The Super Proton–Antiproton Synchrotron (or SppS, 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 SppS-project, which led to the discovery of the W and Z bosons. Other experiments conducted at the SppS were UA4, UA5 and UA8.
Background Around 1968 Sheldon Glashow, Steven Weinberg, and Abdus Salam came up with the electroweak theory, which unified the electromagnetic 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 masses 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, whose construction 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 s = 2 E {\displaystyle {\sqrt {s}}=2E} . 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 would not be sufficient to accelerate protons to a momentum equal to the mass of the boson, as each quark would only carry a portion of the momentum. To produce bosons in the estimated intervals of 60 to 80 GeV/c2 (W boson) and 75 to 92 GeV/c2 (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 SppS was determined by the need to detect the decay Z → e+ + e−. As the cross-section for Z production at ~600 GeV is ~1.6 nb, and the fraction of Z → e+ + e− 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. The first proposal for a proton-antiproton collider seems to have been made by Gersh Budker and Alexander Skrinsky at Orsay in 1966, based on Budker's new idea of electron cooling. In 1972 Simon van der Meer published the theory of stochastic cooling, for which he later received the 1984 Nobel Prize in Physics. The theory was confirmed in the Intersecting Storage Rings at CERN in 1974. While electron cooling might have led to the idea of a proton-antiproton collider, it was eventually stochastic cooling that was used in the preaccelerators to prepare antiprotons for the SppS. Meanwhile, the discovery of neutral currents in the Gargamelle experiment at CERN prompted Carlo Rubbia and collaborators to propose a proton-antiproton collider. In 1978, the project was approved by the CERN Council, and the first collisions occurred in July 1981. The first run lasted until 1986, and after a substantial upgrade it continued operation from 1987 to 1991. The collider was shut down at the end of 1991, as it was no longer competitive with the 1.5 TeV proton-antiproton collider at Fermilab, which had been in operation since 1987.
Operation Between 1981 and 1991 SPS would operate part of the year as a synchrotron, accelerating a single beam for fixed-target experiments, and part of the year as a collider—SppS.
Modifications of the SPS for collider operation The requirements of a storage ring as the SppS, in which beams must circulate for many hours, are much more demanding than those of a pulsed synchrotron, such as the SPS. After the SppS was decided in 1978, the following modifications were done on the SPS:
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