CE30 - Physique de la matière condensée et de la matière diluée

Photodetachment of H- and H¯? ions – Photoplus

Submission summary

The aim of the GBAR collaboration at CERN is to measure the acceleration in the Earth's gravitational field of a free-falling anti-hydrogen atom H¯. To do this, a H¯? ion must be cooled to less than 10 µK and then photodetached, releasing a H¯ atom with a low initial velocity, the fall of which will be timed. This technique of production of slow H¯ atoms is unique to the GBAR project, among other projects with the same objective. However, it requires to master all aspects of the photodetachment process, including temporal and spectral characteristics, power and recoil effects, in order to guarantee a quantitative output of the photodetachment-based project.

The Photoplus project gathers several teams of the Laboratoire Kastler Brossel involved in the GBAR collaboration and the Negative ion team of the Laboratoire de physique des plasmas, specialized in the study of photodetachment. The aims of the project are to build the laser for the GBAR experiment and to study several aspects of photodetachment that need to be controlled in order to ensure the relevance and reliability of the experiment at CERN.

More precisely, the objectives are :
- To bring the experimental knowledge of the electron affinity of H to ±1 µeV, which is the precision required for the GBAR experiment. To do so, a photodetachment experiment will be carried out on a H? beam, inside an optical cavity. The electron affinity will be measured with the photodetachment microscope, which has been the most precise instrument in the world, for atomic electron affinity measurements, since the beginning of the 21st century.
- To carry out an independent calculation of the photodetachment threshold and model all systematic effects, including the threshold shift due to ponderomotive effects, and confront them with the preliminary experiment on H?.
- To build a tunable laser system with a large enough power and a sufficient spectral resolution to reach a photodetachment probability larger than 80% at 30 µeV above the threshold in less than a millisecond, which is a necessary condition for the GBAR experiment.
- To find the optimal parameters of the photodetachment laser to optimize the final accuracy of the GBAR experiment.
- To install and run the laser on the GBAR setup at CERN.

The asymmetry between matter and antimatter observed in the Universe is one of the fundamental problems of modern physics that presently challenge the Standard Model. By measuring the interaction of antimatter with gravity, the GBAR experiment will contribute to understand this problem. Whatever the answer given by the experiment, the ANR project Photoplus will have significant impact on our understanding of fundamental interactions.

Project coordination

Pierre Clade (Laboratoire Kastler Brossel)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


LKB Laboratoire Kastler Brossel
LPP Laboratoire de physique des plasmas

Help of the ANR 554,459 euros
Beginning and duration of the scientific project: September 2021 - 48 Months

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