Simon Van Der Meer (1925–2011) Engineer Whose Invention Enabled the Discovery of the W and Z Particles

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COMMENT OBITUARY OBITUARY COMMENT Simon van der Meer (1925–2011) Engineer whose invention enabled the discovery of the W and Z particles.

imon van der Meer was one of the obey the theorem of the French mathema- the particles back on course. This very handful of truly exceptional people tician Joseph Liouville, and behave like an subtle form of compression of the beam does who contributed to making the Euro- incompressible fluid. In other words, if the not violate the Liouville theorem. Indeed, Spean Organization for Nuclear Research beam is squashed at one end, the distribu- through the synchronized use of a pick-up (CERN) the world’s premier laboratory in tion of particles will bulge out somewhere and a kicker, the very tiny elementary vol- elementary particle physics. His invention of else along its length. Beams of fast-moving umes around which a particle is present stochastic cooling offered a means to nudge electrons and positrons, their antiparticles, are simply moved away from those around protons and their antimatter equivalents into disobey this rule because they emit a form which there are no particles. The density of tightly focused beams, paving the way for the of radiation; in essence, the beams ‘cool’ particles seems to increase simply because discovery of the W and Z particles — two naturally. But in the case of the more mas- the voids have been pushed to the side. fundamental constituents of matter. sive particle beams, the Liouville theorem The approach was originally Van der Meer, who died on 4 March, studied kicks in. This presents a significant obstacle applied to beams of protons, where the technical physics at the University of stochastic cooling effect was visible Technology in Delft, the Netherlands, but very small because the number of from 1945, at a time when Dutch uni- particles in the beams was very large. CERN versities were still recovering after In the late 1970s, David Cline, Peter being closed under the German occu- McIntyre and I used a magnetic field to pation. He eventually obtained his steer head-on protons and antiprotons, engineering degree in 1952. Follow- colliding them many times every sec- ing a short period in Eindhoven at the ond. Provided that we ramped up the Philips Research Laboratory, he was density of the antiprotons as much among the first researchers to move as a billion times, van der Meer’s to the recently founded CERN near cooling process became prominent. Geneva, Switzerland. He arrived at Stochastic antiproton cooling was the age of 31 in 1956, a year before the an essential ingredient for the realiza- laboratory’s first accelerator was built, tion of the CERN ppbar collider and and worked there for the rest of his life. for the subsequent discovery of the In the early 1970s, circumstantial W and Z particles — for which van evidence for the electroweak model der Meer and I shared a Nobel prize was accumulating. This model uni- in 1984. The same technology has fied electromagnetism and the weak been key at the Tevatron collider at nuclear force, two of the four funda- Fermilab in Batavia, Illinois, where the mental forces of nature (gravity and top quark, the last and heaviest of all the strong nuclear force being the elementary quarks, was discovered in other two) in a single mathematical 1995. Indeed, during the past 25 years framework. But no one would be con- or so, stochastic cooling has been a vinced until physicists had actually major tool for all the significant discov observed the W and Z bosons — particles to producing an orderly, narrow beam. eries made from high-energy collisions. that supposedly carried the weak force. Van der Meer’s idea was both simple It is very unusual for a clever idea of an Accelerators at the time could smash and ingenious. The ‘incompressible fluid’ engineer to deeply modify fundamental particles into a stationary object. But pro- condition is strictly valid only when very concepts in physics. Simon himself noted ducing the heavy W and Z bosons would large numbers of particles are present. that his lack of formal training in physics require much more energy — specifically, When small numbers of particles circulate (he never obtained a PhD) probably turned getting protons and antiprotons to collide through an accelerator ring, small fluctua- out to be an asset. Not knowing the rules head on in tightly packed beams. tions from the average course taken by the of the game meant he came up with an Among van der Meer’s many original particles are observed. In the 1970s, these unthinkable idea. and highly practical ideas, stochastic cool- fluctuations — called Schottky noise — With Simon’s departure, we will miss not ing was the most important. This was, in could easily be detected using a device only a remarkable, inventive mind and a giant essence, a subtle and practical realization of called a pick-up electrode. Because every in our field, but also a very kind and univer- the ‘Maxwell’s demon’ thought experiment, particle in the beam has a specific individ- sally respected person. His friends used to say which demonstrates how, under certain ual energy, different amounts of fluctuation that he would never use two words where one circumstances, the second law of thermo- are detected each time the beam of particles would suffice, but that his one word would dynamics (which states that differences in courses through the ring. invariably be the right one. ■ temperature, pressure and so on equilibrate Van der Meer’s idea was to ‘correct’ the over time) can be circumvented. average position of the particles in a beam Carlo Rubbia is at CERN, Geneva 23, It is universally accepted among physicists electronically with the help of a ‘kicker’: a Switzerland. that any beam of elementary particles should magnetic device that continuously nudges e-mail: carlo.rubbia@cern.ch