Speaker
Description
The antihydrogen atom (the antimatter counterpart of a standard hydrogen atom) provides a unique testbed for probes of CPT invariance and the Weak Equivalence Principle (WEP) due to its mathematical simplicity and neutral charge respectively. However, prior to performing any such tests the antiatoms must first be synthesised from their constituent particles.
Presented here are the recent developments in antihydrogen synthesis and trapping made by the ALPHA collaboration in both the ALPHA-2 and ALPHA-g experiments. Novel techniques first reported in [1], (and implemented in ALPHA-2) use laser cooled $\mathrm{Be^{+}}$ ions to sympathetically cool $\mathrm{e^{+}}$ plasmas to around 7 K, compared to 18 K without $\mathrm{Be^{+}}$ ions. The resulting synthesis produces antihydrogen at lower temperatures thereby confining a larger fraction of the produced population within ALPHA’s magnetic minimum trap. Despite only trapping around 0.05% of the produced antihydrogen, ALPHA can repeat the synthesis cycle an arbitrary number of times and is now regularly able to accumulate samples over $10^{4}$ atoms.
Near-linear increases in the antihydrogen trapping with the number of antiprotons used in synthesis have further revealed that the sympathetic cooling provided by the $\mathrm{Be^{+}}$ ions not only reduces the $\mathrm{e^{+}}$ plasma temperature but also help to maintain it throughout synthesis. This additional cooling power counteracts the inherent heating rates associated with the merge of the antiproton and positron plasmas as well as heating due to patch potentials on the electrode surfaces and the radial octupole magnetic field (which provides radial confinement of the antihydrogen atoms in the magnetic trap). [2]
The effects of other parameters that contribute to the 3-body recombination reaction underlying antihydrogen formation at ALPHA [3] such as $\mathrm{e^{+}}$ density, $\mathrm{e^{+}}$ temperature and the shape of the magnetic minimum trap fields have also been studied in efforts to maximise the antiatom yield.
In 2024, the $\mathrm{Be^{+}}$ system was fully integrated into the ALPHA-g experiment where, as expected, it immediately outperformed the traditional synthesis reported in [4]. At present, the collaboration is able to obtain antihydrogen populations comparable to those in ALPHA-2 in the ALPHA-g machine.
The significant increase in antiatom availability for experiments is a paradigm shift in ALPHA’s capabilities which in the future may permit ALPHA to perform stronger tests of CPT invariance via precision spectroscopy of antihydrogen[5] (in the ALPHA-2 apparatus), as well as further probes of the Weak Equivalence Principle through future gravity release protocols (in the ALPHA-g machine).
[1] Akbari, R., de Araujo Azevedo, L.O., Baker, C.J. et al. Be+ assisted, simultaneous confinement of more than 15000 antihydrogen atoms. Nat Commun 16, 10106 (2025)
[2] Goncalves, Maria. Be+-Assisted Antihydrogen Synthesis and Trapping. Swansea, 2026.
[3] Zammit, M. et al. Antihydrogen Chemistry, Phys. Rev. A 111, 050101 (2025)
[4] Anderson, E.K., Baker, C.J., Bertsche, W. et al. Observation of the effect of gravity on the motion of antimatter. Nature 621, 716–722 (2023)
[5] Baker, C.J., Bertsche, W., Capra, A. et al. Precision spectroscopy of the hyperfine components of the 1S–2S transition in antihydrogen. Nat. Phys. 21, 201–207 (2025)
| Academic level | PhD student |
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