Antihydrogen, positronium and positrons

Swansea Physics has long been active in antimatter science and currently Mike Charlton, Stefan Eriksson, Niels Madsen and Dirk Peter van der Werf are key members of the ALPHA antihydrogen collaboration. ALPHA is an international team based at the unique Antiproton Decelerator Facility located at CERN. ALPHA has recently succeeded in trapping antihydrogen atoms in a magnetic minimum neutral atom trap – some have been held for periods in excess of ten minutes. This allowed the first ever experiment involving a quantum transition of an antimatter atom to be performed; a microwave transition was excited between the hyperfine levels of the ground state.

Currently ALPHA is re-tooling and is developing an apparatus to allow laser light to be introduced. Our plan is to excite the two-photon, Doppler free 1S-2S transition for comparison with the same frequency interval in atomic hydrogen. The latter is known with astonishing precision (better than parts in 1014), and the aim is, eventually, to rival this precision with antihydrogen, as a test of some of the fundamental symmetries of nature.

The team have also joined the GBAR collaboration. This group aims to make precision measurements of the acceleration of antihydrogen in the gravitational field of the Earth for tests of the Weak Equivalence Principle. The methodology involves creation and then cooling of trapped antihydrogen positive ions, before photo-detachment to produce the anti-atom – which will then undergo free fall!

Mike, Niels and Dirk were also members of the pioneering ATHENA antihydrogen collaboration. This team was the first to produce (in 2002) cold antihydrogen by the controlled mixture of antiproton and positron clouds.

The positrons for the antihydrogen experiments are controlled using a buffer gas accumulator – essentially a form of charged particle trap – forming part of slow positron beamline. At Swansea we have a similar device, though one that was designed to interface with a laser system (developed with Helmut Telle) with the aim of exciting positronium atoms to Rydberg states. The team has recently seen their first 1S-2P transitions.

This was made possible by a series of advances in positron cloud manipulation in the trap involving novel applications of a rotating electric field. Positron clouds of sub-millimetre dimensions and containing up to 105 particles can be produced for experiments at 1-10 Hz.