Positronium laser cooling

27th February 2024
Paige West

At CERN, the AEgIS (Antimatter Experiment: gravity, Interferometry, Spectroscopy) collaboration, with contributions from Prof. Giovanni Consolati of the Department of Aerospace Science and Technology at Politecnico di Milano, successfully demonstrated positronium (Ps) laser cooling for the first time.

This achievement was made possible using a specially designed alexandrite-based laser system that delivers high intensity, wide bandwidth, and extended pulse duration, essential for effective cooling. The process cooled Ps atoms, emerging from a room-temperature porous target and hit by a positron beam, from 380 to 170°C. This cooling significantly reduced the Ps atoms' transversal root mean square (rms) velocity from 54 to 37km/s.

Positronium is akin to hydrogen but is approximately 2,000 times lighter due to a positron replacing the proton, halving its energy levels. Its instability in vacuum, with a mere 142ns lifespan due to annihilation, makes cooling within its brief existence particularly challenging. The broad bandwidth of the pulsed laser allows for cooling a significant portion of the Ps cloud, thereby extending their effective lifespan, and increasing the Ps quantity available for further experiments.

This advancement is critical for the AEgIS experiment's goal to measure antihydrogen's gravitational acceleration, testing the weak equivalence principle for antimatter. The experiment generates antihydrogen by reacting excited-state Ps with trapped antiprotons, with the formation likelihood increasing as Ps velocity decreases. Thus, producing Ps with minimal kinetic energy is crucial.

The ability to cool Ps atoms significantly benefits fundamental science, such as conducting precision spectroscopy on Ps's excited energy levels to test quantum electrodynamics with unparalleled accuracy or examining the equivalence principle in a purely leptonic system. Moreover, creating a collection of cold Ps atoms may lead to the formation of the first antimatter Bose-Einstein condensate (BEC), a state that makes quantum mechanical phenomena observable on a macroscopic scale. A positronium BEC could facilitate stimulated annihilation, offering a novel method to produce coherent electromagnetic radiation at gamma ray energies.

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