Super Proton–Antiproton Synchrotron

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 , and the Z boson in the range from 75 to —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.{{cite web |last=Darriulat |first=Pierre |date=1 April 2004 |title=The W and Z particles: a personal recollection |url=http://cerncourier.com/cws/article/cern/29053 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 (W boson) and 75 to (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–. The design of the SpS was determined by the need to detect the decay . As the cross-section for Z production at is , and the fraction of decay is ~3%, a luminosity of L = would give an event rate of ~1 per day. To achieve such luminosity, one would need an antiproton source capable of producing 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 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 SpS. 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 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—SpS. Modifications of the SPS for collider operation The requirements of a storage ring as the SpS, in which beams must circulate for many hours, are much more demanding than those of a pulsed synchrotron, such as the SPS. After the SpS was decided in 1978, the following modifications were done on the SPS: The method of reducing the beam dimensions is called stochastic cooling, a method discovered by Simon van der Meer. Simply put, it is a feedback system based on the fact that all beams are particulate and that therefore, on a microscopic level, the density within a given volume will be subject to statistical fluctuations. In the first run, 1981–1986, the SpS accelerated three bunches of protons and three bunches of antiprotons. After the stacking rate of the antiprotons was increased in the upgrade, the number of both protons and antiprotons injected into the collider was increased from three to six. The SpS would accelerate the beams to , keeping them as this energy for a time limited by the heating of the magnets, then decelerate the beams to . The pulsing was operated in such a way that the average dispersion of power in the magnets did not exceed the level of operation at . The SpS occasionally ran pulsed operation after 1985, obtaining collisions at a center-of-mass energy of . ==Findings and Discoveries==

at CERN. From right to left: Carlo Rubbia, spokesperson of the UA1 experiment; Simon van der Meer, responsible for developing the stochastic cooling technique; Herwig Schopper, Director-General of CERN; Erwin Gabathuler, Research Director at CERN, and Pierre Darriulat, spokesperson of the UA2 experiment. The SpS began its operation in July 1981, and by January 1983 the discovery of the W and Z boson by the UA1 and UA2 experiment were announced. Carlo Rubbia, spokesperson for UA1 experiment, and Simon van der Meer received the 1984 Nobel Prize in Physics for, as stated in the press release from the Nobel Committee, "their decisive contribution to the large project, which led to the discovery of the field particles W and Z". The prize was given to Carlo Rubbia for his "idea to convert an existent large accelerator into a storage ring for protons and antiprotons", i.e. the conception of the SpS, and to Simon van der Meer for his "ingenious method for dense packing and storage of proton, now applied for antiprotons", i.e. devise of the technology enabling the Antiproton Accumulator—stochastic cooling. The conception, construction and operation of the SpS was considered a great technical achievement in itself. Before the SpS was commissioned, it was debated whether the machine would work at all, or if beam-beam effects on the bunched beams would prohibit an operation with high luminosity. The SpS proved that the beam-beam effect on bunched beams could be mastered, and that hadron colliders were excellent tools for experiments in particle physics. In such regard, it lay the ground work of LHC, the next-generation hadron collider at CERN. ==See also==