Phase 3: Developments towards HFS measurements with trapped antihydrogen
A further increase in precision can be obtained from using colder beams of antihydrogen, and ultimately by trapping and laser cooling of antihydrogen. A development towards this goal will be started together with the AEgIS collaboration at CERN-AD, whose primary aim is to make a measurement of the gravitational acceleration of antimatter by watching an antihydrogen beam fall [14]. This collaboration intends to produce an ultra-cold antihydrogen beam from a nested Penning trap kept below 1 K (ultimate goal: 100 mK), while the beam from the cusp trap is expected to have a temperature of ~50 K. The beam is not polarized, so an additional sextupole will be needed to polarize the beam. Further simulations performed after submitting the proposal showed that the beam also is expected to have different size than the one of the cusp trap, typically 2-3 cm rather than 10 cm. This and the lower velocity will lead to a very different design of the sextupoles (smaller diameter and shorter length) so that the previously planned copy of the existing large-diameter sextupole is not more reasonable. A new design has to be developed based on first measurements of the beam parameters of AEgIS.
Slower antihydrogen atoms can only be obtained by trapping and laser cooling. This way, temperatures of mK can be reached [15]. For hyperfine spectroscopy, the magnetic gradients of the Ioffe-Pritchard trap used for antihydrogen trapping e.g. by the ALPHA collaboration [10] make it difficult to determine the ground-state hyperfine splitting due to large Zeeman shifts. Therefore the atoms need to be ejected again from the trap into a field-free region for HFS spectroscopy, similar to what has been achieved in atomic fountains of sodium atoms [16]. A measurement using this technique for antihydrogen lies outside the scope of this ERC grant, but initial steps towards this goal will be made. AEgIS plans to trap the antihydrogen beam in a neutral atom trap using e.g. Zeeman slowers outside the magnets used for its formation, where access for laser cooling and spectroscopy will be easier and an apparatus for HFS spectroscopy could be attached.