@article {79,
	title = {In-beam measurement of the hydrogen hyperfine splitting - towards antihydrogen spectroscopy},
	journal = {Nature Communications},
	volume = {8},
	year = {2017},
	month = {12/06/2017},
	pages = {1{\textendash}9},
	abstract = {<p>Antihydrogen, the lightest atom consisting purely of antimatter, is an ideal laboratory to study the CPT symmetry by comparison to hydrogen. With respect to absolute precision, transitions within the ground-state hyperfine structure (GS-HFS) are most appealing by virtue of their small energy separation. ASACUSA proposed employing a beam of cold antihydrogen atoms in a Rabi-type experiment to determine the GS-HFS in a field-free region. Here we present a measurement of the zero-field hydrogen GS-HFS using the spectroscopy apparatus of ASACUSA{\textquoteright}s antihydrogen experiment. The measured value of {\$}\backslashnu{\_}\backslashmathrm{\{}HF{\}}{\$}={\$}1{\~{}}420{\~{}}405{\~{}}748.4(3.4)(1.6){\~{}}\backslashtextrm{\{}Hz{\}}{\$} with a relative precision of {\$}\backslashDelta{\$}{\$}\backslashnu{\_}\backslashmathrm{\{}HF{\}}{\$}/{\$}\backslashnu{\_}\backslashmathrm{\{}HF{\}}{\$}={\$}2.7\backslashtimes10{\^{}}{\{}-9{\}}{\$} constitutes the most precise determination of this quantity in a beam and verifies the developed spectroscopy methods for the antihydrogen HFS experiment to the ppb level. Together with the recently presented observation of antihydrogen atoms {\$}2.7{\~{}}\backslashtextrm{\{}m{\}}{\$} downstream of the production region, the prerequisites for a measurement with antihydrogen are now available within the ASACUSA collaboration.</p>
},
	doi = {10.1038/ncomms15749},
	url = {https://doi.org/10.1038/ncomms15749},
	author = {Diermaier, M. and Jepsen, C. B. and Kolbinger, B. and Malbrunot, C. and Massiczek, O. and Sauerzopf, C. and Simon, M. C. and Zmeskal, J. and Widmann, E.}
}
