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Remembering the W discovery

10 January 2023

Former UA2 spokesperson Luigi Di Lella recalls the events leading to the discovery of the W and Z bosons at CERN 40 years ago.

A W event recorded by UA1 in 1982

When the W and Z bosons were predicted in the mid-to-late 1960s, their masses were not known. Experimentalists therefore had no idea what energy they needed to produce them. That changed in 1973, when Gargamelle discovered neutral-current neutrino interactions and measured the cross-section ratio between neutral- and charged-current interactions. This ratio provided the first direct determination of the weak mixing angle, which, via the electroweak theory, predicted the W-boson mass to lie between 60 and 80 GeV, and the Z mass between 75 and 95 GeV – at least twice the energy of the leading accelerators of the day. 

By then, the world’s first hadron collider – the Intersecting Storage Rings (ISR) at CERN – was working well. Kjell Johnsen proposed a new superconducting ISR in the same tunnel, capable of reaching 240 GeV. A study group was formed. Then, in 1976, Carlo Rubbia, David Cline and Peter McIntyre suggested adding  an antiproton source to a conventional 400 GeV proton accelerator, either at Fermilab or at CERN, to transform it into a pp collider. The problem was that the antiprotons had to be accumulated
and cooled if the target luminosity (1029 cm–2s–1, providing about one Z event per day) was to be reached. Two methods were proposed: stochastic cooling by Simon van der Meer at CERN and electron cooling by Gersh Budker in Novosibirsk. 

CERN Director-General John Adams wasn’t too happy that as soon as the SPS had been built, physicists wanted to convert it into a pp collider. But he accepted the suggestion, and the idea of a superconducting ISR was abandoned. Following the Initial Cooling Experiment, which showed that the luminosity target was achievable with stochastic cooling, the SppS was approved in May 1978 and the construction of the Antiproton Accumulator (AA) by van der Meer and collaborators began. Around that time, the design of the UA1 experiment was also approved. 

A group of us proposed a second, simpler experiment in another interaction region (UA2), but it was put on hold for financial reasons. Then, at the end of 1978, Sam Ting proposed an experiment to go in the same place. His idea was to surround the beam with heavy material so that everything would be absorbed except for muons, making it good at identifying Z → μ+μ but far from good for W bosons decaying to a muon and a neutrino. In a tense atmosphere, Ting’s proposal was turned down and ours was approved.

First sightings

The first low-intensity pp collisions arrived in late 1981. In December 1982 the luminosity reached a sufficient level, and by the following month UA1 had recorded six W candidates and UA2 four. The background was minimal; there was nothing else we could think of that would produce such events. Carlo presented the UA1 events and Pierre Darriulat the UA2 ones at a workshop in Rome on 12–14 January 1983. On 20 January, Carlo announced the W discovery at a CERN seminar, and the next day I presented the UA2 results, confirming UA1. In UA2 we never discussed priority, because we all knew that it was Carlo who had made the whole project possible. 

Luigi Di Lella

The same philosophy guided the discovery of the Z boson. UA2 had recorded a candidate Z → e+e event in December 1982, also presented by Pierre at the Rome workshop. One electron was perfectly clear, whereas the other had produced a shower with many tracks. I had shown the event to Jack Steinberger, who strongly suggested we publish immediately; however, we decided to wait for the first “golden” event with both electrons unambiguously identified. Then, one night in May 1983, UA1 found a Z. As with ours, only one electron satisfied all electron-identification criteria, but Carlo used the event to announce a discovery. The UA1 results (based on four Z → e+e events and one Z → μ+μ) were published that July, followed by the UA2 results (based on eight Z → e+e events, including the 1982 one) a month later. 

The SppS ran until 1990, when it became clear that Fermilab’s Tevatron was going to put us out of business. In 1984–1985 the energy was increased from 546 to 630 GeV and in 1986 another ring was added to the AA, increasing the luminosity 10-fold. Following the 1984 Nobel prize to Rubbia and van der Meer, UA1 embarked on an ambitious new electromagnetic calorimeter that never quite worked. UA2 went on to make a precise measurement of the ratio mW/mZ, which, along with the first precise measurement of mZ at LEP, enabled us to determine the W mass with 0.5% precision and, via radiative corrections, to predict the mass of the top quark (160+50–60 GeV) several years before the Tevatron discovered it. 

Times have certainly changed since then, but the powerful interplay between theory, experiment and machine builders remains essential for progress in particle physics. 

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