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The challenge of

Preparing the at CERN in 1969. In 1973 the chamber took the first historic photographs of interactions. (Photo CERN 409.9.69)

Neutrino book

Gargamelle and Neutral Currents - The Story of a Vital Discovery by Andre Rousset

Andre Rousset's book (in French - Gargamelle et les Courants Neutres - Ecole des Mines de Paris) tells the story of Gargamelle and the discov­ ery at CERN in 1973 of neutral currents, the cornerstone of the electroweak theory. This vital discov­ ery helped to give credence to the of . Rousset is both an observer and one of the key figures in the story. His book is lively and well docu­ mented; in it he uses archive material to ensure the accuracy of his infor­ mation on dates, choices and deci­ sions. ously" to the project was probably in an interesting manner the theo­ After an introduction to particle what swung the decision. rists' "green light", giving the go- physics which puts into perspective Construction took five years, during ahead to the experimentalists. In fact, the electroweak theory unifying weak which many problems were encoun­ the European collaboration (Aachen, and electromagnetic interactions, tered, right up to the fault in the main Brussels, CERN, Ecole Polytechni­ Rousset comes straight to the point. part of the chamber which caused que, Milan, Orsay, UC London) was From the late 1950s onwards he was delays and, a few years later, was to divided between a study of the - involved in the construction of the prove fatal to the detector. As parton model, research into the first heavy liquid bubble chambers by Rousset correctly states, Gargamelle intermediate boson... and research the BP1, BP2 and BP3 teams at the was probably the first big detector into neutral currents. Ecole Polytechnique in Paris. For designed to be built on industrial Gargamelle's first data were taken Gargamelle a bigger laboratory was lines, in direct cooperation with at the CERN PS (the SPS not yet needed, and it was at the CEA industry. The reward: the first neu­ being in operation), and the average (French Atomic Commission) trino interaction was photographed energy of the neutrinos was only a in Saclay that the chamber was on 28 January 1971. few GeV. In a - interac­ designed by teams from the Saturne While it was clear that neutrino tion, either there is a muon among accelerator and the Ecole Polytech­ physics was Gargamelle's main goal the final particles and it is said that nique. However, the decision to build from the start, it was not obvious to there is a W exchange interaction Gargamelle was taken in 1965 all the physicists in the collaboration (), or there is a muon through the impetus of Andre Lagarri- once the detector had been built that neutrino in the final state and there is gue, in defiance of the normal CERN the primary objective was research a Z exchange interaction (neutral procedures. Gargamelle was then in into neutral currents. Rousset points current). Neither the W nor the Z competition with the other big bubble out that the key role played by particles were known in 1972 and chamber project, BEBC; was it really neutral currents in electroweak one of Gargamelle's goals was necessary to build two big cham­ unification did not come to the fore precisely the identification of the W bers? The decision by Francis Perrin until 1971, when Gargamelle went particle (an impossible task, as we and the CEA to contribute "gener­ into operation. Here, he emphasises now know, since the PS did not

40 CERN Courier, September 1995

The challenge of neutrinos

produce the required energy). it was easier to differentiate neutron that really to be expected from such The first neutral current event was interactions, the great neutral current an enthusiastic and ardent observer/ identified in early 1973 in Aachen. simulators, from neutrino interac­ participant? This was a leptonic neutral current, tions. However, the detector was not In conclusion, one may be sorry, where a scatters off easy to understand either. The like Rousset, that this important an , which crosses the HPWF's first results came hot on the discovery was not rewarded by a chamber in a very specific way. heels of the Gargamelle announce­ Nobel Prize. This can probably be However, Andre Rousset somewhat ment. There were fewer neutral ascribed to the early death of Andre disregards leptonic neutral currents, currents, which disappeared alto­ Lagarrigue, the father of Gargamelle, to which he considers history has gether a few weeks later... only to in 1974, followed by the death of paid too much attention. May we be reappear at the beginning of 1974. Paul Musset in an accident in 1985. allowed to disagree, since the beauty This episode of "alternating" neutral The collective scientific venture of the picture is undeniable? currents worried... or kept the com­ which led to the discovery of neutral With perspicuity, Rousset expands munity entertained, for several currents is a wonderful story, to on research into hadronic neutral months. which Andre Rousset makes a very currents, where the final state com­ It was at the London Conference in important contribution. Supplemented prises an escaping neutrino and July 1974 that the existence of by Peter Galison's article ("How the . The difficulty is in separat­ neutral currents was conclusively first neutral current experiments ing such an interaction from that established and accepted by the ended", Review of Modern Physics which could be produced by the scientific community. Even today 55 (1983) 477), it will make a fasci­ interaction of a neutron from the Rousset asks whether neutral cur­ nating read (in French) for all those immediate chamber surroundings. rents had really been observed. This who want to know the details of this Rousset gives a descriptive and question, which is to his credit as a famous chapter in the history of detailed account of this complicated scrupulous physicist, may seem contemporary physics. and painstaking work, without losing surprising; however, there was sight of the main points. clearly no choice at the time: neutral By Michel Spiro The first half of 1973 was highly currents had been predicted, even emotionally charged for the collabo­ expected, by theory, and were by far ration, which took time to understand the most simple interpretation of the data and finalize its analyses. Gargamelle's results. The sociological aspect of the col­ There are few reservations about laboration was also examined. The Rousset's work. The book contains a (Michel Spiro, Michel Cribier and announcement of the discovery was few minor but unimportant errors (the Daniel Vigaud are the authors of a finally made by Paul Musset at detector was new book la lumiere des neutrinos', CERN on 19 July 1973 and the two built in South Dakota and not Utah). published by Editions du Seuil, Paris. papers were published in Physics A little more information on the A review of this book will appear in a Letters on 3 September. Musset had discussion surrounding the possible forthcoming issue.) been the central figure during the existence of neutral currents in the construction period and had played a small chambers before Gargamelle coordinating role in the analysis would have been welcome, and a work. comparison of Gargamelle's results Andre Rousset does not overlook with the present parameter values the competition: at Fermilab, in the would have provided a useful per­ United States, the 'HPWF collabora­ spective. Furthermore, it is perhaps tion placed its 60 tonne neutrino regrettable that Rousset does not detector alongside the new 400 GeV always adopt the necessary detach­ accelerator, which went into opera­ ment, particularly when referring to tion in 1972. The neutrino energy the rival experiment in the United was several tens of GeV and a priori States at Fermilab (HPWF), but is

42 CERN Courier, September 1995