Finding antimatter in the real world


By Roger Highfield Dirac’s peers did not believe in antimatter when he first unveiled his radical ideas. Then, in 1932, physicist Carl Anderson was examining tracks produced by cosmic rays in a cloud chamber, a device that detects high-energy particles by the trails they leave when they whizz through water vapour. One particle made a track like an electron, but the way its path bent in a magnetic field showed that it was positively charged. He named it the positron. This was Dirac’s anti-electron. Today, antimatter is primarily found in cosmic rays – extraterrestrial high-energy particles that form new particles as they zip into the Earth’s atmosphere. It also appears when scientists smash together particles boosted to high energies in machines called accelerators. For over 50 years now, laboratories like the European Organisation for Nuclear Research (CERN) that sits on the Swiss-French border near Geneva have routinely produced antiparticles. In 1995 CERN became the first laboratory to create anti-atoms artificially. One of the more dramatic findings – exploited in many a science fiction adventure – is that antimatter and matter explode on contact. Because these annihilations produce energy in the form of radiation and particles, scientists can use instruments to measure the “wreckage” of these collisions. However, no experiments have yet detected the antigalaxies or vast stretches of antimatter in space that Dirac imagined. Scientists still send observatories into space to look for them though, just in case. Theories of physics say that the universe exploded into existence some 13.7 billion years ago with the big bang. At that instant, matter and antimatter existed in equal amounts because the laws of nature require that matter and antimatter be created in pairs. Yet somehow, an eye-blink after the big bang, matter had begun to predominate, so that for every 1,000,000,000 antiparticles there were 1,000,000,001 particles. Matter and antimatter cannot stand each other’s presence. Instead they destroy each other in a flash of light whenever they come into contact. So within the first second of the universe, every billion antimatter particles had annihilated with a billion matter particles. All that was left behind was the extra smidgen of matter that went on to build stars, galaxies, planets and even you. So far, physicists have not been able to explain this apparent “asymmetry”, or lopsidedness, between matter and antimatter. Some think it was down to a breakdown in the laws of physics in those first few seconds. A few speculate that the annihilation never happened and that antimatter lives on somewhere in a part of the cosmos that we cannot see. Most scientists believe that a subtle difference in the way matter and antimatter interact with the forces of nature may account for why the universe prefers matter. In 1967, Russian theoretical physicist Andrei Sakharov postulated several conditions necessary for the prevalence of matter. One required something called “charge-parity violation”, which is an example of a kind of asymmetry between particles and their antiparticles that describes the way they decay. Physicists have confirmed the existence of this asymmetry in the workings of exotic particles known as kaons and B mesons, but they’ve never found it prevalent enough to explain why the universe is dominated by matter. The latest experiment to investigate, backed by the STFC, is a facility called LHCb. It is one of four large particle physics experiments that monitors particle collisions in the Large Hadron Collider at CERN,
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