tics (a few 107 events per year), the later stage (with a finite crossing tau and charm workshop indicated that the key angle) to attain luminosities beyond element of future precision measure­ 1033. Another option is to reduce the The Standard Model of particle ments will not be statistics but sys­ collision energy spread to 0.1 MeV physics has six quarks, grouped in tematic errors. Here the unique using a monochromator optics, which three pairs (up/down, charm/strange, environment of the TCF, with its is important for resonance and thresh­ top/beauty), each pair being backgrounds that are both small and old running. Longitudinal beam polari­ partnered with a lepton and its experimentally measurable, is likely zation is also possible for the future. corresponding neutrino - respec­ to prove a decisive advantage. The detector concept is technically tively electron, muon, and tau. Among the most challenging TCF sound and incorporates broad Probing the Standard Model in depth goals are: measurement of a possibly experience from previous detectors. to see what makes it work means finite tau-neutrino mass with a At the workshop, test beam results peering into all quark/lepton corners. sensitivity of about 1 MeV, measure­ were presented from several groups While B physics, with its potential at ments of CP-violation asymmetries in which confirmed the design perform­ proton and electron-positron ma­ the decays of D (charmed) mesons ance of certain novel aspects of the chines, is being pushed hard (see and hyperons, precision measure­ detector. These included the tests of previous article), other physicists ments of the space-time structure of the longitudinally segmented cesium underline the need for complemen­ tau decay, and a comprehensive iodide calorimeter by US groups and tary information from other sectors. study of light quark, gluon and hybrid the Valencia tests of 6m-long scintil­ Essential experimental tools for spectroscopy. lating fibre time-of-flight counters exploring out-of-the-ordinary particles There was considerable discussion which achieve 120 ps. A detailed are a Tau-Charm Factory and a at Marbella about new calculations of simulation of the detector has been Beauty Factory. These machines direct CP-violation in D decays. prepared by the Seville group which address similar basic questions in the Seeing CP violation with these will be used to study TCF physics Standard Model, but in complemen­ relatively short-lived particles has performance in detail. A proto­ tary ways: the Beauty Factory is traditionally been written off, but new collaboration representing about optimized for beauty particles and Rome/Naples calculations suggest 20 institutes from 9 countries has CP violation in B decays; and the that this highly constrained sector formed to prepare by mid 1994 a TCF is optimized for the tau lepton, should be worth investigating. proposal for the Tau-Charm Factory. charm particles, and the The calculations indicate decay rate spectroscopy of hidden charm states asymmetries at the 103 level, close and light hadrons. to the TCF's experimental reach. In early June about 100 physicists - In the light meson spectroscopy theorists, experimentalists and sector there was a consensus among Bubbling away accelerator physicists - from Europe the participants that with the detail of and beyond gathered in Marbella, information available from the huge Bubble chambers may have almost Spain, for the 3rd Workshop on the data samples (more than 1010 J/psi vanished from the front line of Tau-Charm Factory (TCF). The decays per year), in an optimized physics research, but the vivid workshop aimed to reassess the detector, a definitive understanding memory of their intricate and some­ TCF physics potential in the light of of the full picture of light quark-, times beautiful patterns of particle recent progress, to develop further hybrid-, and glueball-states should be tracks lives on, and has greatly the designs of the machine and the possible. influenced the computer graphics of detector, and to discuss the experi­ The TCF double storage ring can track reconstruction in today's big mental programme. achieve luminosities of 1033 cm2 s1 experiments. 'Seeing' an interaction At the workshop, particular empha­ with a fairly conservative design makes it more understandable. sis was placed on the prospects for based on head-on collisions, reason­ Bubble chambers, with their big tau and charm physics at current able beam currents, moderate heat collaborations of physicists from machines and at future B Factories. load on vacuum chambers, etc. many widely scattered research Whereas both a BF and a TCF Ideas were explored at the workshop institutes, started another ball rolling. generate large tau and charm statis­ for reducing the bunch spacing at a The groups formed are even now

18 CERN Courier, October 1993 In July, about 130 physicists gathered at Bubble chamber pioneer Donald Glaser - CERN to commemorate the 40th anniversary 'more than watching beer bubble rise up the of the invention of the bubble chamber. side of the glass'. Here participants pose with in front of CERN's two giant bubble chambers - BEBC (centre, with its piston, right) and Gargamelle (left) - now permanently mounted outside the Microcosm exhibition centre. (Photo CERN H124.7.93)

rily at hadron physics, and contrib­ uted to the resonance bonanza of the late 50s and early 60s. Attention then shifted to big chambers for neutrino only surpassed in size by the big recognized in US Laboratories like physics. Later still, they were com­ collaborations working on today's Berkeley and Brookhaven, and soon bined with electronic counters to form major detectors at colliding beam spread to Europe. 'hybrid' spectrometers, or were machines. Bubble chambers covered a wide complemented with electronic From 14-16 July, about 130 physi­ range of technology - complex 'fences' to intercept outgoing muons. cists gathered at CERN to com­ cryogenic installations to handle The wealth of physics contributions memorate the 40th anniversary of the hydrogen, deuterium and neon; large covered at the meeting included high invention of the bubble chamber by and powerful superconducting energy interactions, stable particles, Donald Glaser. The meeting, organ­ magnets cooled by liquid helium; meson resonances, baryon ized by Derek C. Colley from fisheye windows for stereoscopic resonances, and production pro­ Birmingham, gave a comprehensive viewing; advanced holography; cesses, a major milestone having overview of bubble chamber contri­ complex expansion mechanisms; been the historic discovery of the butions to physics, their challenging rapid-cycling expansion; data omega minus at Brookhaven in 1964. technology, and the usefulness of processing; replay machines; sepa­ In weak interactions, many people bubble chamber photographs in rated beams These develop­ point to the vital neutrino physics education, both for physics and the ments brought economic feedback contributions of bubble chambers in public at large. and industrial spinoff. Even a special­ the development of new understand­ After opening remarks by CERN ized conference could not do justice ing, culminating in today's Standard Director Carlo Rubbia, Donald Glaser to all the challenges that were met, Model picture of electroweak pro­ began with a brief review of the work such as innovative expansion sys­ cesses and quark interactions. which led to his invention - there was tems and exotic liquids like xenon. The weak interaction survey began much more to it than idly watching During their lifetime, bubble cham­ with surveys of kaon decays and of beer bubbles rise up the wall of the bers grew in size from the few cubic charmed particles, with the glass - before turning to his present centimetres of the early prototypes to Gargamelle chamber's discovery of line of research, biophysics, also very giants, 35 cubic metres in volume, neutral currents providing a natural visually oriented. but with their accompanying infra­ focus (see following story). After its invention, the usefulness of structure much larger. Historian John Krige's view on the the bubble chamber was quickly Early chambers were aimed prima­ contribution of bubble chambers to

CERN Courier, October 1993 19 John Mulvey - bubble chamber legacy.

The 20th anniversary of this event positron collider is geared to the was marked at CERN on 29 April and Z particle, the carrier of the neutral in the US from 3-5 February (May, current. Neutral current measure­ page 4), and it was natural that it ments are a primary tool for probing should be commemorated also at the the Standard Model to reveal any Paris Ecole Polytechnique, scene of shortcomings. the birth of André Lagarrigue's idea Neutral current behaviour is less for the Gargamelle heavy liquid susceptible than the charged current bubble chamber, the key element in to uncertainties in quark decay the discovery. Thus a major meeting parameters and to hadronic effects. 'Neutral Currents, 20 years on' was Astrophysics too has benefited from held 6-9 July in the institute's former the increased understanding which premises in Paris. has followed from neutrino interac­ 180 physicists from throughout the tions at accelerators, whether it be world took part in the five days of for neutrinos from the sun or from discussions on the role and the more distant sources. European scientific collaboration importance of neutral current transi­ As well as proudly surveying the sparked a lively discussion, while the tions in particle physics, astrophysics enormous progress made in the last impressive legacy of bubble cham­ and atomic physics. Among them twenty years, the meeting could look bers for today's physics was de­ were many from Europe and the US forward to the enormous potential scribed by John Mulvey. who had taken part in the often this physics still offers. The meeting concluding with heated 1973 debates. One day was thoughtful remarks by Victor given over to recalling the underlying Weisskopf (see page 23). theoretical understanding and the As well as the sessions, the meet­ contributions from pioneer experi­ Participants at the 'Neutral currents - 20 years ing was a wonderful opportunity for ments. on' meeting held in July in the old premises of participants to meet old friends and While neutral current measure­ Paris Ecole Polytechnique. It was here that the colleagues. ments are difficult in atomic physics, Gargamelle heavy liquid bubble chamber the impact on the particle physics project was launched, the tool which made the discovery possible. front is impressive. The entire pro­ gramme at CERN's LEP electron- (Photo Ecole Polytechnique) Neutral currents, 20 years on

On 19 July 1973, the late Paul Musset took the microphone in CERN's main auditorium to an­ nounce the existence of electroweak interactions mediated by 'neutral currents'. For the first time, the weak interaction had been seen to operate without permuting electric charges. The Gargamelle bubble chamber collaboration which Musset repre­ sented had measured the rate for these transitions in interactions with neutrinos and antineutrinos. It was one of CERN's major scientific achievements.

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CERN Courier, October 1993