Trapped Antihydrogen, the ALPHA Experiment at CERN
Prof. Jeffrey Hangst (Dept. of Physics and Astronomy, Aarhus, Denmark and CERN)

Antihydrogen, the bound state of an antiproton and a positron, can be used as a test-bed of fundamental symmetries. In particular, the CPT Theorem requires that hydrogen and antihydrogen have the same spectrum. The current experimental precision of measurements of hydrogen transition frequencies approaches one part in 1014. Similarly precise antihydrogen spectroscopy would constitute a unique, model-independent test of CPT symmetry. Antihydrogen atoms have been produced in quantity at CERN since 2002, when the ATHENA collaboration demonstrated1 how to mix cryogenic plasmas of antiprotons and positrons to produce low energy anti-atoms. In this colloquium I will discuss the newest development along the road to antihydrogen spectroscopy: magnetically trapped antihydrogen. In November of 2010 the ALPHA collaboration reported2 the first trapping of antihydrogen atoms in a magnetic multipole trap. The atoms must be produced with an energy - in temperature units - of less than 0.5 K in order to be trapped. Subsequently, we have shown that trapped antihydrogen can be stored3 for up to 1000 s, and we have performed the first resonant quantum interaction experiments with anti-atoms4. I will discuss the many developments necessary to realise trapped antihydrogen, and I will discuss the future with our new apparatus, ALPHA-2, currently being commissioned at CERN.

1. Amoretti, M. et al., Production and detection of cold antihydrogen atoms. Nature 419, 456 (2002).
2. Andresen, G.B. et al., Trapped Antihydrogen, Nature, 468, 673 (2010).
3. Andresen, G. B. et al. Confinement of antihydrogen for 1,000 seconds. Nature Physics 7, 558 (2011).
4. Amole, C. et al., Resonant quantum transitions in trapped antihydrogen atoms, Nature 483, 439 (2012).