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Forerunner of millions to come. The first-ever Z decay, as seen by the UA1 detector at CERN's proton-antiproton collider in May 1983. Top, an high energy electron-positron pair, produced by the decay of a Z particle, emerges from the collision debris. Below, the clean 'lego plot' of the electron-positron pair.

A decade of heavy light

Ten years ago, in May 1983, the UA1 experiment led by Carlo Rubbia at CERN's proton-antiproton collider saw the first Z particle, the heavy (91 GeV) electrically neutral carrier of the weak force. The press announced the discovery of 'heavy light', a highly apt description which has unfortu­ nately fallen into disuse. The weak force comes in two varieties - one which permutes electric charges (the classic example being the beta decay of a neutron into a proton and an electron), and a neutral variant which does not. Each has its carrier particle, and both were discovered at CERN - first the charged W, in January 1983, and then the Z, a few months later. For both experiment and theory, the Z discovery was the culmination of a long and diligent quest without netic radiation. parallel in the history of modern Extending Maxwell's electromag­ physics. The missing piece of the netic unification (in quantum terms 'electroweak' jigsaw finally clicked the emission and absorption of into place, and for ever after electro- photons), the gauge theory ideas of magnetism would be firmly linked twentieth century physics culminated with the weak nuclear force. in the 1960s in the work by Sheldon It was the twentieth-century re­ Glashow, Abdus Salam and Steven make, with a much bigger cast, of the Weinberg which unified electromag- story which began in 1864 when netism with the weak force. This idea James Clerk Maxwell wrote down his had first been proposed in the 1930s four famous equations linking elec­ and had regularly resurfaced, but a tricity and magnetism. This was the successful conclusion had to wait birth of a new science - electromag- until all the necessary techniques the Gargamelle bubble chamber had netism. were firmly in place. jolted other particles in their wake. But Maxwell's equations suggested Until then, all weak interactions The Uncertainty Principle says that also that electromagnetic effects were seen to switch round electric the range of a force is inversely could be transmitted as waves charge, but the Glashow/Salam/ proportional to the mass of its carrier travelling at the speed of light. As Weinberg picture predicted a new particle. The photon, the carrier of well as light itself, a complete spec­ aspect to the weak force, the 'neutral the long range electromagnetic force, trum of wavelengths should exist. current'. In 1973 - exactly twenty is massless, but the W and Z 'radia­ Ten years later Heinrich Hertz' years ago (page 4) - this previously tion' of the short range weak force, famous experiment revealed a new, unseen mechanism was found at had to be heavy, so much so that invisible, component of electromag­ CERN. Neutrinos passing through they were out of reach of conven-

CERN Courier, May 1993 1 A constant driving force in CERN's antiproton project, from inception through scientific discoveries, was Carlo Rubbia, seen here at his historic1983 stamping ground at the UA1 detector. In 1989 he became CERN's Director General.

tional experiments. New techniques were needed, and the CERN proton- antiproton collider, with its huge detectors was the solution. A special session of the European Physical Society's International Conference on High Energy Physics in Geneva in 1979 marked CERN's 25th anniversary. Describing CERN science, CERN's Research Director- General at the time, Leon Van Hove, compared Hertz' discovery of electro­ magnetic radiation with what hun­ dreds of people at CERN were busy doing. 'All you have to do,' joked Van Hove, Is to replace Hertz' rudimen­ tary radiation emitter and coil detec­ tor with, respectively, the proton- antiproton collider now being constructed at CERN's 7-kilometre SPS proton synchrotron and the huge UA1 and UA2 detectors!' As well as embodying the difference between 19th-century and 20th- success, the proton-antiproton several hundred Zs, a large number century science, that big CERN achievements also prepared the when the total count at the two project also marked the dawn of a research community for the next proton-antiproton colliders at the time new era in particle physics. Building stage - colliding beam projects with was less than a thousand. Rather on Simon van der Meer's sugges­ even larger detectors, such as those than simply catching Zs, an electron- tions for beam cooling, it showed that for CERN's LEP electron-positron positron collider can be tuned to the beam gymnastics required for collider. sweep across the Z resonance. The new physics goals required careful From May 1983 to June 1989, high first glimpse of the Z profile at SLC planning and teamwork as well as energy proton-antiproton colliders suggested strongly that there are consummate skill. Physics experi­ were the only source of Zs. Fermilab only three neutrino decay channels ments broke new ground in sheer joined the hunt in 1985, when the big open for the Z. With its three quark scale and complexity, with hundreds CDF experiment captured its first pairs and three types of lepton, the of people involved in the design, events at the Tevatron collider. Standard Model looked to be capped. installation and operation of huge Meanwhile big new projects were Later that summer LEP turned on, detectors and the analysis of the taking shape. To cash in on and before the end of the year its recorded data. electroweak physics, CERN was four experiments - Aleph, Delphi, L3 With Van Hove's challenge met, the building its 27-kilometre LEP elec­ and Opal - had bagged some ten 1984 Nobel Prize for Physics was tron-positron collider, the world's first thousand Zs. The lid was sealed on awarded to Carlo Rubbia and Simon Z factory, while at Stanford the two- Standard Model particles. Since then van der Meer 'for their decisive mile linear accelerator was adapted LEP has not looked back, with the contributions to the large project to become the SLC - Stanford Linear current Z score 4.5 million. which led to the discovery of the field Collider - the world's first electron- Although out of the running for particles W and Z, the carriers of the positron linear collider. sheer numbers of Zs, the SLC has weak interaction'. In the summer of 1989 the up­ the so far unique ability to make Zs As well as being crowned with graded Mark II detector at SLC saw from polarized (spin oriented) beams,

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CERN Courier, May 1993 Ten years of searching preceded the discovery of the neutral current at CERN in 1973.

providing added value to the data sample. Z physics has come a long way since May 1983.

Thirty Years of Weak Neutral Currents!

Twenty years ago at CERN, a new form of interaction, the neutral current, was discovered. However for initial unsuccessful attempts to detect first quark mixing theory. His talk was the preceding ten years physicists WNC at Brookhaven and CERN in followed by George Snow (Maryland) already had been searching for the 1960s and the early search for recounting early data from hyperon variants of this interaction, so a FCWNC. decays and the then new Cabibbo symposium held on February 3-5 by The absence of strange quark model. the Pacific Ocean in Santa Monica, transitions set the stage for the In 1974 came a seminal paper on California reviewed a total of thirty introduction of a fourth quark charm by Ben Lee, Marie K. Gaillard years of neutral current research. ('charm') - the GIM mechanism - and and Jon Rosner. Two of the authors The meeting began with an over­ the subsequent emergence of the were at Santa Monica: Marie Gaillard view of the development of the Standard Model. (Berkeley) described a model of understanding of weak interactions The era of the WNC discovery in strong WW interactions, while Jon from the 1930s to 1950s. Laurie 1973 was described by science Rosner (Chicago) spoke on the Brown (Northwestern) led this discus­ historian Peter Galison (Harvard). current status of mass constraints on sion, which was followed by a tribute Dieter Haidt (DESY) represented the the sixth ('top') quark. to the milestone accomplishments of Gargamelle Collaboration at CERN The meeting then changed direction the late Ben Lee and J.J. Sakurai credited with the discovery, while AI to discuss the implications of neutral (UCLA). Mann (Penn), representing the currents in astrophysics. David In the Weak Neutral Currents Harvard-Penn-Wisconsin-Fermilab Schramm (Chicago) and James (WNC) discovery, neutrinos were (HPWF) collaboration, put the obser­ Wilson (Livermore) explained how seen to interact with target particles vations in the context of Fermilab's supernovae explode, while George but still continued on their way as appearance on the physics scene, Fuller of San Diego looked at how neutrinos. This was the first time that with a new detector in an unexplored supernova data can restrict electron- the weak interaction had revealed a neutrino energy range. neutrino/tau neutrino mixing. disdain for electric charge - previ­ Lively discussion between the For the arrival on the scene of ously all weak interactions had been audience and members of the proton-antiproton colliders and the seen to permute the electric charges Gargamelle and HPWF groups discovery of the W and Z particles at of the participating particles. It recalled the experiences of 1973. CERN by the team led by Carlo opened the door to new synthesis Paul Langacker (Penn) gave an Rubbia, Andy Sessler (Berkeley) and un understanding. overview - "The Five Phases of Weak gave a beautiful review of the history The discovery had followed a Neutral Currents." of colliding beams concepts from decade of careful search, in which Sid Bludman (Penn) described the invention (1957) to the idea of one major target had been Flavor first gauge theory of weak interac­ stochastic cooling by Simon Van der Changing Weak Neutral Currents tions as well as the success of the Meer (1968). A review and discus­ (FCWNC) - in which neutral current Weinberg-Salam-Glashow model. sion of the development of the interactions would be accompanied Nicola Cabibbo (Rome) described proton-antiproton colliders at by transitions of the strange quark. the early days of quark mixing, Fermilab and CERN and the impor­ David Cline (UCLA) looked at the recounting how he came to invent the tant discoveries at CERN was led by

4 CERN Courier, May 1993