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Stochastic CoolingAntiprotonsThe accelerator physicists faced the challenge of producing very intense beams of antiprotons, to ensure a good chance of collisions with protons in the new SPS. An ingenious invention of Simon Van der Meer, called 'stochastic cooling,' made this possible.This technique concentrates particles into a dense beam by reducing their random motion. A slice of the beam is observed at one point of the ring and signals are sent on a short cut across the ring to meet the slice and kick it into line. This process is repeated millions of times, progressively concentrating the particles. It was not known whether antiprotons would survive long enough to be used in experiments. Tests at the Intersecting Storage Rings in 1974 showed that they did and could be controlled by stochastic cooling. The Initial Cooling Experiment went on to achieve cooling rates a thousand times faster. Intense antiproton beams were viable. CollisionsThe crucial new element in the CERN accelerator network was the Antiproton Accumulator (AA). In this ring, stochastic cooling was used to accumulate beams containing over 1011 antiprotons – a hundred thousand million – tens of thousands of times more than ever before.The AA delivered the antiprotons to the Proton Synchrotron (PS) where they were accelerated for the first time in March 1981. They then went to the Super Proton Synchrotron (SPS) for further acceleration. By July, the first proton-antiproton collisions were being detected in the SPS, at a record energy of 540 GeV. All was going well, but could the experiments deliver good physics results? ExperimentsEven if they could be created, Ws and Zs would not live long enough to leave detectable tracks. The experiments would have to capture the particles they decay into.Work on two detectors, UA1 and UA2, started in 1979. Giants of their time, these huge constructions were installed in confined Underground Areas of the SPS. With millimetre precision, they were wrapped around the beam tube in regions where proton-antiproton collisions would occur. The experimental teams were also the largest to date. Carlo Rubbia formed the UA1 team with almost 140 collaborators, while Pierre Darriulat and later Luigi di Lella headed a group of some 60 physicists in UA2. Next: The Results |
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