Spatial confinement of muonium atoms
  We report the achievement of spatial confinement of muonium atoms (the bound state of a positive muon and an electron). Muonium emitted into vacuum from mesoporous silica reflects between two SiO$_2$ confining surfaces separated by 1 mm. From the data, one can extract that the reflection probability on the confining surfaces kept at 100 K is about 90% and the reflection process is well described by a cosine law. This technique enables new experiments with this exotic atomic system and is a very important step towards a measurement of the 1S-2S transition frequency using continuous wave laser spectroscopy. 
K. S. Khaw, A. Antognini, T. Prokscha, K. Kirch, L. Liszkay, Z.   Salman, P. Crivelli
physics.atom-ph hep-ex