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The Gargamelle bubble chamber during its installation at the Proton Synchrotron in 1970

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Gargamelle and the discovery of weak neutral currents

In July 1973, a groundbreaking discovery was announced in CERN’s main auditorium: the Gargamelle group of physicists had found the first direct evidence of the weak neutral current, a process that required the existence of a neutral particle to carry the weak fundamental force.

This particle, called the Z boson, and the associated weak neutral currents, were predicted by electroweak theory, according to which the weak force and the electromagnetic force are different versions of the same force. This was the first great discovery to be made at CERN and opened a new door to the future of particle physics, the unification of the forces.

Gargamelle was the name of the particle detector used to make this discovery at the Proton Synchrotron accelerator. It was a large bubble chamber, a type of particle detector that uses a pressurised transparent liquid to detect electrically charged particles passing through it.

Named after the mother of Gargantua (the giant in the story by François Rabelais), Gargamelle measured 4 m long with a 2 m diameter, weighed 1000 tonnes, and contained 18 tonnes of liquid Freon. It was made especially for detecting neutrinos. These particles have no charge, and would leave no tracks in the detector, so the aim was to reveal any charged particles set in motion by the neutrinos and so reveal their interactions indirectly.

It was one of these interactions, in which a neutrino set an electron in motion, that provided the first direct observation of a weak neutral current interaction.

Ten years later, two CERN experiments, UA1 et UA2 , discovered W and Z particles, carriers of the eletroweak force, a discovery that was awarded the Nobel Prize.

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Gargamelle: the tale of a giant discovery

A look back to the first observation of weak neutral currents.

On 3 September 1973 the Gargamelle collaboration published two papers in the same issue of Physics Letters , revealing the first evidence for weak neutral currents – weak interactions that involve no exchange of electric charge between the particles concerned. These were important observations in support of the theory for the unification of the electromagnetic and weak forces, for which Sheldon Glashow, Abdus Salam and Steven Weinberg were to receive the Nobel Prize in Physics in 1979. Their theory became a pillar of today’s Standard Model of particles and their interactions, but in the early 1970s, it was not so clear that it was the correct approach and that the observation of neutral currents was a done deal.

The story of the discovery has been told in many places by many people, including in the pages of CERN Courier , notably by Don Perkins in the commemorative issue for Willibald Jentschke , who was CERN’s director-general at the time of the discovery, and more recently in the issue that celebrated CERN’s 50th anniversary, in an article by Dieter Haidt, another key member of the Gargamelle Collaboration ( CERN Courier October 2004 p21 ).

The huge bubble chamber, named Gargamelle after the giantess created 400 years earlier in the imagination of François Rabelais, took its first pictures in December 1970 and a study of neutrino interactions soon started under the leadership of André Lagarrigue. The first main quest, triggered by recent hints from SLAC of nucleon structure in terms of “partons”, was to search for evidence of the hard-scattering of muon-neutrinos (and antineutrinos) off nucleons in the 18 tonnes of liquid Freon inside Gargamelle. Charged-current (CC) events in which the neutrino transformed into a muon would be the key. So the collaboration, spread over seven institutes in six European countries, set to work on gathering photographs of neutrino and antineutrino interactions and analysing them for CC events to measure cross-sections and structure functions.

The priorities changed in March 1972, however, when the collaboration saw first hints that hadronic neutral currents might exist. It was then that they decided to make a two-prong attack in the search for neutral-current (NC) candidates. One line would be to seek out potential leptonic NC events, involving the interaction with an electron in the liquid; the other to find hadronic neutral currents in which the neutrino scattered from a hadron (proton or neutron). In both cases the neutrino enters invisibly, as usual, interacts and then moves on, again invisibly. The signal would be a single electron for the leptonic case, while for hadronic neutral currents the event would contain only hadrons and no lepton (figures 1 and 2).

The leptonic NC channel was particularly interesting because previous neutrino experiments had shown that the background was very small and also because Martin Veltman and his student Gerard ‘t Hooft had recently demonstrated that electroweak theory was renormalizable. ‘t Hooft was able to calculate exactly the cross-sections for NC interactions involving only leptons, with the input of a single free parameter, sin 2 θ w , where θ w is the Weinberg angle. Theorists at CERN – Mary K Gaillard, Jacques Prentki and Bruno Zumino – encouraged the Gargamelle Collaboration to hunt down both types of neutral current.

Such leptonic NC interactions would, however, be extremely rare. By contrast hadronic NC events would be more common but it was not yet clear how the theory worked for quarks. In this case the process was not easy to calculate, although Weinberg published some estimates during 1972. In addition there was the problem of a background coming from neutrons that are produced in CC interactions in the surrounding material and could imitate a neutral current signal.

By March 1973 there were as many as 166 hadronic NC candidates

Over the following year various teams carefully measured and analysed candidate events from film produced previously in several runs. The first example of a single-electron event was found in December 1972 by Franz-Josef Hasert, a postgraduate student at Aachen. Fortunately he realized that an event marked by a scanner as “muon plus gamma ray” was in fact something more interesting: the clear signature of an electronic NC interaction written in the tracks of an electron knocked into motion by the punch of the unseen projectile (figure 1). This was a “gold-plated” event because it was found in the muon-antineutrino film in which any background is extremely small. Its discovery gave the collaboration a tremendous boost, strengthening the results that were beginning to roll in from the analyses of the hadronic NC events. However it was only one event, while by March 1973 there were as many as 166 hadronic NC candidates (102 neutrino events and 64 antineutrino events) although the question of the neutron background still hung over their interpretation.

Members of the team then began a final assault on the neutron background, which was finally conquered three months later, as Haidt and Perkins describe in their articles in CERN Courier . On 19 July 1973, Paul Musset presented the results of both hadronic and leptonic analyses in a seminar at CERN. The paper on the electron event had already been received by Physics Letters on 2 July (F J Hasert et al. 1973a); the paper on the hadronic events followed on 23 July (F J Hasert et al. 1973b). They were published together on 3 September.

It was an iconoclastic discovery, leaving many unconvinced. This was mainly because of the stringent limits on strangeness-changing neutral currents and the lack of understanding of the new electroweak theory. Gargamelle continued to increase the amount of data and by the summer of 1974, after the well known controversy described by Haidt and Perkins, several experiments in the US confirmed the discovery. From this time on the scientific community recognized that the Gargamelle Collaboration had discovered both leptonic and hadronic neutral currents.

Thirty-six years later the European Physical Society (EPS) has decided to award its 2009 High Energy and Particle Physics Prize to the Gargamelle Collaboration for the “Observation of weak neutral currents” ( Prize time in Krakow at EPS HEPPP 2009 ). However, it somewhat confounded the collaboration in citing only the authors of the hadronic neutral-current paper, thus neglecting the contributions of the five who signed the electronic paper, but not the hadronic paper (Charles Baltay, Helmut Faissner, Michel Jaffre, Jacques Lemonne and James Pinfold). Though the collaboration is honoured to receive the prize, its members feel that the award should not rewrite history. They feel, and rightly so, that the two papers were of equal importance in the discovery of neutral currents. Also, like many other physicists and the EPS prize committee, they feel that it was perhaps the greatest discovery of CERN. The prize was collected on behalf of the collaboration at the EPS HEP 2009 Conference in Krakow by Antonino Pullia and Jean-Pierre Vialle. Sometime in September the medal will be attached to the Gargamelle chamber, which now stands in CERN’s grounds, and a reunion dinner for the collaboration will follow.

Donald Cundy , CERN/Torino IFSI, and Christine Sutton , CERN.

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Forty years of neutral currents

On 19 July 1973, physicists working with the Gargamelle bubble chamber at CERN presented the first direct evidence of the weak neutral current

19 July, 2013

By Cian O'Luanaigh

The first example of a single-electron neutral current. An incoming antineutrino knocks an electron forwards (towards the left), creating a characteristic electronic shower with electron–positron pairs (Image: Gargamelle/CERN)

Forty years ago today, physicists working with the Gargamelle bubble chamber at CERN presented the first direct evidence of the weak neutral current. The result led to the discovery of the W and Z bosons, which carry the weak force – and ultimately to that of a Higgs boson announced last year.

Gargamelle was a bubble chamber at CERN designed to detect neutrinos. It was 4.8 metres long and 2 metres in diameter, weighed 1000 tonnes and held nearly 12 cubic metres of heavy-liquid freon (CF3Br).

In a seminar at CERN on 19 July 1973, Paul Musset of the Gargamelle collaboration presented the first direct evidence of the weak neutral current - a process predicted in the mid-1960s independently by Sheldon Glashow, Abdus Salam and Steven Weinberg - that required the existence of a neutral particle to carry the weak fundamental force. This particle, called the Z boson, and the associated weak neutral currents, were predicted by electroweak theory, according to which the weak force and the electromagnetic force are different versions of the same force.

The discovery involved the search for two types of events: one involved the interaction of a neutrino with an electron in the liquid, while in the other the neutrino scattered from a hadron (proton or neutron). In the latter case, the signature of a neutral current event was an isolated vertex from which only hadrons were produced. By July 1973 the team had confirmed as many as 166 hadronic events, and one electron event. In both cases, the neutrino enters invisibly, interacts and then moves on, again invisibly.

Paul Musset presented the results of both hadronic and leptonic analyses at the seminar at CERN. The paper on the electron event had already been received by Physics Letters on 2 July ( F J Hasert et al. 1973a ); the paper on the hadronic events followed on 23 July ( F J Hasert et al. 1973b ). They were published together in the same issue of the journal on 3 September.

The discovery of weak neutral currents was a significant step toward the unification of electromagnetism and the weak force into the electroweak force.

In 1983, the UA1 and UA2 experiments at CERN confirmed the existence of the W and Z particles predicted by the theory of neutral currents.

Read more: " Gargamelle: the tale of a giant discovery " – CERN Courier archive

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CERN70: A gargantuan discovery

13 June 2024

Violette Brisson played an active part in the discovery of neutral currents; she was head of the Gargamelle group at the Laboratory of the École Polytechnique in Paris

Source || part 11 of the cern70 series.

It was the first great discovery to be made at CERN. On 19 July 1973, the Gargamelle group announced that they had found evidence of weak neutral currents. It was a groundbreaking discovery that paved the way towards a grand unification theory and the associated new physics. According to this theory, at the high energy levels that existed just after the birth of the Universe, all forces were unified and only became distinct as the Universe cooled.

As early as the 19th century, James Maxwell had demonstrated that the electrical and magnetic forces were just two manifestations of the same interaction. From the ferment of new theories developed in the 1960s emerged the concept that the weak force could be unified with the electromagnetic force as a single interaction, the so-called electroweak interaction. The theory predicted the existence of a specific manifestation of the weak force, referred to as weak neutral currents. Gargamelle was the first experiment to find proof of these currents, thereby confirming the electroweak theory.

The Gargamelle experiment was outlined in 1963 by physicist André Lagarrigue and named after the mother of Gargantua, the giant in François Rabelais’ story. It was a huge bubble chamber, weighing 1000 tonnes and containing 18 tonnes of liquid freon. The chamber’s large volume, for its time, enabled the interaction of neutrinos – particles with no electric charge that interact only weakly. Built by laboratories at the French Atomic Energy Commission (CEA), the Ecole Polytechnique and the Faculty of Science in Orsay, Gargamelle eventually gathered a team of about 50 physicists. The experiment began operation in January 1971, and delivered a first significant result when it confirmed that the proton consists of several particles, the “ quarks “. But its greatest achievement was providing evidence of neutral currents in 1973.

Inside the Gargamelle bubble chamber in 1970 (Image: CERN )

Recollections.

That was when we really began to get excited because, after finding so many semi-precious stones in our experiments over the past ten years, we were now starting to glimpse a genuine diamond! Violette Brisson

Head of the Gargamelle group at the Laboratory of the Ecole Polytechnique in Paris, of which she was deputy director from 1973 to 1985, Violette Brisson played an active part in the discovery of neutral currents. She joined the Linear Accelerator Laboratory in Orsay in 1988, and the Franco-Italian VIRGO experiment in the 1990s.

“I couldn’t begin this account without mentioning my colleagues from the Ecole Polytechnique who are no longer with us: André Lagarrigue, without whom there would be no story to tell, André Rousset, who was in charge of the CERN group at that time and Paul Musset, who also played a very important role in the discovery.

Neutral currents, whose existence was extremely hypothetical in those days, were only eighth on the planned list of experiments for Gargamelle. It was not until 1972 that the theorists drew the Collaboration’s attention to the work of Gerard ’t Hooft, who had predicted their existence in the framework of the new theories unifying the electromagnetic and weak forces.

The associated research was initiated by Antonino Pullia in Milan, who very soon presented us with about 30 possible events, or “candidates”, which generated a lot of excitement. Several physicists, including myself at the Ecole Polytechnique, together with Pierre Petiau and Louis Kluberg, threw themselves into research on the subject. We very quickly found candidate events for neutral currents but I was still very doubtful until a meeting at CERN in May 1972, when all the laboratories involved in the Collaboration presented photos of their candidates. I came away firmly convinced, as did many of my colleagues. That was when we really began to get excited because, after finding so many semi-precious stones in our experiments over the past ten years, we were now starting to glimpse a genuine diamond!

Violette Brisson, a highly respected member of the French particle physics community, played a leading role in the discovery of neutral currents. (Image: École Polytechnique)

This marked the start of an intense phase of work, which involved sorting through some 100 000 photos and analysing events, as well as studying their background (since neutron interactions can be confused with neutral currents). In December 1972, there was great rejoicing when virtually incontestable proof of another type of neutral current, the “leptonic” neutral current, was found in Aachen. The theorists were ecstatic. We re-examined the pictures (four times!) to make sure that we hadn’t missed anything. Later on, our analysis of nine times the number of shots produced only two further candidates, which tied in with the theoretical calculations.

We were euphoric as this confirmed the existing evidence for so-called “hadronic” neutral currents. However, extensive work was needed at the beginning of 1973 to calculate the background for these hadronic currents. Once the simulation programmes of Dieter Haidt and Jean-Pierre Vialle and the more simple calculations of André Rousset, Don Perkins and Antonino Pullia, to name but a few, had been completed, there was no longer any shadow of a doubt. With the unanimous agreement of the whole Collaboration, Paul Musset presented the results at CERN in July 1973 and they were published in early September.

Despite the enthusiasm of the entire community, some our of colleagues remained sceptical. We ourselves were confident, however, especially since the American team at Fermilab had also found candidates. That confidence lasted until one day in October 1973, when André Lagarrigue received a telephone call informing him that Fermilab’s candidates had disappeared. I can still remember the anguish in his voice when he called to tell me that our colleagues “over there” could no longer see anything and wondered whether we could have forgotten something or made a mistake. Doubts began to creep in…

The first example of a single-electron neutral current. An invisible incoming antineutrino knocks an electron forward creating a track that seems to appear from nowhere. (Image: CERN)

In order to convince our detractors, we took data that checked the background results obtained by Dieter Haidt. We even allowed ourselves to make jokes about our American colleagues’ “alternating currents”. All doubt was finally removed with the discovery of two further leptonic neutral currents at the beginning of 1974. The Americans’ neutral currents gradually re-emerged and, in summer 1974, the existence of the phenomenon was officially recognised.

The discovery gave rise to many neutrino experiments at CERN and in the United States, which measured all the parameters of the electroweak interaction with increasing precision. The discovery of the W and Z bosons at CERN in 1983 then confirmed the electroweak theory once and for all.

Unfortunately, Gargamelle was not destined to ring the Nobel chimes for André Lagarrigue, the “father” of the experiment, who passed away in 1975 before he could be considered for the Prize. Having been “put to rust” for several years, the bubble chamber was finally erected at CERN as a piece of sculpture. As for Gargamelle’s “workers”, we were sometimes badly treated or forgotten, but we also experienced the elation and joy of a very great discovery and we have always remained true friends. What more could anyone ask for?”

Gargamelle’s bubble chamber is now on display at Square Van Hove , in the grounds of CERN’s Science Gateway. (Image: CERN)

This interview is adapted from the 2004 book “Infinitely CERN”, published to celebrate CERN’s 50th anniversary. Violette Brisson died in 2018 at the age of 83. In 2023, CERN celebrated the 50th anniversary of the discovery of neutral currents, and the CERN Courier published “ CERN’s neutrino odyssey ” charting CERN’s neutrino programme since Gargamelle.