Antihydrogen ground-state hyperfine structure measurement
The experiment aims at a measurement of the ground-state hyperfine structure of antihydrogen by using a Rabi-type atomic beam setup [1] (see Fig. 1 below (left)). Within the ASACUSA collaboration, groups from RIKEN and Tokyo University are developing a so-called CUSP trap that is expected to produce a polarized beam of antihydrogen atoms by using an anti-Helmholtz type setup consisting of two ring currents running in opposite direction. SMI is in charge of providing the spectrometer line consisting of a spin-flip cavity, and sextupole magnet for spin analysis, and an antihydrogen detector. All necessary devices had been built and were ready to use for the last beam time in 2012.
During 2013, when no beam at CERN was available because of the LHC Long Shutdown 1, the simulation code based on Geant4 for tracking of (anti)hydrogen atoms through the inhomogeneous magnetic fields as well as the detection of antihydrogen annihilations was continued. A central experimental activity was the development of a mono-atomic polarized hydrogen source to characterize the antihydrogen spectroscopy beam line. The hydrogen source was finished at SMI, transportet to CERN and set up there in fall of 2013, tested and connected to the antihydrogen spectrometer line in early 2014. Studies were made on the homogeneity of the constant magnetic holding field to be applied inside the cavity by finite element simulations of the effect of magnetic shielding and magnetic field measurements.
In parallel data taken during the beam time 2012 were analyzed and submitted for publication in 2013, which appeared early 2014 [2]. By placing an antihydrogen detector 2.7 m downstream of the formation region after the spectrometer beam line we could for the first time unambiguously detect antihydrogen atoms in a region free from stray fields (cf. Fig. 1 (right)). This is an important mile stone towards the in-beam measurement of the antihydrogen GS-HFS. The measured parameters of the antihydrogen beam, intensity (about 20 events per hour) and quantum state (a large fraction of the atoms are in states with principal quantum number n≤29) still need optimization before a GS-HFS measurement can be done. This is planned to start from 2014, where new schemes of mixing antiprotons and positrons that create colder antihydrogen will be tried and new field ionization electrodes will be installed to be able to ionize atoms in lower states.
In 2014 the full setup was installed at the AD for the first time and commissioned.
[1] Widmann, E., Diermaier, M., Juhász, B., Malbrunot, C., Massiczek, O., Sauerzopf, C., et al. (2013). Measurement of the hyperfine structure of antihydrogen in a beam. Hyperfine Interactions, 215, 1–8. doi:10.1007/s10751-013-0809-6
[2] Kuroda, N., Ulmer, S., Murtagh, D. J., Van Gorp, S., Nagata, Y., Diermaier, M., et al. (2014). A source of antihydrogen for in-flight hyperfine spectroscopy. Nature Communications, 5. doi:10.1038/ncomms4089