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The AEgIS experiment at CERN's Antiproton Decelerator aims at measuring the gravitational acceleration $\bar{\hbox{g}}$ of antihydrogen ($\bar{\hbox{H}}$) with high precision [1, 2]. A key limitation in these measurements is the $\bar{\hbox{H}}$ temperature: The thermal motion of the $\bar{\hbox{H}}$ atoms blurs their free-fall trajectories and thus limits the achievable $\bar{\hbox{g}}$ precision.
The temperature of antihydrogen, which is formed at AEgIS in a laser-induced charge transfer reaction between positronium and antiprotons ($\bar{\hbox{p}}$), is dominated by the temperature of the antiproton precursors. With the current passive cooling scheme, the achievable $\bar{\hbox{p}}$ temperature is limited to at best tens (but to date a few hundred) kelvin.
Sympathetic cooling of antiprotons through thermalization with co-trapped laser-cooled ions would enable achieving temperatures in the mK range. However, to avoid annihilation, the co-trapped coolant ions must be negatively charged.
The Borealis project at AEgIS aims at realizing Doppler laser cooling of a negative ion. In particular, the diatomic molecular C$_2^-$ ion, a well-suited anion species for laser cooling [3], has been produced, mass-selected and stored in a linear Paul trap [4]. Currently, the capture efficiency of the trap and the lifetime of trapped C$_2^-$ ions are improved and preparations for preliminary in-beam spectroscopic studies are underway.
[1] G. Drobychev et al., Proposal for the AEGIS experiment at the CERN antiproton decelerator, CERN-SPSC-2007-017, SPSC-P-334.
[2] R. Caravita et al., AEgIS/AD-6 annual report 2023, CERN-SPSC-2023-003, SPSC-SR-321.
[3] P. Yzombard et al., Laser Cooling of Molecular Anions, Phys. Rev. Lett. 114, 213001 (2015).
[4] A. Hinterberger et al., Trapping of C$_2^-$ in a digital ion trap, J. Phys. B: At. Mol. Opt. Phys. 52, 225003 (2019).
