The Super Proton–Antiproton Synchrotron (or SpS, also known as the Proton–Antiproton Collider) was a particle accelerator that operated at CERN from 1981 to 1991. To operate as a proton-antiproton collider the Super Proton Synchrotron (SPS) underwent substantial modifications, altering it from a one beam synchrotron to a two-beam collider. The main experiments at the accelerator were UA1 and UA2, where the W and Z bosons were discovered in 1983. Carlo Rubbia and Simon van der Meer received the 1984 Nobel Prize in Physics for their contributions to the SpS-project, which led to the discovery of the W and Z bosons. Other experiments conducted at the SpS were UA4, UA5 and UA8.
Background
Around 1968 Sheldon Glashow, Steven Weinberg, and Abdus Salam came up with the electroweak theory, which unified electromagnetism and weak interactions, and for which they shared the 1979 Nobel Prize in Physics. The theory postulated the existence of W and Z bosons. It was experimentally established in two stages, the first being the discovery of neutral currents in neutrino scattering by the Gargamelle collaboration at CERN, a process that required the existence of a neutral particle to carry the weak force — the Z boson. The results from the Gargamelle collaboration made calculations of the mass of the W and Z bosons possible. It was predicted that the W boson had a mass value in the range of 60 to 80 GeV/c2, and the Z boson in the range from 75 to 92 GeV/c2 – energies too large to be accessible by any accelerator in operation at that time. The second stage of establishing the electroweak theory would be the discovery of the W and Z bosons, requiring the design and construction of a more powerful accelerator.
During the late 70s CERN's prime project was the construction of the Large Electron–Positron Collider (LEP). Such a machine was ideal to produce and measure the properties of W and Z bosons. However, 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 — a new accelerator was needed — the construction of which could not be at the expense of LEP. In 1976 Carlo Rubbia, Peter McIntyre and David Cline proposed to modify a proton accelerator — at that time a proton accelerator was already running at Fermilab and one was under construction at CERN (SPS) — into a proton–antiproton collider. Such machine required only a single vacuum chamber, unlike a proton-proton collider that requires separate chambers due to magnetic fields oppositely directed. Since the protons and antiprotons are of opposite charge, but of same energy E, they can circulate 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 . The scheme was proposed both at Fermilab in the United States, and at CERN, and was ultimately adopted at CERN for the Super Proton Synchrotron (SPS).
W and Z bosons are produced mainly as a result of quark-antiquark annihilation. In the parton model the momentum of a proton is shared between the proton's constituencies: a portion of the proton momentum is carried by the quarks, and the remainder by gluons. It will not be sufficient to accelerate protons to a momentum equal the mass of the boson, as each quark will only carry a portion of the momentum. To produce bosons in the estimated intervals of 60 to 80 GeV (W boson) and 75 to 92 GeV (Z boson), one would therefore need a proton-antiproton collider with a center-of-mass energy of approximately six times the boson masses, about 500-600 GeV. The design of the SpS was determined by the need to detect . As the cross-section for Z production at ~600 GeV is ~1,6 nb, and the fraction of decay is ~3%, a luminosity of L=2,5 · 1029 cm−2s−1 would give an event rate of ~1 per day. To achieve such luminosity one would need an antiproton source capable of producing ~3·1010 antiprotons each day, distributed in a few bunches with angular and momentum acceptance of the SPS.
History
The SPS was originally designed as a synchrotron for protons, to accelerate one proton beam to 450 GeV and extract it from the accelerator for fixed-target experiments. However, already before the construction period of the SPS the idea of using it as a proton-antiproton accelerator came up.