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America goes straight for the Z particle

作者:墨昵出    发布时间:2019-03-04 06:16:01    

By CHRISTINE SUTTON in GENEVA PHYSICISTS at the Stanford Linear Accelerator Center in California (SLAC) have found their first examples of the subatomic particle called the Z° . This is not a new particle, but its appearance marks a long-awaited breakthrough in the laboratory’s struggles to pioneer a new concept in particle accelerators. Studies on the particles will help physicists to answer important questions, relating to astrophysics and cosmology, for example. The SLAC hosts the world’s longest linear accelerator, which takes electrons on a journey 3 kilometres long to reach an energy of 50 gigaelectronvolts (1 gigaelectronvolt is one thousand million electronvolts). For many years, physicists at the SLAC directed beams of energetic electrons at targets of hydrogen or metal. In this way, they found evidence for quarks – still smaller particles – within the protons and neutrons of atomic nuclei in the targets. But in such collisions, much of the energy of the beam is lost in setting the fragmented target in motion. However, by colliding two beams head-on, all the energy would drive the interaction between the particles, creating new kinds of particles. Physicists at the CERN first observed the Z° in this way in 1984 by colliding beams of protons and antiprotons together. The problem is that protons and antiprotons are composite particles. When they smash together, they produce a whole ‘zoo’ of particles which are difficult to analyse. Colliding simpler particles – electrons and positrons (antielectrons) – would produce a ‘cleaner’ result. In 1979, a team at the SLAC led by Burt Richter proposed a way of turning the linear accelerator (linac) into a machine that could collide electrons with positrons. The idea was to build two arcs of magnets at the end of the linac. The whole affair would resemble a tennis racket in shape. Electrons would go along one arc; positrons, following closely behind, would go along the other. And the two sets of particles would collide head-on at the point where the arcs met up. Richter’s dream became the Stanford Linear Collider (SLC). Electrons and positrons first arrived together at the collision point two years ago, but the researchers experienced difficulties with the machine. One important task has been to restrict the width of the beams of particles to a few micrometres. This optimises interactions between the beams because, in the SLC, they vanish after a single meeting. (In circular colliders, the beams can circulate for hours and cross many times.) Focusing the beams in this way proved difficult, partly because all the particles in each beam carry the same electrical charge and so repel one another. But by the end of last year, the team working on the accelerator managed to control the beams so that they met to within 1 micrometre. Earlier this year, however, new problems emerged when physicists began to operate a device called Mark II for detecting the exotic particles produced by the collisions. The detector became swamped by particles called muons, which were originating further back around the arcs. The researchers at the SLAC eventually solved this problem by installing magnetised iron to direct the muons away from the detector. And last week, everyone working on the SLC received their reward in the form of their first Z° particle. It left behind two jets of hadrons (subatomic particles made of quarks). This is the first time that a Z° particle has been seen to decay in this way. It is a neutral particle nearly 100 times as heavy as a proton, and it helps to carry the weak nuclear force between other particles. This is the force that underlies many forms of radioactivity. The SLC was designed to have just enough energy – 100 gigaelectronvolts in total – to create Z° particles freely. By studying the Z° particle in detail, physicists hope to answer questions such as how many types of the particle known as the neutrino exist. This result will have important implications for theories of cosmology and astrophysics. In Europe, CERN is building LEP, the Large Electron-Positron Collider, which is a circular machine 27 kilometres in circumference. It is due to produce its first collisions in July, also at a total energy of 100 gigaelectronvolts. Naturally, there is rivalry between the two laboratories, although Richter likes to play down his competitive spirit. But while LEP is certainly intended as a ‘Z° factory’, and will probably quickly overtake SLAC, the SLC is pioneering the way for a new concept in accelerators – the linear collider. LEP, which should eventually attain energies of 200 gigaelectronvolts, is as big as a circular electron-positron collider can go. But by bringing beams from two linacs to meet head-on, physicists hope to reach energies in the region of 1000 gigaelectronvolts. Such machines will need to work with highly focused, very intense beams,

 

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