Towards a 1% gravity measurement on antihydrogen in ALPHA-g*

Experiments on trapped antihydrogen atoms are often conducted in superimposed Penning-Malmberg (PM) and Ioffe-Pritchard (IP) traps. The PM trap manipulates the charged particles necessary for anti-atom synthesis, while the IP trap confines the neutral antihydrogen produced. Tests of charge-parity-time symmetry through spectroscopy and the weak equivalence principle via controlled releases of anti-atoms in Earth’s gravitational field are performed within the trapping magnetic field. Detailed knowledge of the B-field trapping potential is crucial for understanding and enhancing the precision of these measurements.

The ALPHA experiment at CERN recently published the first results on the effect of gravity on antihydrogen, confirming that antihydrogen exhibits gravitational attraction to Earth, thus ruling out repulsive or zero gravitational interaction [1]. The next goal is to achieve a 1% precision in measuring the acceleration due to gravity on antihydrogen. However, the lack of comprehensive knowledge of the 3D magnetic fields within the trap is a key challenge.

Currently, magnetometry techniques available to ALPHA experiments without invasive hardware within traps are limited to on-axis locations. Techniques include Electron Cyclotron Resonance (ECR) using microwave pulses to illuminate quickly prepared low-density electron plasmas carefully positioned along the PM trap axis [2, 3], or extrapolations of the magnetron frequency of these plasmas [1]. This study presents 3D magnetometry results with additional radial off-axis control of these plasmas under sectored cylindrical PM electrodes, extending over 70% of the trap diameter (29.6 mm). Far off-axis ECR is performed on these displaced plasmas within the superimposed traps. The plasmas can be reproducibly restored on-axis and diagnosed, maintaining acceptable plasma characteristics. Probing the magnetic field with this technique and tuning the 3D shape of the trapping potential can allow to address the outstanding uncertainties in both experimental data and models, significantly enhancing the precision on the gravity measurement on antihydrogen.

[1] ALPHA. Nature, 621(7980):716–722, 2023

[2] ED Hunter. Physics of Plasmas, 27(3), 2020

[3] ED Hunter. Review of Scientific Instruments, 91(10), 2020