* Antihydrogen atom formation and spectroscopy [#j3ccb920]

(cited from ASACUSA proposal)

Since the ground state of antihydrogen has infinite lifetime, its high precision spectroscopy will
give unprecedented accuracies in terms of CPT symmetry tests. In the case of hydrogen, the
ground-state hyperfine splitting (GS-HFS) frequency νHF has been measured in a classic series
of experiments which began in the 1930’s with relatively simple atomic beam experiments, and
culminated with maser experiments in the early 1970s which ultimately achieved a relative
precision of order 10&super{−12};. For the antihydrogen atom, a measurement of νHF with precision equal
to that achieved in the hydrogen case some fifty years ago would constitute a commensurately
precise test of CPT symmetry. It may also be interpreted in terms of the gravitational interaction
of antimatter.
To the leading order, the GS-HFS of antihydrogen is proportional to the spin magnetic
moment of the antiproton, ⃗μp, which is experimentally known only at the level of 0.3%. Below
the level of several ppm accuracy, νHF also depends on the electric and magnetic form factors of
the antiproton. The measurements of νHF(H) to a relative accuracy of better
than 10&super{−6}; as discussed in this letter will therefore yield an improvement of the value of ⃗μp by three
orders of magnitude, and give some insight into the structure of the antiproton. Furthermore, the
only existing phenomenological extension of the standard model that includes CPT violations
(the standard model extension – SME – of Kostelecky’s group) predicts that CPT
violation in the 1S–2S transition is cancelled in first order, while for the hyperfine structure it
is a leading-order effect. In addition, the parameters introduced by Kostelecky et al. have the
dimension of energy (or frequency). Therefore, by measuring a relatively small quantity on an
energy scale (like the 1.4 GHz GS-HFS splitting), a smaller relative accuracy is needed to reach
the same absolute precision for a CPT test.

The key objective for either laser or microwave spectroscopy of antihydrogen atoms is to prepare
an adequate number of them in the 1S ground state at low temperature and to confine these in
a neutral atom trap. The principal mechanisms for antihydrogen atom synthesis are three body
recombination processes in a high density, low temperature plasma. This inevitably results in
the high-Rydberg state atoms which are not suitable for spectroscopic purposes.


:[[Cusp trap]] | It is 
expected that the total electric field of this octupole with the space charge of the trapped 
positrons themselves will be strong enough to confine antiprotons inside the positron cloud, 
where the two particle species will combine to produce antihydrogen atoms spontaneously via 
radiative recombination.

:[[Hbar detector]] | We adopted two type of detectors, 3D tracking detector collaborated with Brescia group (Italy) and MCP detector for detect extracted antihydrogen atoms.

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