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Orsay Tau Workshop Was Situation Is Far from Clear page 3). The RFQ has superseded linear collider ideas aired in reports of the annular coupled structure, a the large high-voltage systems pre­ from SLAC (Stanford), CERN, KEK new coupled-cell configuration. viously necessary in ion accelerator (Japan), and Novosibirsk. Prototyping and construction of injection systems and makes com­ The larger, more exotic and some parts of the JHP linac is un­ pact linacs possible, even for a more expensive these machines derway at KEK. space-based system. Exhibited at become, the more the engineering Linac technology has taken root the conference was the 1 MeV must depart from cut-and-try to ac­ in several fields outside of the par­ RFQ successfully tested aboard a curate design. Thus another tech­ ticle physics laboratories. One new rocket in a Los Alamos defence nological advance is in computer community with more than 16 pro­ technology project. design codes. The newer codes jects represented at Linac90 and Among many other advanced can handle r.f. cavities, beam op­ surveyed by C. Pellegrini (UCLA) is RFQ projects heard at Linac90 tics elements, and beam dynamics developing the free electron laser, were the 250 mA unit at CERN re­ with greater accuracy. This has requiring a high intensity electron li­ ported by M. Weiss; the 100% been important for RFQ and super­ nac. One example, reported by P. duty-factor 75 mA RFQ at Chalk conducting cavity development, O'Shea (Los Alamos), will run 300 River, reported by G. McMichael; and design of any high perfor­ A at 40 MeV. and a variable-energy RFQ for hea­ mance machine. R. Cooper (Los Al­ In possible futuristic applica­ vy ions described by A. Schempp amos) reported on the new and tions, for example the accelerator from Frankfurt. An innovative specialized field of accelerator de­ transmutation of radioactive waste feature which J. Watson said will sign codes, which recently had its or tritium production schemes be tried in front of the RFQ for the second conference. sketched by G. Lawrence (Los Ala­ proposed US Superconducting Su­ Several papers, such as J. mos), a high premium is placed on percollider (SSC) is a helical quadru- Warner's from CERN or Y. Yama- linac power efficiency. I. Hofmann pole. At Argonne, work is under­ zaki's from KEK, indicated that new from GSI (Darmstadt) and R. Ban- way towards a superconducting linac structures could be on the gerter from Berkeley, among oth­ RFQ. way. Since a big advance may be ers, reported on current develop­ Large future accelerator projects necessary for TeV linear colliders, ment of r.f. and induction linacs to will also lean towards supercon­ people at some major Laboratories drive fusion reactors. In his sum­ ducting radiofrequency cavities as are thinking about entirely new mi­ mary P. Grand (Brookhaven) specu­ a design option. The largest scale crowave structures. Linear collider lated whether civilization's need foi application to date is the ATLAS projects at CERN (CLIC - Novem­ energy might eventually become heavy-ion machine at Argonne, ber, page 5) and in Japan (JLC) are the prime force for development of described by K. Shepard, but of looking in this direction, as re­ linac technology. course the real showpiece will be ported by G. Guignard and H. Mat- The main organizers of Linac90 the CEBAF machine with recirculat­ sumoto respectively, with the latter were Stan Schriber and Bob Harde- ing linacs now being built at New­ having achieved accelerating gra­ kopf from Los Alamos. port News, Virginia, where cavities dients of 85 MV/m in a 0.6 m have achieved high fields (Septem­ tank. T. Weiland reported that From Olin van Dyck ber/October, page 43). Summariz­ DESY and Darmstadt are thinking ing the field, H. Padamsee (Cornell) about a 500 GeV (total) electron- reported that about 70 metres of positron linear collider using refine­ superconducting cavities have been ments of existing technology. ORSAY built so far at various institutions, Apparently closer to realization either for development or real use is the Japanese Hadron Project, the Tau workshop (November, page 20). subject of several reports at the From 24-27 September 120 physi­ To reach accelerating gradients conference. The linac stage of this cists met at the French Orsay Lab­ ten times higher may require some machine will deliver a 1 GeV beam; oratory for a meeting devoted to other technological development among the advanced design the tau lepton and its associated such as r.f. pulse compression (No­ features will probably be perma­ neutrino. vember, page 5). New ideas such nent magnet quadrupoles in the The tau was discovered in 1975 as this play a key role in the 1 TeV drift-tube section and possibly use by Martin Perl and his collaborators 22 CERN Courier, December 1990 Tau lepton pioneer Martin Perl (centre) at the recent tau physics workshop at the French Or say Laboratory, flanked by workshop organizers Michel Davier (Orsay Director, right) and Bernard Jean-Marie. their Z profiles from tau pairs and their forward-backward production asymmetries, all measurements in accord with the Standard Model. Aleph and Delphi have looked at tau polarization, with Aleph data suggesting parity violation in both tau production and decay. To­ gether, the L£P results test the 'universality' (family independence) of lepton coupling to the neutral current with ten times better preci­ sion than before. For the future, the Beijing BEPC ring should provide a better meas­ urement of the tau mass, but LEP and lower energy machines should provide the bulk of new tau data. working with the Mark I detector at collaboration at DESY succeeds in High luminosity electron-positron the SPEAR electron-positron ring at arriving at a coherent description rings are being proposed to pro­ Stanford. This unexpected discove­ without any problems, and a pre­ vide bulk data in relatively un- ry heralded a scenario of three fam­ liminary appraisal by the Aleph ex­ probed corners. B-factories geared ilies of leptons and quarks, sub­ periment at LEP, looking at the de­ to look at CP violation are not op­ sequently confirmed by the discov­ cays of Z particles into tau pairs timized for tau physics, although ery of the fifth kind of quark at Fer- and exploiting the detector's pho­ they should open up some new milab in 1977. ton reconstruction and identifica­ reaction channels. A tau-charm fac­ The existence of a third family tion capabilities, confirms these re­ tory would be better adapted for allows the violation of CP symme­ sults. Continuation of these studies tau physics and a design is on the try to be accommodated in (but not and new information from the Cleo drawing board at CERN (but not ne­ explained by) the Standard Model. II detector at Cornell's CESR ring cessarily for construction on the While the sixth ('top') quark has should tidy up the tau decay pic­ CERN site). yet to show itself, experiments at ture. The final topic at the workshop high energy electron-positron col­ In this area, two results were was the tau neutrino. Mass liders have shown that there is presented by the Argus experiment measurements of this elusive parti­ room in Nature for only three at DESY - a high precision meas­ cle are difficult and are attempting quark-lepton families. urement of the Michel parameter to go below the present limit of 35 The Orsay meeting was the first for leptonic tau decays in accord MeV. Tau neutrino interactions are to concentrate exclusively on tau with theory, and the first observa­ difficult to catch, and drive ideas physics, covering from the early tion of parity violation in the decay for new precision detectors. days at SPEAR and at the DORIS of a tau into three pions and a neu­ Overall, the tau is an excellent ring at DESY through to the latest trino, providing another incisive testbed for the Standard Model, results from LEP. probe of the underlying theory. and could be the scene of import­ After an introduction from tau Despite these new results, tau in­ ant new revelations in the coming pioneer Martin Perl, the meeting formation is still meagre compared years. surveyed tau decays, where the with, say, the wealth of data in the The Orsay tau workshop was situation is far from clear. Missing muon sector. organized by Michel Davier, decays could be due to systematic The meeting went on to look at Director of Orsay's Linear Acceler­ experimental underestimations, or the coupling of the tau to the neu­ ator Laboratory, and Bernard to new physics, so clarification is tral current of weak interactions. Jean-Marie. important. The four LEP experiments - Aleph, A global analysis by the Cello Delphi, L3 and Opal - presented From Michel Davier CERN Courier, December 1990 23 .
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