Simon van der Meer - ingenious accelerator ideas.

phase space - a combination of or­ Simon van der Meer dinary space and momentum - and retires their working domain, or accep­ tance, is defined by a phase volume. Secondary particles, such CERN's big Auditorium was packed as antiprotons, produced when a on Friday 23 November for a 'Si­ primary beam hits a target, are mon van der Meer Feest' to mark spread out over a wide phase the formal retirement of the gifted volume, and handling them effi­ Dutch accelerator physicist who ciently means compressing this has made so many valuable contri­ volume. butions to his field and to CERN's It is a fact of life (Liouville's theo­ success. rem) that phase volume cannot be It is no accident that CERN's two compressed using ordinary con­ most important scientific discover­ served forces. The electric and ies - that of the W and Z particles magnetic fields used in accelerators (the carriers of the weak force) in merely distort the phase volume, 1983, and the neutral current of and to change the density calls for the weak interaction ten years ear­ something else. lier - were both made possible by While others looked at schemes ingenious van der Meer inventions based on foils and eventually colli­ enabling unruly particles to be fa­ sions with collinear beams (called These fearsome-looking devices shioned into strong beams. 'smart foils' by Rubbia) which compress charged kaons and pions His idea and subsequent devel­ turned out to be better suited to into a cone so that their resultant opment of stochastic cooling made lower energy projects, Simon van neutrinos, otherwise uncontrolla­ CERN's antiproton project possible der Meer's genius led to a radically ble, are confined in a narrow beam. and opened the way for the W/Z different approach. High energy Steinberger underlined the phy­ discovery, for which he and Carlo particles circulating in a storage sics implications. As well as its Rubbia, now CERN's Director Gen­ ring are monitored by pickups, and neutral current triumph, CERN's eral, were awarded the 1984 Nobel appropriate signals are fed to kick­ early neutrino beam programme Prize for Physics. ers diametrically opposite to catch showed that the deep interior of An earlier invention, the 'mag­ the same particles as they pass the nucleon as seen by neutrinos netic horn', focuses parent charged and give them a small nudge. In was the same as that seen at Stan­ particles before they decay, boost­ this way a wide statistical spread ford in high energy electron ing the intensity of resulting neutri­ can be successively damped and a studies, and went on to enable the nos and greatly increasing their more orderly, or 'cooler', beam tiny 'parton' constituents seen by physics potential, so that rare pro­ built up. these beams to be identified with cesses like neutral currents be­ Despite initial appearances, Liou­ the quarks of the static nucleon come easier to spot. ville's theorem is still OK - the picture. Later came the precision Feest chairman Ugo A maId7 in­ empty phase space around each high energy work at the SPS, troduced the proceedings by recall­ particle is preserved, but these in­ which is still continuing. ing the first time the antiproton dividual phase spaces get rear­ Van der Meer has even a third scheme based on van der Meer's ranged. The stochastic cooling claim to fame, described at the stochastic cooling was described in idea, both elegant and practical, ex­ Feest by Giuseppe Cocconi. At public at CERN, in a 1976 seminar ploits this emptiness of accelera­ CERN's Intersecting Storage Rings by Carlo Rubbia subtitled 'just one tors ' phase space, pushing sparse (closed in 1984) van der Meer de­ of those unthinkable ideas'! At the particles together and empty re­ veloped a precision method for vi­ retirement event, Carlo Rubbia was gions outwards. tal measurements of the machine's on hand with an elegant personal After Carlo Rubbia's tribute, luminosity by determining the verti­ explanation of stochastic cooling. Jack Steinberger turned to van der cal distance between two beams. Particle accelerators work in Meer's magnetic horn brainchild. This exploited the ISR's remarkable

14 CERN Courier, January/February 1991 precision and ability to maintain I have come up with two rea­ and senior theorists. Many of stable conditions, and quickly led sons. The first is the style in which the latter learned for the first time to the early precision determina­ physics is taught essentially every­ how Willis Lamb had actually de­ tions of the proton-proton reaction where. There are two models, A termined 'his' shift, how many bril­ rate (cross-section) in the new en­ and B, both of which fail to convey liant insights he had had to have to ergy range opened up by this pion­ to the students the intellectual con­ achieve his goals. Quite a few peo­ eer machine. tent of important experiments. ple concluded correctly that there In conclusion, Giorgio Brianti In model A, the student is told was as much intellectual content in sketched van der Meer's remarka­ that some great genius, identified the Lamb experiment as in the ble 35-year career at CERN, paying by name, predicted a remarkable quantum electrodynamics explana­ tribute to his remarkable insight dependence y(x) of one observable tion of it. (This example is marred and inventiveness. If there was a upon another. That dependence by the fact Lamb was actually an problem, then Simon could find us was then subsequently brilliantly accomplished theorist!). a solution'. confirmed by experiment - by A second, altogether different With characteristic modesty, van some unspecified person. reason derives from what I might der Meer has shunned publicity In model B, one presents an ob­ call the 'theory of the father after his Nobel triumph, preferring served dependence y(x) that con­ image': in practice, all our physics to work quietly on fresh chal­ stituted at its time a great puzzle. courses are theoretical, whether lenges. Fortunately for CERN and Again a great genius (name given) the title of the course says so or for particle physics as a whole, he came along and presented a theory not. The theorists teaching theory will continue to tinker with these which fitted the observations per­ mostly know what they are talking problems, hopefully for a long time fectly. about, and the experimentalists fre­ to come. In either model, the intellectual quently do not. So the student accomplishment of the experiment­ (who though he may himself not alist is generally not conveyed to understand the subject, still infalli­ the students. I shall illustrate this bly catches the lack of under­ VIEWPOINT by two examples: (1) in Okun's standing of the lecturer!) says to Mind over matter - masterful book 'Leptons and himself: 7 do not want to become Quarks', experiments are rarely like him (insert name of exper­ the intellectual con­ described - although the authors imentalist) but like him (insert name tent of experimental are given - their results are merely of theorist)'. physics quoted, as 'one finds '; (2) I What can we do to remedy this once gave a course 'Great Experi­ situation? Two things: first, we ments in Modern Physics' at MIT. must postpone the difference in According to my experience, the It was attended by young students training of future experimentalists most brilliant physics students at and theorists as far as possible. any university want to become the­ Val Telegdi - mind over matter The difference is one of technique oreticians, and this on both sides and not one of intellectual compe­ of the Atlantic ocean. It is rare that tence. Second, we must teach a person of the intellectual power courses in which brilliant experi­ of, say, a Gell-Mann or a Cabibbo ments of great significance are ana­ decides to embark on a career in lysed in some detail. There is no experimental physics. shortage of candidate material! It is obvious that this fact entails a serious loss for physics, since Val Telegdi physics is primarily a natural science. I have often asked myself (This essay, supplemented by an about the reasons for this regretta­ analysis of several interesting and ble situation; once these are estab­ incisive experiments, is published lished, perhaps remedies could be as a CERN 'Yellow Report', CERN suggested. 90-09.)

CERN Courier, January/February 1991 15