BASE - Annual Report 2021

This report summarizes the progress made by the BASE collaboration in 2021. It contains short summaries of the main output produced, which are three manuscripts published, one in Physical Review Letters and two in Nature. One of these manuscripts sets constraints on the coupling of axions and axion-like particles to photons using the highly sensitive single particle detection systems in the strong magnetic field of the superconducting magnet of the experiment. As a consequence of the successful demonstration of this LC-resonator-based haloscope technique we started constructing the new experiment BASE-CDM. The second manuscript reports on the first demonstration of sympathetic cooling of a single trapped proton by coupling the particle to a cloud of laser-cooled $^9$Be$^+$ ions using a common-endcap Penning trap method and a superconducting tuned circuit that mediates the cooling. This work was selected by "Physics World" as one of the top ten physics breakthroughs made in 2021. The third manuscript reports on a 16-parts-per-trillion comparison of the proton-to-antiproton charge-to-mass ratio. This measurement improves the previous best limit by a factor of 4.3, and contains the first-ever performed clock-based differential test of the weak equivalence principle with protons and antiprotons. In addition, the group focused on the development and implementation of the new ELENA/BASE interface, and the setup of an experiment dedicated to measure the antiproton magnetic moment with a fractional precision at a level of $\approx100$ parts in a trillion. This includes the setup of a new trap stack, the development of revised cryogenic experiment electronics and superconducting single particle detectors, as well as the implementation of a new magnet shimming system and the development of a cooling trap for sub-thermal cooling cycles at drastically reduced thermalization times.