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A Successful Start to Preparations for the Babar Experiment

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A Successful Start to Preparations for the Babar Experiment In June this year, the first part of new collider A successful called PEP-II was commissioned. It will be used in an experiment to observe a rare effect start to in particle physics that could explain why there appears to be only matter and not antimatter in the Universe. The effect is preparations called CP violation, and the experiment will measure for the first time subtle differences in for the BaBar the way fundamental particles called B mesons and their antiparticles decay. experiment According to theory, particles and their antimatter partners should behave like exact mirror images of each other. However, the B by Professor Mike Green mesons and their antiparticles are predicted Royal Holloway, University of London to break this mirror symmetry by a tiny amount. Extract taken from the PPARC Annual Report 1996-97 The so-called BaBar experiment (so named because the symbol for the anti-B meson is a letter 'B' with a bar across the top, and permission was obtained from the author of Babar the Elephant for the use of his name!) takes advantage of the 3-kilometre-long Stanford Linear Accelerator in California which creates and accelerates beams of electrons and their antiparticles, positrons. The beams are then injected into PEP-II. This consists of two concentric rings, 2 kilometres across, which will store the separate beams of electrons and positrons. In the rings, the beams travel close to the speed of light under the influence of electric and magnetic fields which accelerate, guide and focus the beams. The ring being used to store electrons was already in existence, and that is the one which has just been commissioned. The second ring is completely new and is currently under construction. It will be commissioned in April 1998. When the experiment starts in 1999, the two beams will cross in a small region surrounded by a large detector, about 6 metres long and 6 metres in diameter, which will observe the In May, the electron beam from the linear accelerator was particles produced in the electron-positron successfully diverted down a beam-transfer line towards collisions. The energy of PEP-II will be the PEP storage rings- A phosphorescent screen placed at precisely tuned to 10.580 giga-electrovolts the entrance to the ring reveals photographic evidence of (GeV) which is the optimum energy for electrons by glowing brightly where the flat beam of particles hits it close to the central target cross. producing B mesons. A novel feature, essential for the experiment, is Although a small and apparently esoteric effect, that the energy of the particles in the two rings is CP violation is believed to have played an different. The electron ring stores electrons at important role in the first fractions of a second 9GeV while the second ring will store positrons after the Big Bang. The consequence was that at 3.1GeV. This arrangement will ensure that the Universe we know today contains only the B mesons produced will have gained enough matter and no antimatter. If there had been momentum so that their lifetimes can be equal amounts of matter and antimatter, they determined by measuring the distance travelled would have annihilated each other, so without before they decay. CP violation we could not exist! However, particle physicists and cosmologists are The detector is being built by a collaboration currently not able to reconcile the measured including about 30 physicists, engineers and amount of CP violation for the K meson with the PhD students from Rutherford Appleton observed structure of the Universe, and further Laboratory and university research groups at clues are nceded. UK particle physicists are Bristol, Brunel, Edinburgh, Liverpool, looking forward to finding those clues when Manchester, Imperial College, Queen Mary and BaBar detector begins taking data. Westfield College, and Royal Holloway, London. The collaboration will measure detail properties of the B mesons and their antiparticles by detecting about 100 million examples of their decay over a period of five years. The researchers expect CP violation to reveal itself through the different decay rates of the two particles. This effect has been previously observed only in the decay Schematic diagram of the Stamford Linear Accelerator and the PEP-II storage rings. The BaBar detector (not to of a particle called the K meson. scale) surrounds the beam crossing region..
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