Antihydrogen atom formation and spectroscopy

(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.

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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.