CERN Physicists Create Long-Lived Positronium

Physicists from the AEgIS (Antihydrogen Experiment: Gravity, Interferometry, Spectroscopy) Collaboration at CERN have found a new way of making long-lived positronium atoms, which consist of an electron and a positron (antimatter counterpart of the electron), for antimatter gravity experiments.

Positronium is a hydrogen-like atom consisting of a positron and an electron revolving around each other. Image credit: Pete Linforth.

The Universe is almost devoid of antimatter, and physicists haven’t yet figured out why.

Discovering any slight difference between the behavior of antimatter and matter in Earth’s gravitational field could shed light on this question.

Positronium atoms are one type of antimatter atoms being considered to test whether antimatter falls at the same rate as matter in Earth’s gravitational field.

But they are short-lived, lasting a mere 142 nanoseconds — too little to perform an antimatter gravity experiment.

Physicists are therefore actively seeking tricks to make sources of positronium atoms that live longer.

In a paper published this week in the journal Physical Review A, members of the AEgIS Collaboration described a new way of making long-lived positronium.

“To be useful for antimatter gravity experiments, a source of positronium atoms needs to produce long-lived atoms in large numbers, and with known velocities that can be controlled and are unaffected by disturbances such as electric and magnetic fields,” they said.

“Our new source ticks all of these boxes, producing some 80,000 positronium atoms per minute that last 1,140 nanoseconds each and have a known velocity (between 70 and 120 km per second) that can be controlled with a high precision (10 km per second).”

“Using a special positron-to-positronium converter to produce the atoms and a single flash of ultraviolet laser light kills two birds with one stone.”

“The laser brings the atoms from the lowest-energy electronic state to a long-lived higher-energy state and can select among all of the atoms only those with a certain velocity.”

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C. Amsler et al (The AEgIS Collaboration). 2019. Velocity-selected production of 2³S metastable positronium. Phys. Rev. A 99 (3); doi: 10.1103/PhysRevA.99.033405