Among the tests of the Standard Model at low energy, those made with antimatter have a special status, whether they concern precision tests of the CPT theorem or the study of the effects of gravitation on antimatter. Three experiments are being developed for these gravitational tests, ALPHA-g, AeGis and GBAR (Gravitational Behavior of Antimatter at Rest) [1,2]. All three benefit from the use of the new ELENA antiproton cooling storage ring, commissioned in 2018 [3]. Recently, ALPHA-g published a first result [4] showing that the sign of the gravitational constant for antimatter is the same as the one for matter. The GBAR experiment is based on a proposal by J. Walz and T. Hänsch [5] and was accepted at CERN in 2012. The aim is to produce ultra-cold antihydrogen (10µK), much colder than in the other 2 experiments, much below the Doppler limit (2mK) imposed by the direct cooling of antihydrogen by laser. GBAR proposes to manufacture and cool anti-H ions by sympathetic cooling with Be+. These ions when obtained, will be the first 3-body antimatter ions ever produced. They will be created by the interaction between a beam of low-energy antiprotons and positroniums in excited levels. These ions will then be photoionized by laser without any vertical recoil, and their fall time accurately measured. In this seminar, I will describe the present status of GBAR. In particular, I will describe the improvement in positron production and storage [6,7] over the years, and antihydrogen production [8]. I will then describe the method for the production and cooling of anti-H ions. Finally, I will present the methods foreseen to improve the accuracy of the measurement of ultracold antihydrogen once it has been produced. I will also give a brief description of other experiments that will use the GBAR beamline for the study of antiprotonic atoms and the measurement of the Lamb shift of antihydrogen [10].
References
[1] P. Indelicato et al., Hyp. Int. 228, 141 (2014).
[2] P. Perez and Y. Sacquin, Class. Quantum Gravity 29, 184008 (2012).
[3] W. Bartmann et al., Philos. Trans. R. Soc. A 376, 20170266 (2018).
[4] E. K. Anderson et al., Nature 621, 716 (2023).
[5] J. Walz and T. W. Hänsch, Gen. Relativ. Gravit. 36, 561 (2004).
[6] P. Blumer et al., Nucl. Instrum. Methods A 1040, 167263 (2022).
[7] M. Charlton et al., Nucl. Instrum. Methods A 985, 164657 (2021).
[8] P. Adrich et al., Eur. Phys. J. C 83, 1004 (2023).
[9] N. Paul et al., Phys. Rev. Lett. 126, 173001 (2021).
[10] N. Kuroda et al., J. Phys. Conf. Ser. 875, 022054 (2017).
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