Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: In-beam measurement of the hydrogen hyperfine splitting - towards
  antihydrogen spectroscopy   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's
antihydrogen experiment. The measured value of
$\nu_\mathrm{HF}$=$1~420~405~748.4(3.4)(1.6)~\textrm{Hz}$ with a relative
precision of $\Delta$$\nu_\mathrm{HF}$/$\nu_\mathrm{HF}$=$2.7\times10^{-9}$
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 $2.7~\textrm{m}$ downstream of the production region, the
prerequisites for a measurement with antihydrogen are now available within the
ASACUSA collaboration.

36
The categories of this paper are: cmp-lg
4
The categories of this paper are: cond-mat
35
The categories of this paper are: mtrl-th
14
The categories of this paper are: math-ph
21
The categories of this paper are: adap-org
31
The categories of this paper are: ao-sci
24
The categories of this paper are: dg-ga
18
The categories of this paper are: econ
32
The categories of this paper are: atom-ph
37
The categories of this paper are: supr-con
6
The categories of this paper are: astro-ph
8
The categories of this paper are: hep-ex
15
The categories of this paper are: nucl-ex
9
The categories of this paper are: nlin
10
The categories of this paper are: q-bio
7
The categories of this paper are: hep-th