John Mulvey
Willi Jentschke arrived at CERN to take over from Bernard Gregory as director-general at the start of 1971, a year of major developments for the laboratory and the beginning of a period of remarkable progress in particle physics. The developments included significant elements of an improvement programme for the CERN facilities formulated during Victor Weisskopf’s period as director-general and agreed by the member states at the end of 1965 with the aim of ensuring a world-class future for the laboratory. As a member of the CERN Scientific Policy Committee at the time, Jentschke had participated in the discussions defining this programme.
The most ambitious project was the construction of the Intersecting Storage Rings (ISR), a proton-proton (pp) collider fed by the CERN proton synchrotron (PS) and the first machine of its kind. The maximum proton energy reached about 30 GeV, giving a total energy in the collision centre of mass equivalent to a 2 TeV proton hitting a stationary one. Jentschke had been the first chairman of the ISR Experiments Committee, and just a few weeks after his arrival as director-general the first collisions were observed. Experiments began at an equivalent energy far in excess of the highest previously available – 70 GeV at Serpukhov – four months ahead of schedule and within the budgeted cost – a remarkable engineering achievement by Kjell Johnsen and his team. Other improvements included powerful new particle detectors, increased accelerator-beam intensity and improved neutrino beam, new experimental areas and infrastructure. As they came to fruition, these improvements would underlie many of the successes of the CERN experimental programme over the next decade and beyond.
But the Council of 1965 had postponed a decision on the biggest question. To ensure a future role as a leading participant in the advance of particle physics, Europe would need facilities competitive with those in the rest of the world and at that time the US was considering construction of a 200 GeV PS, a machine with much greater potential for discovery than the ISR, which was limited to the exploration of pp interactions only. In 1967 the US approved a 400 GeV PS, which came into operation at Fermilab in 1972. Discussions continued in the CERN Council, the European Committee for Future Accelerators (ECFA), and in the member states until, in December 1968, the Council commissioned John Adams to plan construction of a 300 GeV proton synchrotron and named him director-general designate of the new laboratory where it would be built. His crucial initiative, showing that the accelerator could be realized earlier and cheaper by placing it at Geneva and using the CERN PS as injector, ended long wrangling over the site of the new laboratory and the decision to go ahead was at last taken at a meeting of the CERN Council on 19 February 1971. So when Willi Jentschke made his first appearance before the Council at its mid-year meeting in June 1971, he did so as director-general of CERN I, the existing laboratory, and was joined on the platform by John Adams, director-general of CERN II, just across the road where the new Super Proton Synchrotron (SPS) was to be constructed.
Physics highlights
Jentschke’s annual reports to CERN Council are an excellent source for a brief review of a few research highlights of his time at CERN. In these reports, characteristically, he always endeavoured to make the significance of the discoveries made at CERN, and elsewhere, as accessible as possible to non-specialist Council members.
The early reports described some of the first results from ISR experiments measuring the pp total cross-sections and elastic scattering differential cross-sections. Qualitatively, the elastic-scattering could be described by an optical model of diffraction by a disc of radius about 10-15 m that was increasing slowly in size as the energy rose. The total cross-section, which had flattened off in the region of 70 GeV, was now observed to increase logarithmically up to the highest ISR energy reached, behaviour consistent with asymptotic Reggeon field theory.
The first discovery of fundamental significance – the conclusive identification of quarks as constituents of hadronic matter – came in 1972. It was an early result from the large heavy-liquid bubble chamber Gargamelle, in the new neutrino beam – both items in the 1965 improvement programme. Gargamelle’s unique design, radically different from any other bubble chamber before or since, was the brainchild of André Lagarrigue and his colleagues in Paris who had been members of the collaboration using the CERN 1.2 m heavy-liquid chamber in the first neutrino experiments at the CERN PS. They saw that the length of visible volume was crucial to maximizing the numbers of the rare neutrino interactions and the quality of the data obtainable from those observed. The result was a 12 m3 horizontal cylinder with eight portholes for the cameras, in two rows of four, giving a visible length of nearly 5 m. Its name is attributed to Louis Leprince-Ringuet who, on seeing the monster, was reminded of Gargamelle, the mother of the giant Gargantua in Rabelais’ novel.
Experiments at SLAC in 1968/9 investigating the scattering of 20 GeV electrons on protons had found a much larger than expected number of electrons emerging at large angles, an effect that could be understood if the initial step was the elastic scattering of an electron on a point-like constituent of the nucleon. The SLAC experiment, acting via the electromagnetic force, was able to show that the partons – so named by Feynman – possessed half-integer spin, but could not directly address the next question: did partons have the quark fractional charges? The Gargamelle experiment probed the nucleon with neutrinos and anti-neutrinos, acting via the effectively point-like weak force. The observation of total cross-sections rising linearly with energy then directly indicated an interaction with point-like constituents: partons again. By combining their neutrino with SLAC’s electron data, the Gargamelle collaboration was able to take the next step and show a perfect fit to all the data if nucleons contained three quarks with fractional charges. The data also revealed that the quarks carry only half the nucleon momentum, leaving the rest to be attributed to gluons, mediators of the strong force binding the quarks. As Jentschke remarked in his report: “The observations made in these two totally different approaches, taken together, provide an astonishingly detailed picture of the constitution of the nucleon.”
The first indications of point-like constituents seen via the strong interaction also came in 1972 at CERN, when ISR experiments observed high-momentum pions emerging at large angles from pp collisions. Although very rare, at the top ISR energy pions of 9 GeV/c transverse momentum occurred about 10,000 times more frequently than would be expected from a simple extrapolation of the behaviour at much lower transverse momenta. Later experiments at the ISR saw high-momentum transfer events associated with a back-to-back jet structure similar to that found in electron-positron annihilation at high energy.
Electroweak unification
1973 brought another discovery with profound significance for the understanding of the forces of nature, the most important result from CERN so far. By establishing the existence of a neutral current form of the weak interaction, the neutrino experiment in Gargamelle gave life to the Weinberg-Salam electroweak theory. This theory dates from 1967/8, but had received little serious attention until proved renormalizable by ‘t Hooft in 1971. The most direct and reliable evidence for a weak neutral current would be the observation of neutrino, or anti-neutrino, scattering from an electron. Gargamelle found one candidate in late 1972, but such events were extremely rare so the group turned to investigate neutrino-induced hadronic events with no sign of the emerging muon necessary in the charged-current interaction. Candidates for such events were seen, but their interpretation as evidence for the neutral current suffered from the difficulty of ensuring that a statistically significant number could not be ascribed to the background of neutrons produced by interactions of the neutrino beam in the material ahead of the visible volume. Eventually the group was satisfied that the background problem was understood and with 165 muon-less events, six times the estimated background, the first evidence pointing to the existence of a weak neutral current was published in the summer of 1973.
The violation of charge-parity (CP) conservation in the decay of the K0 meson, a long-standing puzzle, was the subject of a high statistics electronic experiment completed in 1975. In one of the earliest applications of the multiwire proportional chamber (MWPC) technique, developed at CERN by Georges Charpak and his group, this experiment collected data on some 5 x 109 K0 decays, orders of magnitude greater than any previous such experiment, and made very precise measurements of the parameters describing the CP violation mechanism. The results gave strong support to the “super-weak” theory. The MWPC technique revolutionized electronic methods of particle detection, was crucial to the success of experiments at particle colliders and earned Charpak the Nobel Prize for Physics in 1992.
One of the most elegant, most ingenious and fundamental experiments conducted at CERN returned to the experimental floor for the third time in 1974, about six years after its previous visit, with new apparatus and aiming for greater precision. The muon (g-2) experiment measures the anomalous magnetic moment of the muon and provides by far the most precise test of the validity of quantum electrodynamics (QED). The result obtained for the quantity a, the magnitude of the anomaly, was 0.001165924 with an error of 0.000000009; the QED prediction, calculated to the sixth order in e and including a hadronic, strong interaction contribution of 67 x 10-9, was 0.001165921 with an error of 8 x 10-9. The extraordinary precision achieved in this experiment allowed the existence of the hadronic vacuum polarization to be demonstrated for the first time, and at the five standard deviation level. Further muon (g-2) experiments of yet greater precision have since been performed at Brookhaven involving several of the same group.
From 19 February 1971, CERN was in the novel position of having two director-generals. One had responsibility for ensuring the ability of Lab. I to deliver front-line, world-class physics by exploiting the powerful facilities of the improvement programme. The other had direct responsibility for Lab. II and the successful design, construction and first operation of the largest particle accelerator in the world, but was dependent on Lab. I for such critical elements as the injection system – the CERN PS – and much of the basic physical and administrative infrastructure. This seems a situation fraught with potential for damaging friction and inefficiency. In fact, the relationship between the director-generals and their teams – mostly well known to each other – was remarkably effective; regular joint meetings of the two directorates dealt with common concerns. Adams, invariably calm and pragmatic, scrupulously observed the boundaries to his domain and responsibilities; Jentschke was generally content to delegate most administrative and organizational details to his senior staff – his prime interests lay with the physics and what the physicists were doing. Above all, both director-generals and their teams were totally dedicated to making a success of CERN. The joint efforts worked well and by 1976 the SPS came into operation on time, within budget and at 400 GeV – a great tribute to all.
There was, of course, a significant impact on Lab. I. In addition to all the savings inherent in siting the SPS at CERN, Geneva, rather than at a new location, the budgets foreseen for the Lab. I programme were reduced as a contribution to the overall SPS costs. Lab. I was also to bear the costs of any upgrading of the PS, and certain other preparations for the SPS experimental programme. In his report to Council in December 1971, Jentschke estimated a reduction in the sums available for research, compared to previous plans, of about 25% up to 1974. Possible measures, such as limitations on bubble-chamber pictures, longer machine shutdowns and slower implementation of improvements were being considered with the help of advice from an ECFA working group, the Chairmen of the Experiments Committees and CERN’s scientific policy committee. Difficulties experienced by the member states’ economies in the early 1970s exacerbated matters, making the budget an annually recurring problem. The need to find savings was made more difficult by the success of CERN in attracting visiting physicists from the member states and elsewhere to use its outstanding facilities. The number of visitors in 1972 was about four times that in 1966 – far outnumbering the CERN staff physicists – and still rising steadily, so increasing demands on services and space. But Jentschke always insisted that, as CERN’s raison d’être, the users were to be encouraged and supported as far as was possible.
A man of great personal warmth and charm, Jentschke often sought the latest news and views from physicists in the coffee bar and corridors, weighing what he learned against other reports he’d heard. The biggest scientific event during his tenure was the discovery of the weak neutral current. At the time of the publication of the Gargamelle results, in August 1973, it was known that missing-muon events had recently been observed in a neutrino experiment performed by the Harvard, Princeton, Wisconsin and Fermilab (HPWF) group using a massive counter and spark-chamber detector at the new Fermilab 400 GeV accelerator. However, in the late autumn of 1973 HPWF found that new data, taken after changes intended to improve their detector’s muon identification efficiency, appeared no longer to contain significant numbers of muon-less neutrino events. The HPWF group then prepared a paper announcing the failure to see evidence of neutrino-induced missing-muon events to replace their still unpublished first, and positive, result. Not surprisingly this caused some consternation when the news reached CERN, awakening doubts among those sceptical of the existence of neutral currents and in some, including a few in the Gargamelle collaboration, who were not certain that the attribution of the muon-less events to the neutrinos rather than neutron background was reliable.
Hearing these doubts about a discovery perhaps leading, for the first time in the 110 years since Maxwell, to the unification of two fundamental forces of nature, Jentschke naturally felt he should himself take steps to understand the evidence in some detail. By arrangement, and taking two colleagues, he joined three of the Gargamelle group around a table for a tutorial on their data and the all-important question of the neutron background. After discussing the exhaustive analyses performed by the group over several months to reach a reliable estimate of the background, and seeing the clear evidence against neutrons as a significant source of the muon-less events for which a crucial factor was the long length of Lagarrigue’s chamber – about six interaction mean-free-paths for neutrons – Jentschke left fully satisfied that the neutral current discovery was sound and highlighted it in his presentation to Council in December. Meanwhile, at Fermilab, the HPWF group had taken the advice of Bob Wilson, the director, not to publish the negative result before doing more checks on their data. Early in 1974 HPWF published their results in complete agreement with Gargamelle and the Weinberg-Salam theory. Independent confirmation soon followed from three other Fermilab experiments, and Gargamelle had by then accumulated three neutrino scatters on electrons.
Preparing for the future
Given the long, ten years or more, timescale for the planning and implementation of major new facilities for experimental particle physics, one of the important responsibilities of a CERN director-general is to prepare for the future. So starting in 1974, although the SPS was not yet completed, Jentschke encouraged the examination of accelerators that might possibly follow it and so keep CERN in the forefront of world advances. Groups of physicists and accelerator experts began to look at the physics interest and technical feasibility of a variety of machines, including configurations that might be achieved as extensions of the existing base. Examples considered included a 10 TeV proton accelerator, large storage rings for a 400 GeV pp and antiproton-proton (pbarp) collider, and colliding 20 GeV electrons with 400 GeV protons in the SPS. These studies, involving ECFA as well, continued through 1975 and by 1976 developments in physics focused interest on a 100 GeV electron-positron collider, LEP. The same year, Carlo Rubbia came to CERN with his proposal to use the SPS as a 270 GeV storage ring to study pbarp collisions, a possibility made feasible by using the stochastic-cooling mechanism invented by Simon van der Meer at CERN in 1972 to obtain beams of high enough intensity. In 1983, experiments on the SPS operating as a pbarp collider completed the story of the weak neutral current and established the electroweak theory by discovering both the Zº boson, mediator of the neutral current, and the W+ and W– bosons of the charged current. Rubbia and van der Meer shared the 1984 Nobel Prize for Physics.
The two-laboratory structure had worked during SPS construction but it was clear that on its completion in 1976, and as it became the leading facility in the research programme, CERN should return to being one laboratory. To allow time for the new arrangements to be worked out, Jentschke, whose initial appointment had been for four years, was asked to continue as director-general of Lab. I until the end of 1975. This he agreed to do, although it meant one more year living away from his wife, who had remained at home in Hamburg, and so seeing her only at weekends. In March 1975, CERN Council decided that the laboratories would be united at the beginning of 1976, but again under two director-generals: Leon van Hove as director-general for research and John Adams as executive director-general responsible for the administration and running of the laboratory, as well as the completion and early operation of the SPS.
When Willi Jentschke arrived at CERN few could have visualized the dramatic transformations in the landscape of particle physics that would occur by the time he left at the end of 1975. Quarks, from being possibly little more than a convenient representation for the systematics of hadron structure, had become accepted as real physical entities, the primary constituents of hadronic matter. A gauge invariant, calculable, quantum field theory uniting electromagnetism and the weak interaction had received convincing quantitative support from experiment. Equally remarkable, there was for the first time a plausible, candidate theory for the strong force binding quarks to form nucleons, and nucleons in the nucleus: quantum chromodynamics. These are the foundations of the extraordinarily successful, albeit as yet incomplete, “Standard Model” of particle physics. During Jentschke’s years, CERN played, as it continues to play, a leading part in opening this new landscape to view.
John Mulvey, University of Oxford.
