Antihydrogen is the simplest atom consisting entirely of antimatter. Since its hydrogen counterpart is one of the most precisely measured atoms in physics, a comparison of antihydrogen and hydrogen offers one of the most sensitive tests of CPT symmetry. This project proposes to measure the ground state hyperfine splitting (HFS) of antihydrogen (HBAR), which is known in hydrogen with relative precision of 10–12. It is the central part of the program of the ASACUSA collaboration at CERN-AD. The experimental method consists of the formation of an antihydrogen beam and a measurement using a spin-flip cavity and a sextupole magnet as spin analyser like it was done initially for hydrogen. A major milestone was achieved in 2010 when antihydrogen was first synthesized by ASACUSA. In the first phase of this project, an antihydrogen beam will be produced and the HBAR-HFS will be measured to a precision of around 10–7 using a single microwave cavity.
In a second phase, the Ramsey method of separated oscillatory fields will be used to increase the precision by about one order of magnitude. In parallel methods will be developed towards trapping and laser cooling the antihydrogen atoms. Letting the cooled antihydrogen escape in a field free region and performing micro-wave spectroscopy offers the ultimate precision achievable to measure the HBAR-HFS and one of the most sensitive tests of CPT. Assuming the antihydrogen atoms would pass a height difference of 1 m in an atomic fountain, a line width of about 1 Hz will be feasible corresponding to a relative accuracy of 10-9. The time needed to achieve this goal, however, exceeds the scope of the current ERC grant.