Munich conference

Don Perkins - Standard Model, triumph or frustration?

be, at least by its own standards, remarkably well behaved. Measurements of the mass of the electron-type neutrino, the ligh­ test, are coming in from Moscow, Zurich, Los Alamos, Munich, Tokyo and Beijing. While most studies prefer to quote limits (as high as 30 and as low as 18 electronvolts), the Moscow (Institute for Theoreti­ cal and Experimental Physics) group underlines its longstanding positive result (26+6-5 eV). Perkins was pessimistic about the chances of any such experiment being able to rule out a neutrino mass below 10 eV. Other experiments show that the next neutrino, the muon-type, has to be lighter than about 285 keV, while the heaviest known neutrino, the tau, looks to weigh in at less than 35 MeV. While the search goes on for double beta decay without any ac­ companying neutrinos, the past year has seen the sighting of a more conventional double beta de­ cay (January/February issue, page The Standard Model has survived Major conference summarizers 32). Elsewhere in the neutrino sec­ mtact for another year/ declared have got used to singing the tor, evidence for 'oscillations' be­ Don Perkins of Oxford, summariz­ praises of the Standard Model, but tween different neutrino types has ing the 24th International Confer­ this year at Munich even detailed long been sought, using both ter­ ence on High Energy Physics held examination failed to reveal any restrial and extra terrestrial parti­ in Munich from 4-10 August. 'But serious cracks, while looking deep­ cles. At Munich, new evidence is this a triumph or a frustration for er into physics even some anoma­ from the experiment at the French physics?' he added. lous results hinting at gaps in Bugey reactor now rules out oscil­ The twin pillars of the Standard understanding have either gone lations, tying in with the negative Model, the electroweak unification away or have diminished credibility. results from other experiments at of electromagnetism and the weak reactors and particle accelerators. nuclear force, and the field theory With extraterrestrial neutrinos, (quantum chromodynamics) of the Neutrinos earlier this year the Japanese Kam- quark-gluon interactions respons­ ioka experiment had announced a ible for the strong nuclear force , Given the job of summarizing 'non- muon neutrino signal substantially have not trembled since the elec­ accelerator' experiments, Yoji Tot- less than expected (May issue, troweak unification went to the suka of Tokyo had to cover a lot of page 29), suggesting oscillation ef­ textbooks in 1983, but from time material presented in the earlier pa­ fects. However this is not con­ to time small cracks have appeared rallel sessions. The enigmatic neu­ firmed by the Frejus experiment which might have gone on to shake trino has long been the joker in the (France) or from preliminary results the theory severely, if not under­ particle physics pack, but the latest from the 1MB (USA) study. mine it. crop of neutrino results shows it to Covered by Totsuka in both the

CERN Courier, October 1988 1 Is there any longer a problem with solar neu­ trinos ? For a long time, the only experiment monitoring neutrinos from the sun (led by R. Davis using a tank of chlorine-based absor­ ber in a US mine) saw less particles than ex­ pected from confident estimates of solar neutrino activity. However a recent run (right) gives a sharply higher neutrino level, more in line with the predicted value. The dotted line shows sunspot activity - is the correlation superficial or is this a physics message ?.

Heavy quarks

The present picture of particle phy­ sics is based on three families of quark doublets - 'up' and 'down', 'strange' and 'charm', and 'beauty' (or 'bottom', b) and 'top' (t). All these quarks ere known and now well studied, with the exception of top, but even here a series of limit have been charted fixing where it will turn up. The most reliable limit comes from the level of hadronic (strongly interacting) particles seen in elec­ tron-positron annihilations. The world's highest energy fully opera­ tional electron-positron collider is TRISTAN at the Japanese KEK Lab­ oratory. Here the collision energy has gradually been nudged up­ wards since the machine turned on in November 1986. At Munich, plenary speaker Tsuneyoshi Kamae of Tokyo showed how the hadron level shows no unexpected rise at the collision energies reached so plenary and parallel sessions was solar neutrinos could tie in with far (up to about 28 GeV per beam), the 'solar neutrino problem'. An sunspot activity. although the data points push tan- underground experiment led by Ray Participants at the conference talizingly close to the allowed limit Davis of Brookhaven for a long had been intrigued by press reports if the top threshold has not been time reported that the level of neu­ of a new force (the so-called 'fifth reached. trinos reaching a detector from the force'), suggesting that the pull of Another handle on top produc­ sun was only a fraction of what is gravity might depend on the com­ tion comes from the detailed shape expected. However in a recent run, position of a body as well as its analysis of the 'jets' of produced the level has increased to be al­ overall mass. Totsuka also echoed hadrons, showing that TRISTAN is most compatible with expecta­ the sentiments of C. Stubbs of still in territory inhabited only by tions. In parallel, independent solar Washington, one of the experi­ five sorts of quark. Next top limit neutrino results have started to menters in this area, who pointed (41 GeV) comes from the UA1 de­ come in from Kamioka, which, ac­ out the difficulties in reconciling re­ tector at CERN's proton-antiproton cording to Totsuka, are 'compati­ sults from different, and frequently collider, followed by about 60 GeV ble' with the more recent Davis highly ingeneous, experiments, from the observed 'mixing' of elec­ findings. In the meantime, a big concluding that 'no reasonable phe- trically neutral B mesons (contain­ push continues to prepare new so­ nomenological picture can account ing b quarks). After an initial report lar neutrino detectors (see for ex­ for all the data,' and that although on this mixing from UA1 in 1986, ample May 1987 issue, page 26) there was 'no compelling evidence more information came last year and additional results in this sector for new physics', there was still from the ARGUS experiment using are eagerly awaited. some room left to manoeuvre be­ the DORIS ring at the German Several speakers pointed to sug­ fore a null result could be con­ DESY Laboratory in Hamburg. The gestions that the measured level of cluded. level of neutral B mixing looked to

2 CERN Courier, October 1988 At the opening of the plenary sessions of the International Conference on High Energy Physics in Munich on 8 August. Left to right, Organizing Committee Chairman G. Busch- horn, Bavarian State Government Minister for Science and Culture W. Wild, CERN Di­ rector General Herwig Schopper and CERN Council President J. Rembser.

(Photos Uni Munchen P.M. Schmidt) be surprisingly large. At Munich, this result was confirmed by the CLEO group working at Cornell's CESR electron-positron collider. The mixing of the neutral B me­ sons is indeed huge/ commented Henning Schroder of DESY, in his report on the spectroscopy and de­ cays of heavy quarks. An upper limit for the mass of articles containing the top quark comes from comparison of Stan­ dard Model parameters coming from different approaches. The particles have to be lighter than about 250 GeV, and with some hopeful assumptions the limit can be clawed down to 180 GeV. This means that top particles could be within reach of the pro- ton-antiproton colliders at CERN and Fermilab. The latter has higher collision energy on its side (1800 GeV compared with CERN's routine 630 GeV), but CERN is pushing hard to boost the collision rate (lu­ deterred, he looked at what a few many specialists. After a further re­ minosity). 'We could find top by assumptions could do for predic­ port from ARGUS in a Munich pa­ next year, lei: alone the next tions in this sector. rallel session underlining their ear­ Rochester Conference, com­ Putting undiscovered particles lier findings, David Kreinick of Cor­ mented Perkins. aside, Schroder underlined the im­ nell described a 'diligent' search at The impressive results from pro- portance of measurements on par­ the CLEO detector at CESR which ton-antiproton collision physics at ticles containing the b quark. Over failed to find any evidence for CERN and Fermilab were summar­ twenty years ago, it was discov­ charmless B transitions - 'clear di­ ized at Munich by Melvin Shochet ered that electrically neutral kaons sagreement'. In his plenary talk, of Chicago, staying close to the have no respect for the classical in- Schroder called for more data to findings of the recent proton-anti- variance principle of particle/anti- resolve this 'discrepancy', looking proton physics workshop at Fermi­ particle permutation combined with particularly for simpler B decays lab (September issue page 4). left/right reversal. This 'CP viola­ more easily interpreted theoretical­ Besides the top quark, the other tion' is still not understood, but can ly. missing ingredient of the Standard be accommodated into a six quark In his talk on CP violation and re­ Model is the 'Higgs' mechanism re­ model. For this Cabibbo/Kobaya- lated matters, Konrad Kleinknecht sponsible for the delicate symme­ shi/Maskawa scheme to be right, B of Mainz stressed the need to find try breaking driving the electro- particles (containing the b quark) charmless B decays, pointing out weak unification. 'The Higgs is the should be able to decay into parti­ also that the long-awaited top most arbitrary part of the Model,' cles containing lighter quarks with­ quark should also decay predomi­ said Standard Model summarizer out passing through an interme­ nantly into the b variety. He pre­ Paul Langacker of DESY. 'The only diate stage involving charm quarks. sented the latest CP violation re­ thing that can be said with com­ Last year, the ARGUS experi­ sults from the NA31 experiment at plete certainty is that the mass of ment reported such 'charmless' B CERN (July/August issue, page 7). the Higgs particle, if it exists, must decay, (September 1987, page 4) Earlier Hitoshi Yamamoto of Fermi­ be between zero and infinity!' Un­ but the level of the signal surprised lab had sketched the Fermilab

CERN Courier, October 1988 3 At Munich, CERN Research Director John Thresher covered the impressive progress being made at CERN for the 27 kilometre LEP electron-positron collider and its four big experiments. Here the Aleph detector starts to take shape 150 metres below ground. In front and on either side of the assembly is the shielding where the LEP beams will enter the detector.

(Photo CERN 210.6.88)

E731 experiment also aiming for a precision fix on CP violation. After a substantial period of data-taking, initial results could emerge before the end of the year. Plenary speaker Roland Wind- molders of CERN, echoing what emerged from the parallel sessions, reported on the latest measure­ ments from CERN, Fermilab and Stanford of the distributions of quarks and gluons inside nucleons (structure functions). While many results from different experiments appear to converge, certain struc­ ture functions measured at CERN using muon beams by the Euro­ pean Muon Collaboration (EMC) and the Bologna/CERN/Dubna/Mu- nich/Saclay group do not agree. This makes problems for calcula­ tions using these structure func­ tions as input. Ongoing studies at CERN and Fermilab could help re­ solve the problem, but Perkins re­ marked 'it is a shame that after ten years of work there are such clear discrepancies'. hadrons. Earlier Steve Olson of the Rembser sketched the world scen­ Debuting at a big conference AMY experiment at TRISTAN had ario of major machine projects, was the EMC measurement show­ drawn attention to 'dramatic' ef­ calling for a maximum of inter­ ing that between them the consti­ fects in the distribution of the lead­ national collaboration. In special tuent quarks of the proton appear ing particles in jets produced by sessions, John Thresher of CERN to carry almost none of the parti­ gluons. described the impressive progress cle's intrinsic spin angular momen­ If enough energy is available, the being made with the 27 kilometre tum (June issue, page 9). quarks and gluons locked inside LEP electron-positron collider, and Structure functions provide input nucleons should eventually fuse its four big experiments, with a po­ to quark calculations, summarized into a quark-gluon plasma. Seeing sitron beam having been steered by Keith Ellis of Fermilab. While be­ hints of this plasma is one of the round the first completed octant of moaning the disagreement be­ aims of the experiments at CERN the ring (September issue, page 7). tween structure function measure­ and Brookhaven using high energy LEP is well on course for first col­ ments, Ellis was able to summarize nuclear beams. Louis Kluberg of liding beams next summer. 'a bumper year' of calculations the Ecole Polytechnique, Paris, sur­ After LEP, it was the turn of which help to explain many of the veyed the results so far and the at­ Bjorn Wiik of DESY to cover the details of particle production. One tempts to unravel possible quark- status of the HERA electron-proton outstanding problem was to ex­ gluon plasma signals. collider. Injection into the com­ plain the EMC quark spin result. pleted electron ring was imminent Additional high energy input is (see page 20), and the first octant Machines coming in from the production of of the 6.3 kilometre superconduct­ hadron jets at TRISTAN, throwing ing proton ring is scheduled for further light on the way produced Introducing the Munich plenary ses­ testing next spring en route to first quarks and gluons materialize as sions, CERN Council President J. colliding beams in October 1990.

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CERN Courier, October 1988 7