Trapped antihydrogen: A new frontier in fundamental physics

Antihydrogen, the bound state of an antiproton and a positron, has been produced at low energies at CERN since 2002. Antihydrogen is of interest for use in precision tests of nature's fundamental symmetries. The charge conjugation/parity/time reversal (CPT) theorem, a crucial part of the foundation of the standard model of elementary particles and interactions, demands that hydrogen and antihydrogen have the same spectrum. Given the current experimental precision of measurements on the hydrogen atom, subjecting antihydrogen to rigorous spectroscopic examination would constitute a compelling, model-independent test of CPT. Antihydrogen could also be used to study the gravitational behaviour of antimatter. However, until recently, experiments have produced antihydrogen that was not confined, precluding detailed study of its structure. Experimenters working to trap antihydrogen have faced the challenge of trapping and cooling relativistic antiprotons and using them to make antihydrogen cold enough to be trapped in a magnetic minimum trap with a depth of only 50 μeV. In November 2010 the ALPHA collaboration demonstrated the first trapping of antihydrogen, thus opening the door to precision measurements on anti-atoms which can soon be subjected to many of the same techniques as developed for atoms. The prospect for such measurements improved further with ALPHA's demonstration of 1000 s confinement of the anti-atoms in the summer of 2011 and the recent first detection of resonant quantum interactions in antihydrogen.