Antiprotons and antihydrogen

Positron emission in the decay of radioactive elements is not the only source of antimatter. At particle accelerators high-energy protons can be made to collide with nuclei in a target, or indeed head-on with one another. The resultant energy release creates many new particles, and sometimes particle-antiparticle pairs such as protons and antiprotons. These emerge at high speed and can be separated and channelled into beams. One such beam is the beam of antiprotons at the centre of European nuclear research (CERN) near Geneva.

Atoms of antihydrogen

In August 2002, physicists at CERN announced the first controlled production of large numbers of atoms of antihydrogen. Antiprotons were slowed down until they were very slow by the standards of accelerators, and then trapped in an electromagnetic cage. They were then mixed with about 75 million positrons from radioactive decay of sodium-22, caught in a second trap. There was now created an environment of antimatter, in which the positrons and antiprotons were 'cold', i.e. moving slowly enough to have a significant chance of joining up as atoms. The team estimated that about 50,000 antihydrogen atoms had been produced in the first few weeks of the experiment.*

The next step will be to study the light emitted from the antihydrogen, and compare its spectrum with that of normal hydrogen. If there is a difference, no matter how small, it will

* Some more details of the experiment are given in Appendix 17.1.

mean that nature has come up with yet another major surprise: a fundamental basic asymmetry between matter and antimatter.*

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