UA2 experiment

The Underground Area 2 (UA2) experiment was a high-energy physics experiment that ran at CERN’s Super Proton Synchrotron (SPS) from 1981 until 1990. Its main objective was to discover the W and Z bosons. UA2, together with the UA1 experiment, succeeded in discovering these particles in 1983, leading to the 1984 Nobel Prize in Physics being awarded to Carlo Rubbia and Simon van der Meer. The UA2 experiment was also involved in the search of the top quark.

Around 1968 Sheldon Glashow, Steven Weinberg, and Abdus Salam came up with a theory that unified electromagnetism and weak interactions, for which they shared the 1979 Nobel Prize in Physics. This theory, known as the electroweak theory, postulated the existence of W and Z bosons. The theoretical discovery put pressure on experimental physicists to show the existence of the W and Z bosons. It was predicted that the W boson had a mass value in the range of 60 to 80 GeV, and the Z boson in the range from 75 to 92 GeV – energies too large to be accessible by any accelerator in operation at that time.

During those years the prime project for CERN was the construction of an electron-positron-collider, known as LEP. Such a machine is ideal to produce and measure the properties of W and Z bosons. But due to the pressure to find the W and Z bosons, the CERN community felt like it could not wait for the construction of LEP. In 1976 Carlo Rubbia, Peter McIntyre and David Cline proposed to modify the existing Super Proton Synchrotron (SPS). The SPS was originally designed and built for fixed-target experiments, meaning it would accelerate one proton beam. The modification meant making it into a two-beam proton-antiproton-collider, which became known as the SppS. In such a scheme, a proton and an anti-proton beam, each of energy E, circulated in the same magnetic field in opposite directions, providing head-on collisions between the protons and the antiprotons at a total center-of-mass energy ${\displaystyle {\sqrt {s}}=2E}$. The scheme was adopted at CERN in 1978. Eventually the SppS could run with a ${\displaystyle {\sqrt {s}}}$ up to 630 GeV. By the end 1982, the machine had reached high enough energies to permit the observation of ${\displaystyle W\rightarrow e\nu }$ decays. The UA2 and UA1 collaboration chose to detect the W boson by identifying its leptonic decay, because even though the W boson predominantly (≈70%) decays to quark-antiquark pairs, which appear as two hadronic jets, such jets are overwhelmed by other jet production.

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