NEUTRAL CURRENTS Gargamelle: the tale of a giant discovery
The discovery of neutral currents in the Gargamelle bubble chamber at CERN has been honoured with many prizes to members of the collaboration, including most recently the EPS High Energy and Particle Physics Prize. The key results were published in September 1973 after an intense period of work that had begun nearly two years earlier.
On 3 September 1973 the Gargamelle collaboration published two papers in the same issue of Physics Letters, revealing the first evi- dence 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 Shel- don 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 Col- laboration (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 neu- Fig. 1. The first example of the leptonic neutral current. An incoming trino and antineutrino interactions and analysing them for CC events muon-antineutrino knocks an electron forwards (towards the left), to measure cross-sections and structure functions. creating a characteristic electronic shower with electron–positron pairs. The priorities changed in March 1972, however, when the col- laboration saw first hints that hadronic neutral currents might exist. seek out potential leptonic NC events, involving the interaction with It was then that they decided to make a two-prong attack in the an electron in the liquid; the other to find hadronic neutral currents search for neutral-current (NC) candidates. One line would be to in which the neutrino scattered from a hadron (proton or neutron). s
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2 of a single free parameter, sin θw, where θw 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. Over the following year various teams carefully measured and Fig. 2. A beautiful hadronic neutral current event, where the interaction analysed candidate events from film produced previously in sev- of the neutrino coming from the left produces three secondary particles, eral runs. The first example of a single-electron event was found in all clearly identifiable as hadrons, as they interact with other nuclei in December 1972 by Franz-Josef Hasert, a postgraduate student at the liquid. There is no charged lepton (muon or electron). Aachen. Fortunately he realized that an event marked by a scanner as “muon plus gamma ray” was in fact something more interest- In both cases the neutrino enters invisibly, as usual, interacts and ing: the clear signature of an electronic NC interaction written in then moves on, again invisibly. The signal would be a single electron the tracks of an electron knocked into motion by the punch of the for the leptonic case, while for hadronic neutral currents the event unseen projectile (figure 1, p25). This was a “gold-plated” event would contain only hadrons and no lepton (figures 1 and 2). because it was found in the muon-antineutrino film in which any The leptonic NC channel was particularly interesting because background is extremely small. Its discovery gave the collabo- previous neutrino experiments had shown that the background ration a tremendous boost, strengthening the results that were was very small and also because Martin Veltman and his student beginning to roll in from the analyses of the hadronic NC events. Gerard ’t Hooft had recently demonstrated that electroweak theory However it was only one event, while by March 1973 there were was renormalizable. ’t Hooft was able to calculate exactly the cross- as many as 166 hadronic NC candidates (102 neutrino events sections for NC interactions involving only leptons, with the input and 64 antineutrino events) although the question of the neutron
The two papers from the 23 September 1973 issue of Physics Letters together showed that Gargamelle had discovered weak neutral currents.
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background still hung over their interpretation. Members of the team then began a final assault on the neutron %%#&".015*$4 background, which was finally conquered three months later, as Haidt and Perkins describe in their articles in CERN Courier. On &$)"3(&%1"35*$-& 19 July 1973, Paul Musset presented the results of both hadronic 53"+&$503:"/"-:4*4 and leptonic analyses in a seminar at CERN. The paper on the elec- tron event had already been received by Physics Letters on 2 July 40'58"3& (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- ?DJ;=H7J;:[^ZaYhdakZgh]VkZWZZcegdkZc^cVY^kZghZ changing neutral currents and the lack of understanding of the new gVc\Zd[Veea^XVi^dch[dg'*nZVgh#Djg7djcYVgn electroweak theory. Gargamelle continued to increase the amount :aZbZciBZi]dY 8;C !;^c^iZ:aZbZciBZi]dY <;C ! of data and by the summer of 1974, after the well known controversy VcY>O8H?:hdakZghVgZVkV^aVWaZ^ci]ZhVbZeVX`V\Z described by Haidt and Perkins, several experiments in the US con- id\^kZndjV\gZViZgX]d^XZ#Cdl!ndjXVc^cYZeZcYZcian firmed the discovery. From this time on the scientific community recognized that the Gargamelle Collaboration had discovered both kZg^[ni]Zhdaji^dcl^i]^cdcZegd\gVb!VcYVkd^Yi]Z leptonic and hadronic neutral currents. i^bZVcYZ[[dgid[kZg^[n^c\i]gdj\]VhZXdcYegd\gVb# Thirty-six years later the European Physical Society (EPS) has decided to award its 2009 High Energy and Particle Physics Prize to
Further reading D;M0 F J Hasert et al. 1973a Phys. Lett. 46 121. B^c^bjb&%m[VhiZgheZZY F J Hasert et al. 1973b Phys. Lett. 46 138. d[XVaXjaVi^dc BZc^hXjheaVhbV Résumé XVaXjaVi^dc^c'9 Gargamelle : histoire d’une découverte géante 53:06340'58"3&'03%":4 La découverte des courants neutres dans la chambre à bulles 86AA;DG6
CERN Courier September 2009 27
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