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Stanford Lepton- Symposium

Stanford Lepton-Hhoton Symposium sum- marizer Frank Sciulli of Columbia looked for• ward to an 'era of discovery' in the 1990s.

With CERN's new LEP electron- come from the big experiments at positron collider poised to make its the proton-antiproton colliders at physics debut (September, page 1), CERN and Fermilab. (The particles the physics at the 14th Inter• were discovered at the CERN col• national Symposium on Lepton and lider in 1983.) Here, the Ws and Zs Photon Interactions, held at Stan• are fingerprinted through decays ford from 7-12 August, featured a producing easily identifiable leptons ripple of new results on the Z and (electrons and ). Their de• W bosons, the carriers of respect• cays into strongly interacting parti• ively the electrically neutral and cles () are difficult to sepa• :harged components of the weak rate from the mainstream proton- nuclear force. antiproton collision products. How• These new W and Z results, ever these hadronic decays in fact combined with refined measure• dominate the Mark II Z sample. ments in other sectors, have inter• Alan Weinstein of Caltech showed esting implications for expected how this provides a new window but as yet unseen particles, notably on underlying mechanisms. the sixth ('top') quark. Clearly this will be a feature of the The Mark II detector at Stan• physics emerging over the next ford's SLC linear collider has made few months. its initial scan of the Z, the first The Mark II Z sample shows a time this particle has been seen in bit of scatter among the three lep• electron-positron annihilations ton channels (electron, and (September, page 12). By the time tau). For instance 15 tau pairs are of the Stanford meeting, Mark ll's seen when only about half this sample of 106 Z events had climbed to 233, and Gary Feldman reported a mass centred around 91.17 GeV and a width of 1.95 eV, giving an upper limit for the possible number of different types of neutrino as 4.4 and a value as low as 3 ± 0.9. In other words, there is very little room for more types of neutrino in Nature other than the three currently known - electron-, muon-, and tau-type. However the limit still gives a bit of additional neutrino elbow room and it will take a lot of Zs before a fourth type of neutrino can be ruled out. (The Mark II limit has been subsequently tightened - more news next month.) Until the arrival of the SLC, all di• rect W and Z information had

A scan of the Z particle, the electrically neu• tral carrier of the weak nuclear force, as seen by the Mark II detector working at the SLC Stanford Linear Collider. The curve is a ' Fit', using five , three charged leptons and three neutrinos.

CERN Courier, October 1989

A good fix on the Z particle by the Mark II detector at Stanford's SLC linear collider was reported by Gary Feldman. number are expected, but this is the fifth ('beauty'-b) quark behaves being played down. like one of a pair. With big data samples from their Once thought to be just round recent lengthy runs (September, the corner, the stubborn• page 13), improved Z measure• ly refuses to come out into the ments came from the UA2 (Antho• open. Strict limits from CDF (Pekka ny Weidberg, CERN) and CDF (My• Sinervo, Pennsylvania) put it above ron Campbell, Chicago) experi• 80 GeV, however consistency ar• ments at CERN and Fermilab guments using the new Z mass va• pspectively. With 132 decays into lues together with data from preci• nuon pairs and 64 into electrons, sion studies of neutrino scattering the latter had already reported a Z (results reported by Jaap Panman mass of 90.9 ± 0.3 GeV. At Stan• of CERN) and other electroweak in• ford, UA2 gave a Z mass of 90.5 put now suggest that the top quark ± 0.5 from an 85-event sample. could be considerably heavier than These values complement the new 100 GeV. 'This is depressing news fix from SLC. if it is taken seriously', commented However these proton-antipro- Conference summarizer Frank Sciul- ton collider experiments also home li of Columbia, intimating that some in on the W, the electrically calculations and predictions might charged partner of the Z, produced need to be overhauled to take ac• in pairs in particle-antiparticle anni• count of a heavyweight quark. hilations. Although the W is slightly production levels at lighter than the Z, W pairs are out TRISTAN show a slight surge of reach of SLC and LEP, at least around 60 GeV, and this had once for the time being. New W mass been a talking point. Thoughts of values announced at Stanford were an early top have now been dis• and from Tokyo's Institute for Nu• 80.0 ± 0.4 from 1185 UA2 ev• pelled, even though the surge is clear Study (less than 11 eV). Ac• ents and 80.2 ± 0.2 from CDF ev- still there. cording to some observers, these its with electrons. Results from However the top quark is being new numbers must be approaching the CDF muon sample should fol• squeezed from all sides. Guido Al- the absolute limit with which the low soon. tarelli of CERN explained how an neutrino mass can be measured di• Until LEP's is boosted, upper limit of around 200 GeV for rectly. proton-antiproton studies have a the missing quark comes from tak• There are still not enough neutri• monopoly on the W, and new ing account of the subtle radiative nos seen coming from the sun (the measurements will provide a useful corrections to electroweak effects. 'solar neutrino problem'). This complement to the new Z informa• Combined with precision electro• long-standing enigma had shown tion. weak measurements, this cramps signs last year of resolving itself, With just three types of neutrino the space remaining for the top with bursts of activity reported looking increasingly probable, an quark. from new solar neutrino measure• underlying physics picture of six ments at both the Japanese Kamio- types of quark grouped pairwise kande underground detector and Lepton sector into three 'generations' continues from the classic study led by Ray to sell well. Particles containing five Davies of Brookhaven (October types of quark are known, and ex• At Stanford, lepton (weakly in• 1988, page 2). However this year periments at the TRISTAN elec• teracting particle) properties were the Kamiokande neutrino levels tie tron-positron collider at the Japa• covered by Marty Perl of SLAC, in with the previous enigmatic lev• nese KEK Laboratory (Akihiro Maki, who had news of fresh limits on els. Further solar neutrino informa• KEK), although not seeing the sixth the mass of the electron-type neu• tion will come from a new genera• ('top') quark at collision trino from experiments at Los Ala• tion of experiments being prepared exceeding 60 GeV, suggest that mos (less than 13.4 electron volts) using special detector materials.

CERN Courier, October 1989 3 A talking point. CP violation parameter (ver• tical axis) as measured by the NA31 experi• ment at CERN and the E731 experiment at Fermilab (preliminary result only), compared with the theoretically expected value, which depends on the mass of the so-far unseen sixth ('top') quark.

In inter-quark transformations, ARGUS at the DORIS ring at the German DESY Laboratory in Ham• burg (Michael Danilov, Moscow) and CLEO at Cornell's CESR ring (David Kreinick) could both report that in analysis of decays (containing the b quark) around the broad (4S) upSilon resonance, the level of transformations of b quark to light ('up') quarks was about ter per cent that of beauty to charm. This is the level needed to accom• modate CP violation in a six-quark picture. Several years ago an initial ARGUS measurement had seen an anomalously large effect. Like the neutral , the neu-

Stanford Symposium

In keeping with the tradition Singly-charged particle decay at CERN reported a new slant on of the Lepton-Photon Sympo• modes of the heavy tau lepton are CP violation (July/August 1988, sia, the Stanford meeting in• not seen quite as frequently as ex• page 7), with the decays of short• cluded only plenary sessions, pected, but Perl speculated as to lived and long-lived neutral kaons making for simpler coverage how this 'Gilman anomaly' might into charged and neutral pairs but with a single speaker on be fixed up. In looking to build up showing a small but significant each topic there is an ever- an accurate picture of the tau, Perl bias. This result was reiterated at present risk of overindulgent underlined the need for a Stanford by Daniel Fournier of Or- subjectivity. tau/charm factory to mass produce say, however the E731 study at In addition to a solid scien• this rare particle (July/August, Fermilab, reported by Bruce Win- tific content and good partici• page 29). stein of Chicago, sees a much pation, the imagination of the smaller effect in a preliminary sam• Stanford organizers resulted ple of 20 per cent of the data col• in a number of innovations, lected in a recent long run. The including TV retransmission data should be completely analysed of talks to a nearby room Twenty-five years after the dis• next year. In the meantime NA31 is with lots of desk space and a covery of the subtle violation in also analysing new data and is zoom lens making over• neutral decays of the com• busy collecting even more with crowded transparencies ea• bined 'CP' symmetry of particle- CERN's SPS sier to follow. A boon for antiparticle switching and left-right running for fixed-target experi• avid notetakers. reversal, the origin of this effect re• ments in parallel with feeding elec• The Symposium dinner in mains a mystery, and new trons and positrons to LEP. Sum- the Monterey Aquarium is measurements of additional CP-vio• marizer Sciulli was unfazed by the beyond the scope of this re• lation parameters are needed to apparent disagreement, but pointed port, but was an experience provide fresh insights. out the implications for the mass of few delegates will forget. Last year, the NA31 experiment the top quark.

4 CERN Courier, October 198£

Confined sprays ('jets') of hadrons at the CDF detector working at the Fermilab Teva- tron proton-antiproton collider extend out to transverse energies of 400 GeV.

sharply-defined showers ('jets') of produced hadrons, which for the CDF experiment extend out to 450 GeV of transverse momentum.

New machines

Had his talk been scheduled just a few days later, Friedrich Dydak of CERN would have been able to il• lustrate his LEP progress report with some examples of LEP Zs (September, page 1). Mass produc• tion of Zs at LEP should get under• way later this year. News of initial operation of another electron-positron collider came from Ye Minghan of Beijing. The 240-metre BEPC ring at the Chinese Institute of High Energy Physics is well on its way to attain• ing its intended collision rate, hav• ing exceeded luminosities of 1030 per sq cm per s, and Ye was op• timistic that 1031 could be sur• tral B mesons are expected to mix, Feltesse of Saclay, the structure passed. The BES detector has and ARGUS and CLEO see compa• function measurements from differ• started work in the ring, and al• tible signals for B mesons contain• ent experiments are now compati• though initial magnetic interference ing the light 'down' quark. The ble, and looked a few years into between the ring and the detector UA1 experiment at CERN's proton- the future to new structure function reduced the available electron-posi antiproton collider homes in on B measurements at the HERA elec• tron collision rate, 50,000 J/psi mixing by comparing levels of like- tron-proton collider now nearing particles have been reconstructed. and unlike-sign charged muon completion at DESY, opening up Looking to the electron-positron pairs. At these energies, B mesons new kinematical areas. collider future, Gustav-A. Voss of containing strange quarks can also Another structure function re• DESY reminded the audience of be intercepted, and with a lot more sult, initiated by spin-dependent in• ongoing plans tabled for LEP and data now available, Karsten Eggert vestigations by the European Muon SLC. While LEP will push for in• (CERN) reviewed the evidence for B Collaboration, throws new light on creased energy using supercon• mixing in the strange-quark sector. how the spin of the proton is car• ducting radiofrequency cavities and Last year, the analysis of quark ried by its constituent quarks (June increased collision rates using interactions had been confused by 1988, page 9), with apparently lit• beam separators, SLC's plan in• differences in the quark structure of tle spin being carried by the val• cludes increasing the rate from 60 protons (structure functions) as ence quarks. Graham Ross of Ox• to 120 Hz and boosting the num• measured in different experiments ford looked into this problem, but ber of bunches handled. This could at CERN using muon beams and a did not think that it implied the im• increase luminosity tenfold over the hydrogen target. This year, a re- mediate downfall of quark-gluon present 1.6 x 1028 figure. Polarized analysis at Stanford (SLAC) of ex• field theory (quantum chromody- beams also figure high on the isting data from electron beam ex• namics - QCD). agenda. Longer term investments periments has helped to resolve Impressive evidence for QCD is in rebuilding the damping rings and the confusion. According to Joel now contained in the spectra of the final focus could push the lumi-

6 CERN Courier, October 1989 Conferencing

Traditionally, the biennial issue). This double heading (even-year) international means that physicists have a 'Rochester' choice of venue, but has led meetings alternate in odd- to some conflict of interests

Gustav-A. Voss of DESY - 'most de• number years with the Lep- and a good deal of repetition. pressed' by the support for the R and D ef• ton-Photon Symposia, anoth• Next year's Rochester fort for new machine ideas. er IUPAP line, and the inter• meeting will be held in Singa• (Photos Harvey Lynch) national 'Europhysics' meet• pore from 2-8 August (details ing organized by the European page 29), but as an experi• Physical Society. ment, 1991 will feature a This year saw the Lepton- single major meeting, to be Phot on Symposium held in held in Geneva from 23 July Stanford from 6-13 August to 1 August. In contrast with back-to-back with the Euro- the Lepton-Photon Sympo• physics meeting in Madrid sium tradition of plenary talks from 6-13 September (a re• only, this meeting will also in• port on the Madrid meeting clude parallel sessions. will feature in a forthcoming

'most depressed' by the lack of petition, collaboration and camer- current research and development aderie', and with many new ma• effort, apparently limited more by chines in the pipeline, Sciulli looked funding than by ideas. forward to an 'era of discovery'. Concluding his summary, Sciulli hoped that the final decade of the century would reveal the limitations behind today's Standard Model. Omnipotent yet stuffed full of un• knowns that have to be plugged in Of fish and physicists on Cannery Row - the from experiment, the Standard Stanford Symposium programme included a Model can only be the crust of memorable evening at the Monterey something deeper. With the tradi• Aquarium

nosity towards 1030. tional mix of particle physics 'com- (Photo Harvey Lynch) Voss was sceptical of ideas for an additional ring, for electrons and positrons, in the 84-kilometre tun• nel to be built in Texas for the US Superconducting Supercollider (SSC) proton-proton machine. Al• though benefiting from a prebuilt tunnel, such an electron-positron collider would cost a lot of money, calling for its own distributed su• perconducting acceleration system, and would yield only a modest markup on the energy available from LEP. However a separate el• ectron ring might be worth looking at to provide the SSC with an elec• tron-proton option, commented Voss. Looking further ahead, Voss drew attention to the difficulties in opening up a new electron energy domain. 'All the exotic ideas of the past 20 years have evaporated', he declared, and admitted to feeling

CERN Courier, October 1989 7