Atoms of antimatter uring 1995 an experiment at under the action of the electromag­ In January, an explosion of DCERN synthesized nine atoms netic force, anti-atoms, composed of publicity greeted the news that of anti-hydrogen, the first time that positively charged anti-electrons atomic antimatter had been true atomic antimatter has been (positrons) orbiting around negatively synthesized in an experiment at seen. While physicists are naturally charged anti-nuclei, will behave like CERN. For scientists, the fact excited about the discovery, the real conventional chemical atoms, where that atomic (chemical) antimatter business has yet to come - precision negative electrons orbit around can exist has long been taken for spectroscopy to see whether matter positively charged nuclei. granted, but now the door is open and antimatter really behave in the While the list of known antiparticles to its scientific exploration. For same way. has grown steadily, the difficulties of particle physicists, the challenge The underlying rules of physics say synthesizing such atomic antimatter is to make precision tests of that every elementary particle have meant that physicists have had matter and antimatter symmetry constituent of the Universe has a to wait until now for the first sighting to test that their understanding of corresponding antiparticle, carrying of the simplest anti-atom - the underlying principles is valid. equal and opposite quantum antihydrogen, with a lone positron With everyday language ill- numbers (charges). When a particle orbiting around a nuclear antiproton. suited to describing the bizarre and its antiparticle meet, they most During 1995 a Julich (IKP quantum world, conveying the likely annihilate, disappearing in a Forschungszentrum)/Darmstadt ideas of particle physics to a puff of radiation. (GSI)/Erlangen-Numberg/Genoa general audience is a continual In principle a whole 'mirror' Uni­ (University and INFN)team challenge. Even though particle verse of antimatter could be built up (spokesman Walter Oelert) working physicists read the more lurid of from these antiparticles in the same at CERN's LEAR low energy the mass-media antimatter way as our Universe of ordinary antiproton ring has for the first time reports sceptically, they were matter is made of particles. If our constructed antihydrogen. However encouraged to know that at least understanding of physics is correct, the experiment only saw the decay some aspects of their work are then most of the time this antimatter products of antihydrogen, and the widely appreciated. will behave in exactly the same way ultimate goal remains to isolate However media suggestions as the matter we know. For example 'chemical' antimatter. that antimatter could soon be exploited outside the laboratory are absurd. It is just too difficult and expensive to make. Without radical (and as yet inconceivable) improvements in technique, the Universe has not been in existence long enough for any laboratory in it to amass enough chemical antimatter to produce any startling effect. Walter Oelert (left), spokesman of the PS210 experiment at CERN's LEAR low energy antiproton ring which saw first evidence for chemical antimatter, with Mario Macri, spokesman for the 'parent' JETSET experiment. CERN Courier, March 1996 1 Atoms of antimatter History of antimatter In 1898 Arthur Schuster in the UK anticipated the existence of antimat­ ter, surmising that there might be atoms with properties exactly oppo­ site to those of ordinary atoms. However the most compelling argu­ ments that antiparticles had to exist emerged from Paul Dirac's work in the late 1920s which reconciled quantum mechanics and special relativity. His electron equations had two solutions, one corresponding to the known electron, the other, with equal but opposite electric charge, to something else. At first Dirac tried to explain the second solution as the proton, but as this is nearly 2,000 times heavier than the electron, the idea would not work. Other quantum mechanics pioneers, for example Werner The payoff came almost immedi­ Schematic of the CERN antimatter experiment. Heisenberg, had always been im­ ately. In 1955 Owen Chamberlain, Collisions between gas nuclei and antiprotons form by-product electron-positron pairs, and if pressed by the power of Dirac's Emilio Segre, Clyde Wiegand and the conditions are right (antiproton and reasoning, but found the unexplained Thomas Ypsilantis discovered the positron velocities very close), the emerging new solutions difficult to swallow. long-awaited antiproton, a feat which positron can be captured by a neighbouring Undeterred, Dirac suspected that his brought Chamberlain and Segre the antiproton to form a neutral atom of antihydrogen. This flies off in a straight line, equations predicted the existence of 1959 Nobel Physics Prize. escaping from the magnetic antiproton orbit of a shadow 'antimatter' world. To synthesize atomic antihydrogen the LEAR ring. Fortunately he did not have long to means bringing together artificial wait. In 1932 Carl Anderson discov­ sources of antiprotons and of ered the positron, the antimatter positrons - neither of which exist counterpart of the electron, in cosmic naturally. In addition, antihydrogen, To make antihydrogen requires two ray tracks. Having sold the idea, the like its matter counterpart, is only things - a lot of antiprotons, and a next big antimatter prize was the stable at low energies. Once the way of 'cooling' them to low enough antiproton, however manufacturing 'temperature' exceeds about 14 temperatures that antihydrogen does these - which have to be produced as electronvolts (140,000 degrees), not immediately evaporate into its proton-antiproton pairs - requires a hydrogen and antihydrogen component particles. big step up in energy. The 2 GeV of dissociate (ionize) into their The world's first antiproton 'factory' rest energy needed to create such a component particles. is at CERN, where since 1980 a pair requires a beam energy of at In our cool everyday world, atomic complex of specially built machines least 5.6 GeV, and the Bevatron at hydrogen can exist quite happily. has provided dense beams of Berkeley was built with this goal in However in the high energy antiprotons for physics experiments. mind. When this machine came into conditions needed to create With these antiprotons, CERN was operation in 1954, and until the antiprotons, any chance positrons able to operate the SPS proton- CERN PS came on in 1959, it was are immediately ripped from the antiproton collider which in 1983 the highest energy particle accelera­ antiprotons before atoms have time discovered the W and Z carriers of tor in the world. to form. the weak nuclear force. 2 CERN Courier, March 1996 Atoms of antimatter Another feature of CERN's downstream of the jet target. The new electromagnetic 'bottles' and provide antiproton programme is the LEAR experiment used a jet of a heavy gas, detailed comparisons of the low energy antiproton ring, in action xenon, to boost the production rate. behaviour of matter and antimatter. since 1983, which slows the particles Antihydrogen is difficult to detect The world's other antiproton factory down to explore proton-antiproton directly, because the antiatoms is at Fermilab, where a new experi­ annihilation at lower energies. immediately annihilate with their ment, E862, a Fermilab/lrvine/Penn LEAR was in fact designed with the matter surroundings. Passing them State/SLAC collaboration, will soon possibility of antihydrogen production through thin silicon detectors (which produce antihydrogen by passing an in mind! Antihydrogen atoms, being 'strip' away the positron) immediately 8.9 GeV antiproton beam through a electrically neutral, could escape the dissociates the antihydrogen back gas jet target. magnetic clutch of the normal into an antiproton and a positron. antiproton orbit of the LEAR ring: exit However the two particles remain in windows were incorporated in the close proximity and the trick is to pick ring's straight sections, while the up the tell-tale simultaneous bending magnet design ensured that antiproton/positron pairs which have any such particles would not be escaped from the normal antiproton blocked. orbit of the LEAR ring. The nine antiproton atoms each remained intact for some 40 billionths of a The experiment second, travelling some ten metres. One hundred years They were carefully sifted out from The CERN antihydrogen 23,000 counts, with particular care to the day experiment uses apparatus originally being taken to eliminate 'rogue' built for 'JETSET', an experiment by counts due to antineutrons. The recent discovery at CERN of a Bari/CERN/Erlangen-Nurnberg/ Experiments at LEAR and antihydrogen caught the attention Freiburg/Genoa/lllinois/Julich/Oslo/ elsewhere have long studied of the media more than any other Uppsala collaboration (spokesman antiprotonic atoms, where, instead of physics development in recent Mario Macri) which collides the LEAR an electron, a negatively-charged years. The CERN press release antiproton beam with a jet of gas to antiproton orbits round a conven­ which started the bandwagon search for new particles formed in tional nucleus. With the orbit of the rolling went out on 4 January. the proton-antiproton annihilations. heavy antiproton passing much Broadcast over the Internet via The widely-acclaimed gas jet closer to the nucleus than that of an the World Wide Web, it made technology developed by JETSET is electron, comparing the spectroscopy prime time TV and headlines all