutrino and TASSO. German government approval hypothesis (see Fig. 2). scattering as obtained from QCD fits to the for constructing PETRA was obtained while The search for gluons, the proposed structure functions F2 and F3. It can be H. Schopper was Chairman of the DESY mediators of the strong interaction, was seen that the gluons carry more than 50% Directorate. begun in 1979 by PLUTO using data from of the momentum. 88 Europhys. News 26(1995) Fig. 3 - Histograms of the number of events plotted as a function of the square of the transverse momentum PT for momentum components perpendicular to (‘‘out’’) and in the plane of (“in”) the event plane for e+e--collisions at PETRA as reported by the TASSO Collaboration in June 1979. The results (curves) of Monte Carlo statistical fluctuations of q-qbar jets. The simulations assuming only two-jet events accurately planar events exhibit three axes, the aver­ predicted the “out” distribution (top panel) but not age transverse momentum of the hadrons the “in” distribution (middle panel). For the latter one with respect to these axes being 0.30 needed three-jet events (bottom panel) for an accurate GeV/c... The data are most naturally ex­ prediction. plained by hard noncollinear bremsstrah­ lung... (of gluons)”. such as sphericity, triplicity, thrust, planarity, The early data clearly suffered from low aplanarity, oblateness, and the JADE jet­ statistics, and subsequent textbook repre­ finding algorithm. sentations of hard gluon emission in clear- All four Collaborations reported data at cut events with well-separated jets did not the 1979 Lepton-Photon Conference (held accumulate very rapidly. Nonetheless, the at the Fermi National Accelerator Labo­ first such events indicating a well-resolved ratory, USA) at the end of August 1979 three-jet topology stimulated much interest. In the summer of 1978 G. Wolf, in showing convincing independent evidence Since the amount of data reported in the describing the PETRA’s programme at a for an admixture of planar-event topologies first presentations was limited it was very summer school at Les Houches, pointed out which fitted well to a three-jet topology (pre­ important to see if an unusual topology that going further up in energy was of great sentations were made by S. Orito for JADE, could be produced in simulations of two-jet interest because one can then “differentiate H.B. Newman for MARK-J, Ch. Berger for events. So the rather sophisticated analyses between the ‘naive’ jet picture, whereby jet PLUTO, and G. Wolf for TASSO). A session of jet broadening carried out from the members have a fixed transverse momen­ summary by E. Lohrmann stressed the beginning by the Collaboration represented tum with respect to the jet axis, from expec­ deviation from rotational symmetry around an important step. The planarity of the tations based on QCD according to which the jet axis at the highest PETRA energy, events clearly pointed to an underlying gluon emission leads to a widening of the jet i.e., a developing planar structure, as evi­ three-particle process, but careful Monte with increasing centre-of-mass energy". dence for gluon bremsstrahlung. Carlo calculations were needed to rule out While all four Collaborations were very the possibility that these topologies could be A Photofinish close in obtaining and evaluating data, faked by, for instance, heavy-quark decays. TASSO was able to report interesting re­ Such calculations were included in the early PETRA was commissioned very quickly sults in June 1979. It had developed early TASSO analyses (and eventually by all the under the supervision of Gustav-Adolf Voss on a method to analyze three-jet events Collaborations in their analyses). The 1995 and gave, in the early summer of 1979, [Wu & Zobemigj along with Monte Carlo HEPP Prize acknowledges TASSO’s careful sufficient luminosity at high energy to supply models to simulate the hadronization effects study of planar events. The special Prize experiments with data on the production of of the quarks and gluons. It became clear honours the four Collaborations’ major on­ hadrons. The bulk of the hadronic events that the effects of QCD were large — energy going efforts. showed the typical two-jet pattern mani­ was more important than statistics — and festing quark-antiquark pair production. At as soon as PETRA reached energies well Understanding the Strong Interaction the higher PETRA energies, the jets above 20 GeV in the centre of mass, became more collimated than at lower At the 1978 Les Houches summer school, TASSO immediately saw the footprints of B. Wiik in a talk on the PETRA experiments, energies and were fairly easy to see. This QCD in its data. allowed a more efficient study of jet when touching upon the issue of QCD, The TASSO Collaboration disclosed re­ pointed out that observing three-jet events broadening as well as a search for three-jet sults indicating planar events in June 1979 patterns that would provide direct evidence did not prove QCD since such a feature at conferences in Bergen (presented by B. arose in any field theory of the strong for gluon emission by one of the quarks. Wiik) and Geneva (presented by P. Söding) However, since the energy region opened interaction. To test QCD additional infor­ which were confirmed on the basis of mation was required about the gluon, up a few years earlier by SLAC’s SPEAR improved statistics at the Fermilab confe­ and DESY's DORIS colliders had shown namely its vector nature (spin 1), flavour, rence. Analyses of the planar events de­ and non-abelian nature (i.e., the existence so many surprises, attention was focussed monstrated that they could not be fitted with more on possible new discoveries (e.g., to- of gluon-gluon coupling). His best bet at the only two jets: one of the jets was broader time for such studies were top-antitop ponium, open-top production, new charge- than the other indicating an abnormal 1/3 quarks, heavy leptons) as anything resonances (so-called toponium), where transverse momentum structure in the event one hoped for the existence of sufficiently could reveal itself in the vast energy region plane. The evidence, referring mainly to the which was now accessible. heavy toponium giving clear jet signatures tail of the distribution of the square of the in the decays. The four Collaborations lined up in the transverse momentum PT, which was pre­ race to be first to find evidence for new It turned out that the decays of the ypsilon sented in Geneva is shown in Fig. 3. On the states were difficult to analyze, and that phenomena. PLUTO had a certain ad­ other hand, selected planar events could be vantage at the start since the apparatus had toponium did not show up. So it took some accurately fitted by three-jet axes, where all time before ypsilon decays could be used to been running at DORIS and was already jets now showed similar and normal frag­ well tested. MARK-J and JADE had a more demonstrate the gluon, and indeed the mentation behaviour. TASSO submitted a vector nature of the gluon [PLUTO Collabo­ complete coverage than TASSO, including publication detailing the work on 29 August neutral-particle detection capabilities, but ration, Phys. Lett B. 88 (1979) 119 & DESY [Phys. Lett. B 86 (1979) 243], slightly before Rep. 80-117 (1980); LENA Collaboration, one needed to understand fully how their the other collaborations submitted their own calorimeters worked. The TASSO Collabo­ DESY Rep. 81-008 (1981)]. Among the reports. It described a planar event rate PETRA experiments, TASSO and PLUTO ration was still in the process of completing “which is well above the rate computed for the calorimeter and hadron identification both presented evidence for the vector systems. In order to have an early start, the FURTHER READING nature of the gluon [Phys. Lett. B 97 (1980) detector had been moved into the beams 453, 459 respectively]. The gluon self­ with only the tracking and triggering systems “Experimental evidence on QCD” by P. Söding coupling, showing the gluon’s non-abelian & G. Wolf, Ann. Rev. of Nucl. & Particle nature, was much harder to observe and operational and TASSO could therefore Science 31 (1981). concentrate on charged-particle detection. “Review of e+e- physics at PETRA” by P. required another energy upgrade, in fact an­ Novel methods for topology analysis were Duinker, Rev. Mod. Phys. 54 (1982). other collider, namely CERN’s LEP machine “e+e- physics at PETRA - The first five years” that started taking data in 1989. developed by groups working with all four by Sau Lan Wu, Phys. Rep. 107 (1984). detectors, and one had to get used to words G. Jarlskog,Chairman, HEPP Division Europhys. News 26 (1995) 89