LEP dominates LP-HEP Janet Carter of Cambridge and Opal - Preci­ sion tests of the Standard Model with LEP. CERN's LEP electron-positron col­ lider was the star of this year's ma­ jor physics meeting - the Joint In­ ternational Lepton-Photon Sympo­ sium and Europhysics Conference on High Energy Physics (LP-HEP) - held in Geneva from 25 July - 1 August. All major results so far from LEP, and there are plenty of them, are in accord with the Standard Model of physics - a dual picture with the el- ectroweak unification of the elec­ tromagnetic and weak nuclear forces on one hand coupled with the quantum chromodynamics (QCD) field theory of inter-quark forces on the other. Summarizing the meeting, CERN Director General Carlo Rubbia pointed out the need to probe this picture in as much detail as pos­ sible. Far from being a closed book, the Standard Model covers a lot of uncharted territory, with the long-awaited sixth ('top') quark and the neutrino sector still being 'terra incognita', while the spontaneous symmetry breaking ('Higgs') me­ chanism at the heart of the elec- troweak unification, and the details of QCD dynamics, are still far from clear. LEP apart, the meeting reflected the continuing reluctance of the neutrino to give up all its secrets. The long-standing difficulty of pin­ ning down the delicate violation of combined particle/antiparticle 'beauty') quark, and the contribu­ Closing the meeting, Sheldon left/right switching (CP) symmetry, tions which could be made with the Glashow of Harvard highlighted the known for more than a quarter of a next generation of proton colliders. changing face of physics. For a century but still not understood, In their keynote talks on the final century, researchers had been used was also a talking point. For both day, Rubbia and neutrino summariz- to a steady stream of surprises to neutrino and CP-violation physics, er Rudolf Mossbauer surmised that keep them on their toes and stimu­ insights from new areas are eagerly the lower energy region, relying on late thinking. According to Gla­ awaited. reactors and the Sun, would supply show, the last major particle phy­ With CP violation effects so far the bulk of new neutrino informa­ sics surprise dates back to 1977, confined to the neutral kaon sector, tion. Mossbauer also looked for­ with the discovery of the fifth Rubbia pointed out the need for ad­ ward to the advent of cryogenic quark through the upsilon particle ditional CP violation investigations detectors to open up new neutrino at Fermilab. using B mesons (carrying the fifth fields of study. 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Janet Carter opened the plenary sessions, setting the tone with a mass of precision data on the Z resonance, together with di­ rectional effects in lepton pair pro­ duction, tau spin orientation, charged hadron production in gen­ eral and B meson production in particular all supporting the Stan­ dard Model and sharpening knowl­ edge of specific parameters. The consistency of this informa­ tion, combined with precision measurements from other experi­ ments, provides powerful limits on the top quark, now confidently pre­ dicted to live somewhere between 120 and 160 GeV. One suggestion of a non-stan­ dard effect had come recently from Aleph, where the level of electron- positron annihilations into two tau leptons accompanied by a pair of charged particles was significantly higher than the analogous channels with two muons and with two elec­ trons. In a parallel session, Sau-Lan Wu of Aleph revealed that the latest data do not continue this trend, which moreover is not seen by Al­ eph and Opal at LEP, nor by the Mark II detector at Stanford's SLC Linear Collider (S. Wagner). Another non-standard hint had ers - LHC at CERN. and the US Su­ to summarize results from all come from L3 (Juan Alcaraz) in the perconducting Supercollider (SSC) around the ring. parallels, where high energy pho­ - will surely reach beyond the For the subsequent plenaries, tons accompanying an electron-po­ Standard Model, declared Glashow, the LEP presentations featured a sitron pair invited an explanation. but until then the task is to exploit speaker from each of the four ex­ Standard Model theorists were un­ and test the existing picture. periments - Janet Carter of Cam­ convinced. Nowhere is this happening better bridge and Opal (Precision tests of Speaking on LEP QCD results, than at LEP. The four big experi­ the Standard Model), Thomas Heb­ Hebbeker pointed out the advan­ ments - Aleph, Delphi, L3 and Opal beker of Aachen and L3 (QCD tages of LEP for QCD measure­ - at CERN's big ring were deftly studies), Michel Davier of Orsay ments - high energy, high precision treated at the meeting, with parallel and Aleph (Searches for new parti­ and relative freedom from hadronic session speakers using their own cles), and Patrick Roudeau of Orsay masking (fragmentation) effects. results as pointers but being careful and Delphi (Heavy flavour physics). The coupling strength of quark in- 4 CERN Courier, September 1991 Rudolf Mossbauer -15 major neutrino ques­ tions still to be answered. gy upgrade. Also preoccupied with Higgs were John Ellis of CERN and Graham Ross of Oxford, investigat­ ing possible Higgs dynamics in talks on the Status of the Electro- weak Sector and Beyond the Stan­ dard Model respectively. Looking at heavy quark produc­ tion at LEP, Roudeau showed how b quarks are a powerful probe of directional effects (forward-back­ ward asymmetry), although ac­ count has to be taken of B particle mixing. Charmed particles too are also a good source of Standard Model information and are begin­ ning to be studied, while the para­ meters of B mesons and tau lep- tons now benefit from LEP data. Guido Martinelli of Rome des­ cribed the status of QCD, showing how precision is improving and how more and more processes are becoming amenable to calculation. In this difficult work, approxima­ tions are always attractive, but Martinelli remarked that perhaps teractions (alpha-s) is measured in but highly important, mechanism. only the unseen sixth quark is hea­ both Z hadronic decays and in the Whatever it might be, and there is vy enough to use this approxima­ production of collimated 'jets' of precious little indication, LEP exper­ tion confidently. hadrons. Comparison of the pro­ iments have now ruled out a lot of Neutrino sessions can always be duction rates of particles containing lower energy territory, and the counted on for controversy. With heavy and light quarks shows that Higgs must be heavier than 57 reports of 17 keV neutrinos from alpha-s is independent of quark fla­ GeV. 'The Higgs hunt is now on in several experiments (John Simp­ vour, while the variation of jet pro­ earnest,' affirmed Davier. son, Guelph) in conflict with a duction shows how alpha-s is 'run­ When the top quark is finally range of null results, the neutrino ning' with energy.